Ask the Experts: All Questions

Ask the Experts is one of our most popular destinations for healthcare professionals. Our experts provide clear, easy-to-understand answers to commonly asked questions about vaccines and their use.

How to Find Your Answer

Note: We are experiencing a technical issue. Selecting a filter subcategory may also return results from the primary category. We are working with our vendor to resolve this error.

  • Some filter options may become disabled as you narrow your selections.
  • Filters are easily added and removed. Click or tap on the X to remove individual filters or choose “Reset All Filters” to remove all filters.
Results (1316)

Stand-alone units that only refrigerate or only freeze are recommended by CDC. Household-style combination refrigerator/freezer units are less capable of simultaneously maintaining proper storage temperatures in both compartments. In addition, some areas of the refrigerator space may also be unusable due to uneven temperatures in the refrigerator section interior. If a household-style combination refrigerator/freezer must be used, only refrigerated vaccines should be stored in the unit: a separate stand-alone freezer should be used if the clinic also provides frozen vaccines. Pharmaceutical grade combination units designed for vaccine storage may be acceptable for use because they are engineered not to circulate air from the freezer directly into the refrigerator compartment in the way that a household-style unit does. Stand-alone units can vary in size from compact, under-the-counter (not dormitory) style to large, stand-alone, pharmaceutical grade units (which may be labeled as purpose-built for vaccine storage). For additional information see the CDC Storage and Handling Toolkit, page 9, at www.cdc.gov/vaccines/hcp/admin/storage/toolkit/storage-handling-toolkit.pdf.

One way to have confidence that the refrigerator or freezer unit you purchase will reliably maintain proper vaccine storage temperatures is to look for a unit labeled as meeting the NSF/ANSI 456 standard for vaccine storage. This voluntary certification indicates that the model has been tested and certified to maintain proper vaccine storage conditions under a range of normal clinic conditions.

Last reviewed: July 26, 2023

Appropriate site and needle length depends on age, route of injection, and body mass. Most injected vaccines are administered by the intramuscular route.

Please refer for details to the Immunize.org handouts on administering intramuscular and subcutaneous vaccines to children and adults at www.immunize.org/catg.d/p2020.pdf and to adults only at www.immunize.org/catg.d/p2020a.pdf.

A summary of needle length and site selection by age is below.

For intramuscular injections (use a 22- to 25-gauge needle for all ages):

  • For neonates (first 28 days of life) and preterm infants the anterolateral thigh should be used. A ⅝-inch needle usually is adequate to penetrate the thigh muscle if the skin is stretched flat between the thumb and forefinger and the needle is inserted at a 90-degree angle to the skin.
  • The anterolateral thigh is preferred for infants younger than age 12 months. For the majority of infants a 1-inch needle is sufficient.
  • For toddlers age 12 months through 2 years the anterolateral thigh muscle is preferred. The needle should be at least 1 inch long. The deltoid muscle can be used if the muscle mass is adequate.
  • For children age 3 through 10 years, the deltoid muscle is preferred; the needle length for deltoid site injections can range from ⅝ to 1 inch on the basis of technique. The anterolateral thigh can also be used. In this case the needle length should be 1 inch to 1.25 inches.
  • For adolescents 11 through 18 years, the deltoid muscle is preferred. The anterolateral thigh can also be used. For injection into the anterolateral thigh, most adolescents will require a 1-1.5-inch needle.
  • For adults age 19 years and older, the deltoid muscle is preferred. The anterolateral thigh also can be used.
    • For men and women who weigh less than 130 pounds (less than 60 kg), a ⅝-inch needle is sufficient to ensure intramuscular injection in the deltoid muscle if the injection is made at a 90-degree angle and the tissue is not bunched.
    • For men and women who weigh 130–152 pounds (60–70 kg), a 1-inch needle is sufficient.
    • For women who weigh 152–200 pounds (70–90 kg) and men who weigh 152–260 pounds (70–118 kg), a 1- to 1½-inch needle is recommended.
    • For women who weigh more than 200 pounds (more than 90 kg) or men who weigh more than 260 pounds (more than 118 kg), a 1½-inch needle is recommended.

For subcutaneous injections (use a 23- to 25-gauge needle for all ages):

Subcutaneous injections are administered at a 45-degree angle, usually into the thigh for infants younger than age 12 months and in the upper-outer triceps area of people age 12 months and older. Subcutaneous injections may be administered into the upper-outer triceps area of an infant if necessary. A ⅝-inch needle length should be used for all ages.

More information on injection technique is in the ACIP “General Best Practices Guidelines for Immunization”, available at www.cdc.gov/vaccines/hcp/acip-recs/general-recs/administration.html.

Last reviewed: December 28, 2022

Pneumococcal disease can be invasive, meaning a normally sterile part of the body is infected, or non-invasive. There are two major clinical syndromes of invasive pneumococcal disease: bacteremia (blood stream infection), and meningitis (infection of the meninges that surround the brain). They are both caused by infection with the same bacteria, but produce different signs and symptoms.

Pneumococcal pneumonia is a common disease caused by pneumococcal infection. Symptoms include abrupt onset of fever, shaking chills or rigors, chest pain, cough, shortness of breath, rapid breathing and heart rate, and weakness. The case-fatality rate is 5%–7% and is higher in adults 65 years and older and people with certain underlying medical conditions.

Pneumococcal pneumonia can occur in combination with bacteremia and/or meningitis (invasive pneumococcal pneumonia), or it can occur alone (non-invasive pneumococcal pneumonia). Before the COVID-19 pandemic, at least 100,000 people were hospitalized each year for pneumococcal pneumonia. At least 30,000 people were hospitalized each year for invasive pneumococcal disease, and about 3,000 people died. Rates of pneumococcal disease declined during the COVID-19 pandemic, but rates have begun to return to pre-pandemic levels.

About 4,000 cases of pneumococcal bacteremia without pneumonia occur each year in the United States. Bacteremia is the most common clinical presentation among children less than two years, accounting for up to 70% of invasive disease in this age group. The overall case-fatality rate of pneumococcal bacteremia is about 20% and may be as high as 60% among older adults.

Pneumococci cause 50% of all cases of bacterial meningitis in the United States. There are an estimated 2,000 cases of pneumococcal meningitis each year. Symptoms and signs may include headache, tiredness, vomiting, irritability, fever, seizures, and coma. The case-fatality rate of pneumococcal meningitis is about 8% among children and 22% among adults. Permanent neurological damage is common among survivors.

Last reviewed: April 5, 2024

Zoster is caused by reactivation of a latent varicella virus infection (from having chickenpox in the past). Zoster is not passed from one person to another through exposure to another person with zoster. However, if a person who is susceptible to chickenpox (i.e., they had never had chickenpox and were not vaccinated against chickenpox) comes in direct contact with a person with a zoster rash, the virus could be transmitted to the susceptible person. The exposed person would develop chickenpox, not zoster. Covering the zoster rash reduces the chances of transmitting varicella zoster virus.

