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.

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Results (1313)

Prior to the availability of varicella vaccine there were approximately 4 million cases of varicella a year in the U.S. Though usually a mild disease in healthy children, an estimated 150,000 to 200,000 people developed complications, about 11,000 people required hospitalization and 100 people died each year from varicella. Varicella tends to be more severe in infants, adolescents and adults than in young children. The most common complications from varicella include bacterial superinfection of skin lesions, pneumonia, central nervous system involvement, and thrombocytopenia.

Last reviewed: May 16, 2023

Since the 1980s, the number of reported pertussis cases has increased in children, adolescents and adults. The most recent overall peaks in disease were seen between 2010 and 2014. The incidence in all age groups has decreased since 2014 and preliminary surveillance data indicate that incidence declined precipitously during the COVID-19 pandemic in 2020 and 2021. In 2019, CDC received reports of more than 18,000 cases of pertussis, with preliminary estimates of 5,398 reported cases in 2020 and just 1,609 in 2021. COVID-19 control measures such as wearing masks in public, social distancing, and other interventions designed to reduce the spread of COVID-19 also helped reduce the spread of pertussis and other respiratory infections. However, with the discontinuation of these measures, the prevalence of infections like pertussis that are transmitted by respiratory droplets is likely to increase.

The incidence of pertussis remains highest in young infants. Approximately 1 in 10 U.S. pertussis cases were among infants. Infants are at greatest risk for serious disease and death from pertussis. An increase in the number of reported deaths from pertussis among very young infants has paralleled the increase in the number of reported cases.

Reasons for the increases in pertussis seen since the 1980s are not completely clear; however, multiple factors have likely contributed to the increase, including waning immunity from the pediatric acellular vaccine (DTaP), increased recognition of pertussis, and improved diagnostic testing and reporting.

Last reviewed: March 31, 2022

Before rotavirus vaccines were available, rotavirus was the most common cause of severe gastroenteritis in infants and young children in the United States and worldwide. Almost all children were infected by age 5 years. Before vaccine was introduced in the United States, rotavirus was responsible each year for about 3 million episodes of gastroenteritis, 410,000 physician visits, 205,000–272,000 emergency department visits, 55,000–70,000 hospitalizations, and between 20 and 60 deaths among children younger than age 5 years.

Last reviewed: June 7, 2023

Dengue is an infectious disease caused by dengue viruses (DENV) spread among people through the bite of an infected mosquito from the Aedes species (Ae. aegypti or Ae. albopictus). DENV are members of the genus Flavivirus in the family Flaviviridae.

There are four dengue virus serotypes (DENV-1, DENV-2, DENV-3, and DENV-4), all of which circulate globally. The four dengue virus serotypes are closely related but can be differentiated on serologic tests. Infection with one serotype generally produces long-lived immunity to that serotype, but only provides short-lived protection against infection with other serotypes. For this reason, a person can be infected with DENV as many as four times in their lifetime.

Symptoms of dengue range from asymptomatic or very mild (75% of cases) to severe disease complicated by shock, bleeding, or severe organ impairment. Fever is the most common symptom of dengue. Other symptoms can include sudden onset of headache, pain behind the eyes, loss of appetite, abdominal pain, nausea, vomiting, muscle aches, bone and joint pain, and flushing of the face. A generalized flat, red rash is often seen within 3–4 days of fever onset. Symptoms typically last 2–7 days, with most people recovering after about a week.

About 1 in 20 patients with dengue progress to severe dengue, typically 24-48 hours after the fever resolves. Severe dengue can become life-threatening within hours, often as a result of hypovolemic shock due to plasma leakage from the vasculature. There is no specific treatment for dengue. With proper supportive care in a hospital or intensive care unit setting, fewer than one in 100 people with severe dengue may die; fatality rates have been reported as high as 13% in the absence of adequate supportive care.

A person’s risk for progression to severe dengue varies based on several factors. Age (over 60), comorbidities (including pregnancy), host genetics, and the infecting virus strain are risk factors for severe dengue. For any individual, the second infection with a different DENV serotype is the most likely to cause severe dengue compared with the first, third, or fourth infections.

CDC has a website with additional details on the clinical presentation of dengue:

Last reviewed: February 16, 2022

Hepatitis A is a liver disease common in many parts of the world and caused by hepatitis A virus (HAV), a picornavirus that causes acute inflammation of the liver. It is not related to the common viruses that cause hepatitis B or C.