Last reviewed: March 9, 2022

Persons with chronic HBV infection (those with persistent hepatitis B surface antigen [HBsAg] in the serum for at least 6 months) serve as the main reservoir for HBV transmission.

HBV is transmitted through percutaneous (through the skin), mucosal, or non-intact skin exposure to infectious blood or body fluids. HBV is concentrated most highly in blood, and percutaneous exposure is an efficient mode of transmission. Semen and vaginal secretions are infectious, and HBV also can be detected in saliva, tears, and bile. Cerebrospinal fluid, synovial fluid, pleural fluid, peritoneal fluid, pericardial fluid, and amniotic fluid are also considered potentially infectious. Urine, feces, vomitus, nasopharyngeal washings, sputum, and sweat are not efficient vehicles of transmission unless they contain blood because they contain low quantities of infectious HBV. Hepatitis B surface antigen (HBsAg) found in breast milk is also unlikely to lead to transmission so HBV infection is not a contraindication to breastfeeding.

Among adults in the U.S., HBV is transmitted primarily by percutaneous exposure to blood (for example, injection drug use) and sexual contact. HBV is transmitted efficiently by sexual contact both among heterosexuals and among men who have sex with men (MSM). Transmission can occur from interpersonal contact (e.g., sharing a toothbrush or razor, contact with exudates from dermatologic lesions, or contact with HBsAg-contaminated surfaces) and in settings such as schools, child care centers, and facilities for developmentally disabled persons. Transmission of HBV from transfusion of blood or blood products is rare because of donor screening and viral inactivation procedures. Other possible sources of infection include contaminated medical or dental instruments, unsafe injections, needle-stick injuries, organ transplantation, and dialysis.

In 2019, a total of 3,192 cases of acute hepatitis B were reported to CDC, corresponding to 20,700 estimated acute infections (based on the estimated ratio of acute cases reported to actual acute cases). The most commonly reported risk behaviors and exposures were injection drug use (35%), multiple sex partners (23%), and surgery (10%), followed by other sexual and bloodborne risk behaviors; risk behavior and exposure information were missing for 37.1% of cases.

Last reviewed: July 21, 2023

Most commercial insurance plans provide coverage of routine vaccinations with no out-of-pocket cost to the family. However, a child whose health insurance plan covers the cost of vaccinations as a benefit is not eligible for VFC vaccines, even if the plan requires a copay or requires that they meet a deductible for the year.

Last reviewed: August 26, 2022

Healthcare providers should suspect measles in patients with a febrile rash illness and the clinically compatible symptoms of cough, coryza (runny nose), and/or conjunctivitis (red, watery eyes). The illness begins with a prodrome of fever and malaise before rash onset. A clinical case of measles is defined as an illness characterized by

  • a generalized rash lasting 3 or more days, and
  • a temperature of 101°F or higher (38.3°C or higher), and
  • cough, coryza, and/or conjunctivitis.

Koplik spots, a rash present on mucous membranes, are considered pathognomonic for measles. Koplik spots occur from 1 to 2 days before the measles rash appears to 1 to 2 days afterward. They appear as punctate blue-white spots on the bright red background of the buccal mucosa. Pictures of measles rash and Koplik spots can be found at www.cdc.gov/measles/symptoms/photos.html.

Providers should be especially aware of the possibility of measles in people with fever and rash who have recently traveled abroad or who have had contact with international travelers.  Providers should immediately isolate and report suspected measles cases to their local health department and obtain specimens for measles testing, including viral specimens for confirmation and genotyping. Providers should also collect blood for serologic testing during the first clinical encounter with a person who has suspected or probable measles.

Last reviewed: June 19, 2023

The timing and severity of influenza seasons are always unpredictable. Influenza viruses circulated at very low levels while measures to prevent the spread of COVID-19 were widely adopted, including social distancing, mask wearing, and reduction in travel. As the use of these COVID-19 mitigation measures decreased, there was an increase in the circulation of influenza and other respiratory viruses. Current information on influenza virus circulation can be found at www.cdc.gov/flu/weekly/index.htm.

Last reviewed: September 10, 2023

Almost all U.S. infants and toddlers contract RSV illness within the first two years of life. RSV causes a mild respiratory illness in most, with symptoms including cough, runny nose, fever, and fatigue. Illness is more likely to be mild if the child is older at the time of first infection. Infants with RSV infection frequently develop bronchiolitis, a lower respiratory tract disease (LRTD) that can be severe.

RSV LRTD is the leading cause of hospitalization among U.S. infants, who may require supplemental oxygen, treatment for dehydration, or mechanical ventilation. Approximately 50,000–80,000 RSV-associated hospitalizations and 100–300 RSV-associated deaths occur each year among U.S. infants and children younger than age 5 years. The risk of severe RSV disease is increased by prematurity and lung disease, among other health conditions. However, RSV is also the leading cause of hospitalization among healthy, full-term infants. The large majority (almost 80%) of infants and children younger than age 2 who are hospitalized with RSV are otherwise healthy and have no underlying medical conditions.

Some otherwise healthy American Indian or Alaska Native (AI/AN) children experience higher rates of severe RSV disease than the general population. One study found that the incidence of RSV-associated hospitalization among children in their second RSV season in some AI/AN communities was 4 to 10 times higher than that of similar-aged children elsewhere in the United States. AI/AN children living in remote areas also may have difficulty accessing adequate medical care when they develop LRTD.

Last reviewed: January 22, 2024

One way to handle this is to indicate if the vaccination was given either in the “upper” or “lower” portion of the injection area selected (e.g., DTaP: right thigh, upper; Hib: right thigh, lower; or PCV13: left thigh, upper; HepB: left thigh, lower). It is helpful if everyone in your office or clinic uses the same sites for each vaccine. Use of a standardized site map can facilitate this. Here are some helpful site maps for different ages so you can record where shots were given:

For infants and toddlers: www.eziz.org/PDF/IMM-718.pdf

For adults: eziz.org/assets/docs/ADA/IMM-718A.pdf

Last reviewed: June 6, 2023

CDC estimated that 36,500 people developed cancers attributable to HPV infections each year between 2015 to 2019. Of these annual cases, about 94% could have been prevented by the 9-valent HPV vaccine, including about 30,100 cases caused by HPV types 16 and 18 and 4,300 cases caused by HPV types 31, 33, 45, 52, and 58.

HPV types 6 or 11 cause 90% of anogenital warts (condylomata) and most cases of recurrent respiratory papillomatosis.

For additional details, see www.cdc.gov/cancer/uscs/about/data-briefs/no31-hpv-assoc-cancers-UnitedStates-2015-2019.htm.

Last reviewed: March 2, 2024

In addition to risk based on age, non-specific risk factors for serogroups A, C, W and Y include having a previous viral infection, living in a crowded household, having an underlying chronic illness, and being exposed to cigarette smoke (either directly or second-hand).

The following groups are at increased risk for all meningococcal serogroups:

  • People with persistent (genetic) complement component deficiencies (a type of immune system disorder)
  • People who use complement inhibitors such as eculizumab (Soliris, Alexion Pharmaceuticals) and ravulizumab (Ultomiris, Alexion Pharmaceuticals) for treatment of atypical hemolytic uremic syndrome or paroxysmal nocturnal hemoglobinuria
  • People with anatomic or functional asplenia
  • Microbiologists routinely exposed to meningococcal isolates in a laboratory
  • People at increased risk during an outbreak of meningococcal disease
  • Military recruits
  • College students

Certain groups are at increased risk of serogroups A, C, W and Y, but not serogroup B:

  • People living with HIV
  • Men who have sex with men (MSM)
  • Travelers to countries where meningococcal disease is endemic or hyperendemic, such as the meningitis belt of sub-Saharan Africa
Last reviewed: March 24, 2024

Multiple complex mechanisms likely contribute to increased disease severity during a second DENV infection. The published ACIP recommendation provides a detailed description of these mechanisms at www.cdc.gov/mmwr/volumes/70/rr/pdfs/rr7006a1-H.pdf. In addition, CDC has developed simple illustrations and descriptions to explain this phenomenon at www.cdc.gov/dengue/vaccine/hcp/eligibility/index.html.

Last reviewed: February 16, 2022

Immunity to pertussis following infection is not life-long. Persons with a history of pertussis should continue to receive pertussis-containing vaccines according to the recommended schedule. (Note: This answer is based upon recommendations of the AAP’s Committee on Infectious Diseases.)

Last reviewed: March 31, 2022

Person-to-person spread through the fecal-oral route is the primary means of HAV transmission. Peak infectivity in infected people occurs during the two-week period before the onset of jaundice when the concentration of virus in the stool is highest and most people are no longer infectious one week after jaundice onset. Before routine vaccination of children was recommended, children were a key source of infection because most infected children had no symptoms and could shed virus in stool for weeks or months. Transmission currently occurs primarily among susceptible adults.

Common-source outbreaks and sporadic cases can occur from exposure to fecally-contaminated food or water. Uncooked HAV-contaminated foods have been recognized as a source of outbreaks. Cooked foods also can transmit HAV if the temperature during food preparation is inadequate to kill the virus or if food is contaminated after cooking, as occurs in outbreaks associated with infected food handlers. Transmission of the virus from infected food handlers to food service establishment patrons is rare, accounting for 0.2% of the nearly 23,000 outbreak-associated cases of hepatitis A investigated by state health departments during 2016-2019. In 2020, there were 9,952 cases reported to public health, although CDC estimates that the actual number of cases was closer to 19,900. This represents a 47% decrease from 2019.

Until 2017, US incidence rates of hepatitis A were driven by occasional outbreaks, often linked to viral contamination of imported food. Since 2017, communitywide outbreaks have occurred more frequently, predominantly among people who are connected by specific risk factors, such as drug use or homelessness, and their close contacts.

Last reviewed: June 25, 2023

You should record the generic abbreviation for the type of vaccine given (e.g., DTaP-IPV-HepB) in each of the sections that correspond to the separate antigens listed on the record (e.g., DTaP section, polio section, hepatitis B section). If possible, avoid using trade names, since trade names could be misinterpreted or discontinued.

Last reviewed: July 15, 2023

CDC maintains a webpage with critical interim clinical considerations for vaccination of eligible recipients: www.cdc.gov/vaccines/covid-19/clinical-considerations/covid-19-vaccines-us.html. This covers important clinical details about COVID-19 vaccination. It is the first content to be updated soon after any announced change to CDC recommendations.

All of the CDC’s Advisory Committee on Immunization Practices (ACIP) vaccine recommendations published in MMWR can be accessed here: www.cdc.gov/vaccines/hcp/acip-recs/vacc-specific/covid-19.html.

CDC has created infographic documents with the current recommended COVID-19 vaccination schedules for each age group:

Last reviewed: March 19, 2024

Antibiotics administered before or after receiving the CVD 103-HgR vaccine might diminish the effectiveness of the vaccine because the immune response to the vaccine relies on the live attenuated vaccine organisms replicating within the small intestine. We do not know what the optimal interval is between receipt of antibiotics and the Vaxchora vaccine. CDC refers clinicians to the package insert which specifies that CVD 103-HgR should not be given to patients who have received oral or parenteral antibiotics during the preceding 14 days. A duration of fewer than 14 days between stopping antibiotics and giving CVD 103-HgR might be acceptable under certain circumstances, such as if travel cannot be avoided during that 14-day interval.

The package insert, however, does not specify an optimal minimum duration between the completion of CVD 103-HgR vaccination and starting antibiotics. In certain circumstances, antibiotics might be clinically necessary after the vaccine (to treat an unrelated infection), thus clinical discretion is recommended.

Last reviewed: August 21, 2023

Chloroquine might diminish the immune response to CVD 103-HgR. The vaccine manufacturer, Emergent BioSolutions, recommends that CVD 103-HgR be administered 10 days or more before starting chloroquine. Doxycycline, a tetracycline antibiotic, is often used for malaria prophylaxis. The manufacturer does not recommend administration of the vaccine with oral antibiotics and does not recommend administering CVD 103-HgR to a person within 14 days prior to vaccination. The optimal duration between completion of CVD 103-HgR and then starting doxycycline is unknown.

Last reviewed: August 21, 2023

The significance of non-standard intervals probably depends on the vaccine and the dose. This is a complex issue—studies have not been done to examine the effect of various intervals between doses on the immunogenicity of those doses. But ACIP has examined the available data and made recommendations about the minimum acceptable interval between doses for that dose to be considered valid (there is no maximum interval between doses). These minimum intervals are published as Table 3-2 in ACIP’s “General Best Practice Guidelines on Immunization”, available at www.cdc.gov/vaccines/hcp/acip-recs/general-recs/timing.html. Doses with a minimum interval less than the recommended minimum, as described in Table 3-2, should not be counted as valid. More details on this topic can be found in the General Best Practice Guidelines.

Last reviewed: June 6, 2023

There are several very easy to read pieces that can be downloaded from the Immunize.org website. This includes “Vaccinations for Infants and Children, Age 0–10 Years”, “When do children and teens need vaccinations?”, “Vaccinations for Preteens and Teens, Age 11–19 Years”. These handouts can be found at www.immunize.org/handouts/discussing-vaccines-parents.asp

Last reviewed: August 22, 2020

In May 2022, CDC published new ACIP recommendations reducing the primary PrEP rabies vaccination series from 3 doses to 2 doses (administered intramuscularly on days 0 and 7) for all people at elevated risk for exposure to rabies. This was done based on strong evidence that the 2-dose schedule would provide the same protection as the previously recommended 3-dose primary series for up to three years. The less costly 2-dose schedule conserves rabies vaccine supplies, which have been subject to national shortages at times, and may increase adherence with PrEP recommendations.

Five categories of risk were created based upon variables including elevated risk of unusual, unrecognized, or recognized exposures and whether elevated risk is sustained over time, or not. For individuals in risk categories 1–3 (all with sustained elevated risk expected to last more than 3 years after the primary series), ACIP recommends periodic rabies vaccine antibody titer checks at different intervals, and/or booster vaccination. See the table on pages 622–623 of the MMWR published on May 6, 2022, (www.cdc.gov/mmwr/volumes/71/wr/pdfs/mm7118a2-H.pdf) for each risk category’s antibody titer and/or booster dose recommendations for sustained pre-exposure prophylaxis over time.

Last reviewed: May 14, 2023

Haemophilus influenzae type b is a polysaccharide-encapsulated bacteria that causes a variety of invasive diseases, such as meningitis, epiglottitis, and pneumonia. Influenza is a virus that causes the disease influenza.

Historical note: Haemophilus influenzae was first isolated in 1889 from the sputum of a patient who died of (viral) influenza disease, and the isolated organism (then called the Pfeiffer bacillus) was incorrectly assumed to have caused the patient’s illness. Haemophilus influenzae received its name in 1920, to acknowledge its historical association with influenza illness. The viral cause of influenza was not discovered until 1933.

Last reviewed: July 31, 2022

Advisory Committee on Immunization Practices (ACIP) recommendations for use of rotavirus vaccines are available at www.cdc.gov/mmwr/PDF/rr/rr5802.pdf.

Last reviewed: June 7, 2023

Following a multi-year shortage, in April 2021, the FDA-licensed yellow fever vaccine, YF-Vax (Sanofi) resumed availability for purchase in the United States. Providers with a current Yellow Fever Vaccination Stamp issued by their state or territorial health department may order YF-VAX from the manufacturer.

Locations that administer yellow fever vaccine can be found on CDC’s yellow fever vaccination clinic search page at wwwnc.cdc.gov/travel/yellow-fever-vaccination-clinics/search.

Healthcare providers should refer to the section titled “Yellow Fever and Malaria Information, by Country,” in CDC Health Information for International Travel 2024 (“The Yellow Book”) for information about the countries that require yellow fever vaccination for entry and the countries where CDC recommends yellow fever vaccination. If a country does not have an entry requirement, CDC does not recommend yellow fever vaccination if the traveler’s itinerary does not include travel to a yellow fever–endemic area. This section is available at wwwnc.cdc.gov/travel/yellowbook/2024/preparing/yellow-fever-vaccine-malaria-prevention-by-country.

Last reviewed: August 21, 2023

In most temperate regions of the continental United States and other areas with similar climates, RSV season typically begins in the fall and peaks in the winter, generally circulating between October and the end of March. However, the timing and severity of RSV season in a given community can vary from year to year. RSV seasonality was temporarily disrupted by the COVID-19 pandemic and widespread changes in social interactions and infection control practices (e.g., mask wearing, social distancing).

U.S. states and territories with very different weather patterns from the continental United States (e.g., Alaska, and areas with tropical weather patterns, such as Hawaii, southern Florida, Puerto Rico, and other island territories) may experience very different patterns of local RSV circulation. For this reason, public health and healthcare authorities in those areas may issue different recommendations for appropriate timing of RSV prevention strategies (vaccines, monoclonal antibodies).

Last reviewed: January 22, 2024

Pneumococcal disease is a serious disease that causes much sickness and death. Before the dip in pneumococcal disease observed in the first two years of the COVID-19 pandemic (when measures to control COVID-19 reduced the incidence of several infectious diseases), an estimated 30,000 cases and 3,000 deaths from invasive pneumococcal diseases (bacteremia and meningitis) occurred in the United States. Children younger than age two years and adults age 50 years and older have the highest incidence of serious disease. Case-fatality rates are highest for pneumococcal meningitis and bacteremia, and the highest mortality occurs among older adults and patients who have underlying medical conditions.

Last reviewed: April 5, 2024

Measles is highly contagious. A person with measles is infectious up to 4 days before through 4 days after the day of rash onset. Patients with suspected measles should be isolated for 4 days after they develop a rash. Airborne precautions should be followed in healthcare settings by all healthcare personnel. The preferred placement for patients who require airborne precautions is in a single-patient airborne infection isolation room. Providers should immediately isolate and report suspected measles cases to their local health department and obtain specimens for measles testing, including serum sample for measles serologic testing and a throat swab (or nasopharyngeal swab) for viral confirmation.

Measles is a nationally notifiable disease in the U.S.; healthcare providers should report all cases of suspected measles to public health authorities immediately to help reduce the number of secondary cases. Do not wait for the results of laboratory testing to report clinically-suspected measles to the local health department.

More information on measles disease, diagnostic testing, and infection control can be found at www.cdc.gov/measles/hcp/index.html.

Last reviewed: June 19, 2023

In a school setting, an immunocompetent person with zoster (staff or students) can remain at school as long as the lesions can be completely covered. People with zoster should be careful about personal hygiene, wash their hands after touching their lesions, and avoid close contact with others. If the lesions cannot be completely covered and close contact avoided, the person should be excluded from the school setting until the zoster lesions have crusted over. See www.cdc.gov/chickenpox/outbreaks/manual.html for more information. If your program is licensed by a state or county, you should check their regulations as well.

Last reviewed: March 9, 2022

Yes. Both viruses can circulate at the same time, and a person can be infected with both viruses at the same time. Illnesses experienced by people co-infected with influenza and SARS-CoV-2 are more likely to be severe compared with those infected with only SARS-CoV-2 or influenza alone. The extent to which SARS-CoV-2 and influenza viruses will co-circulate during the upcoming 2023–24 fall and winter respiratory virus season is unknown.

Last reviewed: September 10, 2023

Yes. Vaccines that are stored in the refrigerator portion of a household-style combination refrigerator/freezer should be moved away from the vent located in the refrigerator compartment. The cold air from the freezer is circulated into the refrigerator compartment to cool it, which can cause your vaccines to freeze. Inactivated vaccines must be kept between 2° and 8°C (between 36° and 46°F) and not frozen.

Last reviewed: July 26, 2023

The production of a few vaccines, including those for varicella, rubella, hepatitis A, and one of the COVID-19 vaccines (Janssen, Johnson & Johnson) involves growing the viruses in human cell culture. Two human cell lines provide the cell cultures needed for producing vaccines; these lines were developed from two legally aborted fetuses in the 1960s. These cell lines are maintained to have an indefinite life span. No fetal tissue has been added since the cell lines were originally created.

Some parents are concerned about this issue because of misinformation they have encountered on the Internet. Two such untrue statements are that ongoing abortions are needed to manufacture vaccines and vaccines are contaminated with fetal tissue.

Parents can read the facts and several thought-provoking articles about this issue at www.immunize.org/talking-about-vaccines/religious-concerns.asp and then make an informed decision.

A Catholic Pontifical Academy for Life statement that Catholic parents have no general obligation to refuse permission for these vaccines is summarized at www.immunize.org/talking-about-vaccines/vaticandocument.htm.

Last reviewed: August 31, 2022

Most adults residing in the United States are presumed to be protected against polio because they received routine childhood immunization and have only a small risk of exposure to poliovirus in the United States. For decades, routine polio vaccination of U.S. residents 18 years of age and older, including those working in healthcare or in healthcare-related training, was not recommended.

In June 2022, a case of paralytic polio caused by vaccine-derived poliovirus type 2 (VDPV2) was confirmed in an unvaccinated young adult from Rockland County, New York, and wastewater surveillance in the region repeatedly detected evidence of poliovirus over the following months, suggesting the presence of an unknown number of asymptomatically infected people in the community. People who are fully vaccinated with IPV are protected from polio illness but may shed virus in their feces, if exposed. This experience underscored the ongoing risk, however small, of paralytic polio among incompletely vaccinated people in the United States.

CDC’s ACIP recommended in June 2023, that all adults (18 years and older) who are known or suspected to be unvaccinated or incompletely vaccinated against polio should complete a primary 3-dose vaccination series with IPV: 2 doses of IPV administered at an interval of 4–8 weeks; and a third dose 6–12 months after the second. If an adult previously received only one or two documented doses of polio vaccine (either trivalent OPV or IPV), the person should receive the remaining dose(s) of IPV necessary to complete the series, regardless of the interval since the last dose. It is not necessary to restart the vaccination series.

Polio vaccination with a complete primary series is recommended for all travelers to countries with wild poliovirus (WPV) or vaccine-derived poliovirus (VDPV) circulation. WHO recommends that countries affected by WPV or cVDPV require long-term visitors (4 weeks or longer) to provide proof of polio vaccination before leaving the country. For additional information on countries with WPV or VDPV circulation, as well as country-specific requirements for documentation of vaccination, consult the CDC Travelers’ Health website (wwwnc.cdc.gov/travel/).

Adults who have completed a routine series of polio vaccine (with trivalent OPV or IPV in any combination) are considered to have lifelong immunity to poliomyelitis, but data on duration of immunity are lacking. In June 2023, ACIP affirmed and clarified its longstanding recommendation that certain adults at increased risk of exposure to poliovirus may receive a single lifetime adult booster dose of IPV. Adults in situations that put them at increased risk of poliovirus exposure include: travelers going to countries where polio is epidemic or endemic; laboratory and healthcare workers who handle specimens that might contain polioviruses; and, healthcare workers or other caregivers who have close contact with a person who could be infected with poliovirus.

Last reviewed: July 23, 2023

There is no treatment for infection with the HPV virus itself. Only HPV-associated lesions including genital warts, recurrent respiratory papillomatosis, precancers, and cancers are treated. Recommended treatments vary depending on the diagnosis, size, and location of the lesion. Local treatment of lesions might not eradicate all HPV-containing cells fully. It is unclear whether treating the lesion reduces the risk that the infected person could transmit the HPV infection to others.

Last reviewed: March 2, 2024

All people age 6 months and older in the United States are recommended to receive an age-appropriate updated (2023–2024 Formula) COVID-19 vaccination. Schedules vary by age and immunocompromised status. CDC has created infographic documents with the current recommended COVID-19 vaccination schedules for each age group:

Last reviewed: March 19, 2024

The vaccines for meningococcal serogroups A, C, W, and Y (MenACWY; MenQuadfi by Sanofi; Menveo by GSK) contain meningococcal conjugate in which the surface polysaccharide is chemically bonded (“conjugated”) to a protein to produce a robust immune response to the polysaccharide. The MenACWY vaccine products are considered interchangeable; the same vaccine product is recommended, but not required, for all doses.

Two meningococcal vaccines were used in the recent past but are no longer available. Menactra (Sanofi) is a discontinued MenACWY conjugate vaccine. The last doses of Menactra expired in 2023. Menactra was considered interchangeable with Menveo or MenQuadfi. An older meningococcal polysaccharide vaccine (MPSV4, Menomune, Sanofi) was available in the United States until the last doses expired in 2017: it was never routinely recommended for children or teens.

Since late 2014, vaccines have become available that offer protection from meningococcal serogroup B disease (MenB; Bexsero by GSK; Trumenba by Pfizer). These vaccines are composed of proteins found on the surface of the bacteria. Bexsero and Trumenba are not interchangeable; the same vaccine product is required for all doses.

A pentavalent MenACWY and MenB vaccine, abbreviated MenABCWY (Penbraya, Pfizer) contains a conjugated MenACWY vaccine mixed with the MenB vaccine contained in Trumenba (Pfizer).

MenACWY vaccines provide no protection against serogroup B disease, and MenB vaccines provide no protection against serogroup A, C, W, or Y disease. For protection against all 5 serogroups of meningococcus, it is necessary to receive both MenACWY and MenB, either as separate vaccines or as the combination MenABCWY vaccine, Penbraya.

Trade Name Type of Vaccine Serogroups Year Licensed Approved Ages
Penbraya Conjugate A, B, C, W, Y 2023 10–25 years*
Menveo (two vial)
Menveo (one vial)		 Conjugate
Conjugate		 A, C, W, Y
A, C, W, Y		 2010
2022		 2 mos.–55 years*
10–55 years
MenQuadfi Conjugate A, C, W, Y 2020 2 years and older
Trumenba Protein B 2014 10–25 years*
Bexsero Protein B 2015 10–25 years*

*May be given to adults at increased risk older than the FDA-approved upper age limit (see ACIP recommendations, Table 11, page 41, www.cdc.gov/mmwr/volumes/69/rr/pdfs/rr6909a1-H.pdf)

Last reviewed: March 24, 2024

Dengvaxia (Sanofi Pasteur) is a live-attenuated, chimeric tetravalent dengue vaccine built on a yellow fever 17D backbone. It is designed to protect against all four DENV serotypes. It was licensed by the FDA in May 2019 for children and adolescents age 9 through 16 years with a confirmed history of previous dengue infection. The recommendation for vaccination is restricted to people with confirmed previous DENV infection because Dengvaxia is associated with an increased risk for severe dengue in those who experience their first natural infection (i.e., primary infection) after vaccination.

Last reviewed: February 16, 2022

In general, vaccines containing adjuvants (a component that enhances the antigenic response) are administered IM to avoid irritation, induration, skin discoloration, inflammation, and granuloma formation if injected into subcutaneous tissue. This includes most of the inactivated vaccines, with a few exceptions (such as IPV and pneumococcal polysaccharide vaccines, which may be given either SC or IM). Vaccine efficacy may also be reduced if not given by the recommended route.

Last reviewed: December 28, 2022

Yes. Adolescents or adults who have a history of pertussis disease generally should receive Tdap according to the routine recommendation. This practice is recommended because the duration of protection induced by pertussis disease is unknown (waning might begin as early as 7 years after infection) and because diagnosis of pertussis can be difficult to confirm. Administering pertussis vaccine to people with a history of pertussis presents no theoretical risk. For details, visit CDC’s published recommendations on this topic at www.cdc.gov/mmwr/volumes/67/rr/pdfs/rr6702a1-H.pdf.

Last reviewed: March 31, 2022

Vaccination providers frequently encounter people who do not have adequate documentation of vaccinations. Providers should only accept written, dated records as evidence of vaccination. CDC recommends that, with the exception of influenza and pneumococcal polysaccharide vaccines, self-reported doses of vaccine without written documentation should generally not be accepted. An attempt to locate missing records should be made whenever possible by contacting previous healthcare providers, reviewing state or local immunization information systems, and searching for a personally held record. However, if records cannot be located or will definitely not be available anywhere because of the patient’s circumstances, people without adequate documentation should be considered susceptible and should be started on the age-appropriate vaccination schedule. Serologic testing for immunity is an alternative to vaccination for certain antigens (e.g., measles, rubella, or hepatitis A).

In general, although it is not ideal, receiving extra doses of vaccine poses no medical problem. Receiving excessive doses of tetanus toxoid (DTaP, DT, Tdap, or Td) can increase the risk of a local adverse reaction, however. For details, consult the ACIP’s “Best Practice Guidelines for Immunization” chapter titled Timing and Spacing of Immunobiologics, available at www.cdc.gov/vaccines/hcp/acip-recs/general-recs/timing.html.

Last reviewed: June 6, 2023

Yes. Even though tattooing and body piercing are not thought to be a significant mode of transmission for HBV, tattooing and body piercing have the potential to transmit bloodborne infections, including HBV, hepatitis C virus (HCV), and human immunodeficiency virus (HIV), if the person doing the tattoos or body piercing does not use good infection control practices. The Centers for Disease Control and Prevention (CDC) recommends that instruments or materials (including ink), intended to penetrate the skin be used once, then disposed of or thoroughly cleaned and sterilized between clients. Personal service workers who do tattooing or body piercing should be educated about the transmission of bloodborne pathogens and what precautions are needed to prevent transmission.

People considering getting a tattoo or having a body part pierced should ask staff at the establishment what procedures they use to prevent the spread of bloodborne infections. They also might call the local health department to find out what sterilization procedures are required by law or ordinance for tattooing and body piercing establishments.

Last reviewed: July 21, 2023

The minimum age for varicella vaccine is 12 months. Vaccination is not recommended for infants younger than 12 months of age even as post-exposure prophylaxis. CDC recommends that a healthy infant should receive no specific treatment or vaccination after exposure to VZV. The child can be treated with an appropriate antiviral medication if chickenpox occurs.

See the Varicella Zoster Immune Globulin section for details on the recommended use of VariZIG in immunocompromised children, infants exposed to varicella around the time of birth and some hospitalized preterm infants.

Last reviewed: May 16, 2023

HAV can produce either asymptomatic or symptomatic infection in humans after an average incubation period of 28 days (range: 15–50 days).

Last reviewed: June 25, 2023

No. The CDC states that the only criteria are age (age 18 years or younger) and the four eligibility criteria listed previously. No other factors (for example, residency status) can be considered when screening for eligibility requirements for the VFC program.

Last reviewed: August 26, 2022

The hyphen (-) is intended to indicate that the antigens are mixed together by the manufacturer before the product is sold and the forward slash (/) indicates that the two products are to be reconstituted by the user.

Last reviewed: July 15, 2023

This is not true. Pregnant healthcare personnel may administer any vaccine except the live, replication-competent smallpox vaccine (ACAM2000, Emergent Biosolutions).

Last reviewed: August 29, 2022

As a general rule, infants or children who are more than 1 month or 1 dose behind schedule should be on an accelerated schedule, which means the intervals between doses should be reduced to the minimum allowable. Catch-up schedules for children ages 4 months through 18 years are included in Table 2 of each year’s recommended immunization schedule for children and adolescents, approved by the ACIP, CDC, and all other major professional organizations of healthcare providers who care for children. To review Table 2, go to www.cdc.gov/vaccines/schedules/hcp/index.html and open the current schedule for children and adolescents.

Last reviewed: June 6, 2023

There are many excellent websites that have information about vaccine safety, including the American Academy of Pediatrics, the CDC, Vaccinate Your Family, Immunize.org, the National Academies of Science, Engineering, and Medicine, and the Vaccine Education Center of the Children’s Hospital of Philadelphia.

Last reviewed: August 31, 2022

No. Doses of rabies vaccine given in the gluteus should not be counted as valid and should be repeated. If repeating the invalid dose results in an interval between doses more than 3 days longer than the recommended interval, then you should perform a rabies serology 7–14 days after administration of the final dose in the series to ensure an adequate immune response to the series. For more information about rabies serology, see www.cdc.gov/rabies/specific_groups/doctors/serology.html.

Last reviewed: May 14, 2023

The most recent Advisory Committee on Immunization Practices (ACIP) recommendations for Hib vaccination were published in 2014 and are available on the CDC website at www.cdc.gov/vaccines/hcp/acip-recs/vacc-specific/hib.html. Guidance for Hib vaccination is also provided in the annual childhood immunization schedule, available at www.cdc.gov/vaccines/schedules/hcp/child-adolescent.html.

Last reviewed: July 31, 2022

With some exceptions, there aren’t VISs for combination vaccines. Instead, providers should provide a separate VIS for each vaccine component in the combination (e.g., DTaP-IPV-HepB or DTaP-IPV/Hib). There is a combined VIS (the multi-vaccine VIS) that can substitute for any or all of the routine vaccines given from birth–6 months (DTaP, IPV, Hib, PCV and HepB vaccines). VISs in English and many other languages are available at www.immunize.org/vis/.

Last reviewed: July 15, 2023

Immunity to one of the serotypes of polio does not produce significant immunity to the other serotypes. A history of having recovered from polio disease should not be considered evidence of immunity to polio. You should complete a primary series of IPV if he is known or suspected to have been unvaccinated or incompletely vaccinated against polio. If he received a primary series in childhood, a single adult booster dose may be given before travel to an area where polio vaccination is recommended.

Last reviewed: July 23, 2023

Generally speaking, CDC recommends avoiding the top shelf and the areas near vents due to temperature fluctuations. However, most purpose-built or pharmaceutical-grade units use a fan to circulate air within the storage area and create more uniform temperatures than household units. During a power outage, the top shelf is an area of caution for all units as the temperatures increase most quickly there. In this instance, it would be best to check with the manufacturer or owner’s manual to see if the top shelf is appropriate for storage in your unit. Units that meet the NSF/ANSI 456 voluntary certification standard are designed to deter the user from placing vaccines in areas where proper storage temperatures cannot be maintained, so any shelf available storage in a certified unit would be usable.

Last reviewed: July 26, 2023

Two rotavirus vaccines are available in the United States. RotaTeq (RV5; Merck) is recommended for routine oral administration for all infants as a 3-dose series. The usual schedule is at ages 2, 4, and 6 months. Rotarix (RV1; GlaxoSmithKline) is recommended as a 2-dose series at ages 2 and 4 months.

The minimum interval between doses of rotavirus vaccine is 4 weeks. The minimum age for the first dose is 6 weeks and the maximum age for dose #1 is 14 weeks 6 days. Vaccination should not be initiated for infants age 15 weeks 0 days or older because there are insufficient data on the safety of dose #1 in older infants. The maximum age for the last dose of rotavirus vaccine is 8 months and 0 days.

Last reviewed: June 7, 2023

The most current comprehensive recommendations from the Advisory Committee on Immunization Practices (ACIP) for meningococcal vaccines is available on the MMWR website at www.cdc.gov/mmwr/volumes/69/rr/pdfs/rr6909a1-H.pdf. This document replaces all previously published reports and policy notes.

The use of MenABCWY (Penbraya) is discussed in the notes of the current CDC Recommended Immunization Schedules available at: www.cdc.gov/vaccines/schedules/hcp/index.html.

Last reviewed: March 24, 2024

The CDC’s General Best Practice Guidelines for Immunization generally recommends that live parenterally or nasally administered vaccines not given on the same day should be separated by at least 28 days. Despite this general principle, limited data suggest that coadministration of yellow fever vaccine and MMR may diminish the immune response. The CDC travel health website recommends that yellow fever vaccine and other parenteral or nasal live vaccines should be separated by at least 30 days, if possible. If yellow fever vaccine and another injectable live-virus vaccine are not administered either simultaneously or at least 30 days apart, CDC advises that providers might consider measuring the patient’s neutralizing antibody response to vaccination before travel. CDC recommends contacting the state health department or the CDC Arboviral Disease Branch (970-221-6400) to discuss serologic testing. For details, see the 2024 Yellow Book section on spacing of vaccines and immunobiologics: wwwnc.cdc.gov/travel/yellowbook/2024/preparing/vaccination-and-immunoprophylaxis-general-principles#spacing.

Last reviewed: August 21, 2023

Immunize.org has developed several screening questionnaires for patient use. These include:

  • Do I Need Any Vaccinations Today? (adult vaccination assessment checklist)
  • Vaccinations for Adults—You’re never too old to get immunized!
  • Vaccinations Needed During Pregnancy
  • Vaccinations for Adults with Chronic Liver Disease or Infection
  • Vaccinations for Adults with Diabetes
  • Vaccinations for Adults with Heart Disease
  • Vaccinations for Adults with HIV Infection
  • Vaccinations for Adults with Lung Disease
  • Vaccinations for Adults without a Spleen
  • Vaccinations for Men Who Have Sex with Men
  • Should You Be Vaccinated Against Hepatitis B? A screening questionnaire for adults, and
  • Should You Be Vaccinated for Hepatitis A? A screening questionnaire for adults.

These handouts can be found at www.immunize.org/handouts/view-all.asp

In addition to these printed pieces, there are several interactive tools on CDC’s website. For children, go to www2a.cdc.gov/vaccines/childquiz/, and for adults, go to www2.cdc.gov/nip/adultImmSched/.

Last reviewed: November 5, 2023

One pneumococcal polysaccharide vaccine (PPSV23, Pneumovax 23, Merck) and two pneumococcal conjugate vaccines [PCV15 (Vaxneuvance, Merck), and PCV20 (Prevnar 20, Pfizer)] are FDA-licensed and recommended by CDC for use in the United States. PCV13 (Prevnar 13, Pfizer) is FDA-licensed and may still be available in some clinics. It is no longer routinely recommended; however, CDC guidance allows for its use as previously recommended in situations where PCV15 or 20 is indicated but unavailable and the alternative is that the patient would not be vaccinated.

PPSV23 is licensed for age 2 years and older. It was first licensed in 1983. It is an option for use in series with PCV15 for children and adults ages 2 through 64 years with specified risk factors for pneumococcal disease depending on their prior pneumococcal vaccination history. A PCV15 + PPSV23 series also is recommended as an option for pneumococcal disease prevention in adults 65 years and older. Following the 2022 changes to the pneumococcal vaccination schedule for adults, PPSV23 is no longer recommended alone, however PPSV23 is recommended for adults following PCV13 or PCV15 vaccination. It is not recommended for people who have previously received a PCV20 vaccination.

PCV15 is licensed for people age 6 weeks and older. CDC recommends the use of PCV15 as an option for the routine vaccination of children younger than age 5 years and certain children 6 through 18 years who have conditions that put them at high risk of invasive pneumococcal disease. CDC recommends PCV15 in series with PPSV23 as an option for pneumococcal disease prevention in adults age 19 through 64 years who are at increased risk for invasive pneumococcal disease due to behavioral or medical risk factors and for adults age 65 or older. When used in adults, it is always recommended to be used as part of a vaccination series with PPSV23 typically given 1 year later (a minimum interval of 8 weeks may be considered for people with immunocompromising medical conditions, cochlear implant or cerebrospinal fluid leak).

PCV20 is licensed for people age 6 weeks and older. CDC recommends the use of PCV20 as an option for the routine vaccination of children younger than 5 years of age and certain children 6 through 18 years who have conditions that put them at high risk of invasive pneumococcal disease. CDC recommends it as an option for pneumococcal disease prevention in adults age 19 through 64 years who are at increased risk for invasive pneumococcal disease due to behavioral or medical risk factors and for adults age 65 or older. If PCV20 is given, no further pneumococcal vaccination is recommended.

For details of recommendations for these vaccines, see Recommendations for Pneumococcal Vaccines Use in Children and Teens, www.immunize.org/wp-content/uploads/catg.d/p2016.pdf, or Standing Orders for Administering Pneumococcal Vaccines (PCV15, PCV20, PPSV23) to Adults: www.immunize.org/wp-content/uploads/catg.d/p3075.pdf.

Last reviewed: April 5, 2024

There are no specific data on transmission of bloodborne viruses through oral-genital sex. Saliva has not been associated with HBV transmission unless biting has taken place. HBV is not spread by kissing, hugging, sneezing, coughing, food or water, sharing eating utensils or drinking glasses, or casual contact.

Last reviewed: July 21, 2023

Information regarding influenza surveillance is available year-round from CDC at www.cdc.gov/flu/weekly/fluactivitysurv.htm. The full FluView surveillance report is published each Friday afternoon from October through mid-May and an abbreviated FluView report is published Mid-May through September. In addition, periodic updates about influenza are published in MMWR.

State and local health departments should be consulted regarding local availability of influenza vaccine, access to vaccination programs, information about state or local influenza activity, and for reporting influenza outbreaks and receiving advice regarding their control.

Last reviewed: September 10, 2023

Tdap vaccination status does not change the approach to evaluating postexposure prophylaxis when HCWs are exposed to pertussis. Tdap vaccines have an uncertain role in the prevention of transmission of pertussis and herd protection. Antipertussis antibody levels begin to decline precipitously after the first year following a single Tdap vaccination. Healthcare facilities should follow the post-exposure prophylaxis protocol for pertussis exposure recommended by CDC regardless of a HCW’s vaccination status (see www.cdc.gov/pertussis/pep.html). HCW can either receive postexposure prophylaxis or be carefully monitored for 21 days after pertussis exposure. Health care personnel should be treated with antibiotics at the onset of signs and symptoms of pertussis and excluded from work for the first 5 days while receiving appropriate antibiotics. CDC supports targeting postexposure antibiotic use to people at high risk of developing severe pertussis, as well as people who will have close contact with others at high risk of developing severe pertussis.

Last reviewed: October 31, 2023

Occupational infection with HPV is possible. Some HPV-associated conditions (including anogenital and oral warts, anogenital intraepithelial neoplasia, and recurrent respiratory papillomatosis) are treated with laser or electrosurgical procedures that could produce airborne particles. These procedures should be performed in an appropriately ventilated room using standard precautions and local exhaust ventilation. Workers in HPV research laboratories who handle wild-type viruses or “quasi virions” might be at risk of acquiring HPV from occupational exposures. In the laboratory setting, proper infection control should be instituted, including, at minimum, biosafety level 2. Whether HPV vaccination would be of benefit in these settings is unclear because no data exist on transmission risk or vaccine efficacy in this situation.

Last reviewed: March 2, 2024

CDC recommends that all people receive an updated (2023–2024 Formula) COVID-19 vaccine, as authorized or approved by FDA. There are three options: Moderna or Pfizer-BioNTech mRNA vaccine (both FDA-authorized beginning at age 6 months and FDA-licensed for age 12 years and older), or Novavax adjuvanted protein subunit vaccine (authorized for age 12 years or older). Most people age 5 years through 64 years should receive a single dose of vaccine.

Certain people need, or have the option to receive, more than one 2023–2024 COVID-19 vaccine dose:

  • All adults age 65 years and older should receive a second dose of any 2023–2024 Formula COVID-19 vaccine at least 4 months after receiving their first dose of 2023–2024 Formula COVID-19 vaccine. The first and second doses do not need to be the same brand. If administering Novavax to a previously unvaccinated person in this age group, administer the recommended 2-dose primary series, followed by a third dose of 2023–2024 COVID-19 vaccine at least 4 months after completing the 2-dose primary series.
  • All previously unvaccinated individuals age 12 years or older receiving Novavax vaccine should receive two doses separated by at least 3 weeks. If age 65 years or older and previously unvaccinated, administer the 2-dose primary series, followed by a third dose of 2023–2024 COVID-19 vaccine at least 4 months after completing the 2-dose primary series.
  • Children younger than age 5 years are recommended to receive at least one updated 2023–2024 Formula mRNA vaccine dose. If not previously vaccinated, or incompletely vaccinated, they should complete a 2-dose (Moderna) or 3-dose (Pfizer-BioNTech) primary series, using the same brand for all doses. CDC recommends that children younger than age 5 years receive the same brand for all doses (also referred to as homologous doses), even after completing the primary series. If it is not feasible to administer the same brand for one of the following reasons, it is acceptable to use a different brand: the brand is unavailable at the clinic at the time of the vaccination visit; the previous brand is unknown; the child would not otherwise receive a recommended vaccine; or, the child cannot continue with the previous brand due to a contraindication.
  • Individuals who are moderately or severely immunocompromised and who have not already completed a 3-dose primary series, should complete a 3-dose primary vaccination series with the updated 2023–2024 Formula. Use the same brand for all doses of the primary series, unless the brand is unavailable at the time of the clinic visit, the patient would otherwise choose not to complete the series, or the patient has a contraindication to vaccination with the original product. If the primary series is already complete, then a single dose of any age-appropriate updated (2023–2024 Formula) COVID-19 vaccine should be administered at least 2 months after the most recent dose of COVID-19 vaccine. Additional doses may be given, spaced at least 2 months apart, as determined by these individuals with their clinical teams.

CDC has created infographic documents with the current recommended COVID-19 vaccination schedules for each age group:

Last reviewed: March 19, 2024

Vaccine efficacy (VE) during clinical trials in multiple countries ranged from 76% to 82%. VE varied by serotype and was highest for DENV-4 (89% point estimate) and lowest for DENV-1 and DENV-2 (67% point estimate). Among study participants ages 9 through 16 years who developed protective antibodies, the vaccine reduced their risk of hospitalization with dengue by 79%, and reduced their risk of severe dengue by 84% over the 5-year follow-up period after vaccination. Studies are ongoing to determine how long protection from hospitalization or severe disease may last, but current evidence show protection lasting at least 6 years after the last dose of vaccine.

Last reviewed: February 16, 2022

Yes. There is no age limit for use of the anterolateral thigh for either subcutaneous or intramuscular vaccines.

Last reviewed: December 28, 2022

This page was updated on .

Our Affiliated Sites

For the Public: VaccineInformation.org
For Coalitions: ImmunizationCoalitions.org
65+ Flu Defense
Protect vulnerable, older adult patients from influenza.
Give2MenACWY.org
Improve protection from meningococcal serogroups ACWY.
Mass Vaccination Resources
Access guidance documents, toolkits, and helpful resources.
NAIIS
Home of the National Adult and Influenza Immunization Summit.