Last reviewed: June 25, 2023

Never! A “dormitory-style” refrigerator is a small combination refrigerator/freezer unit that is outfitted with one exterior door and an evaporator plate (cooling coil), which is usually located inside an icemaker compartment (freezer) within the refrigerator. Dormitory-style (bar-style) units pose a significant risk of freezing vaccine even when used only for temporary storage. During testing, dormitory-style refrigerators demonstrated consistently unacceptable performance, regardless of where the vaccine was placed inside the unit. The use of dormitory-style refrigerators is specifically prohibited for storage of VFC vaccines or other vaccines purchased with public funds.

Last reviewed: July 26, 2023

This is not true. Pregnant healthcare personnel may administer any vaccine except the ACAM2000 smallpox vaccine.

Last reviewed: December 28, 2022

COVID-19 is the name given to the disease caused by infection with the SARS-CoV-2 coronavirus. This virus was first detected as a cause of human illness in late 2019 in Wuhan, China, and triggered a global pandemic that began in 2020. The virus is thought to spread mainly from person to person through respiratory droplets and small particles produced when an infected person coughs, sneezes, or talks. The virus spreads easily in crowded or poorly ventilated indoor settings. Some people who are infected have no symptoms of illness, while the severity of symptomatic illness ranges from mild to life-threatening. Older adults and people of any age with underlying medical conditions are at higher risk for severe illness.

The incubation period after exposure ranges from 2–14 days, with an average of about 5 days. People with COVID-19 are generally considered potentially infectious up to 48 hours before symptom onset through 10 days after onset, though people with severe illness may be infectious longer. Symptoms may include fever or chills, cough, shortness of breath or difficulty breathing, fatigue, muscle or body aches, headache, new loss of taste or smell, sore throat, congestion or runny nose, nausea or vomiting, and diarrhea.

Last reviewed: October 26, 2023

Herpes zoster is a painful rash that occurs along one or more dermatomes. Zoster is caused by reactivation of latent varicella zoster virus infection from a prior chickenpox infection. People who have had a prior infection with varicella zoster virus (chickenpox) are at risk of shingles.

Last reviewed: March 9, 2022

Pneumococcal disease is caused by Streptococcus pneumoniae, a bacterium that has more than 100 serotypes. Most serotypes cause disease, but only a few produce the majority of invasive pneumococcal disease.

Last reviewed: July 26, 2022

VFC is a program designed to reduce or eliminate vaccine cost as a barrier to childhood vaccination. The program purchases vaccines from manufacturers at federal contract prices and provides them at no cost to participating public and private healthcare providers for use in children through age 18 years who are eligible for Medicaid, are uninsured, or are American Indian or Native American. Children whose health insurance benefit plan does not cover one or more vaccines included in VFC, or whose insurance plan has a fixed dollar cap for vaccines (after the cap is reached) are considered “underinsured” and are also able to receive VFC vaccine at a Federally Qualified Health Center (FQHC) or Rural Health Clinic (RHC).

If you are interested in becoming a VFC provider, you should contact your state immunization program. For more information on the VFC program in general, go to the CDC’s website at

Last reviewed: August 26, 2022

About 30%–50% people who are 5 years of age or older with acute (recently acquired) hepatitis B have initial signs or symptoms when infected with hepatitis B virus (HBV). Children younger than age 5 years and newly infected immunosuppressed adults rarely show any symptoms. When present, signs and symptoms of hepatitis B might include nausea, lack of appetite, tiredness, muscle, joint, or abdominal pain, fever, diarrhea or vomiting, headache, dark urine, clay-colored stools, and yellowing of the skin and whites of the eyes (jaundice). People who have such signs or symptoms generally feel quite ill and might need to be hospitalized. People with chronic (life-long) HBV infection might have no symptoms, have no evidence of liver disease, or have a range of disease from chronic hepatitis to cirrhosis or hepatocellular carcinoma, a type of liver cancer.

Last reviewed: July 21, 2023

HPV is the most common sexually transmitted infection in the United States. In the United States, an estimated 79 million persons are infected, and an estimated 14 million new HPV infections occur every year among persons age 15 through 59 years. Approximately half of new infections occur among persons age 15 through 24 years and the first HPV infection typically occurs within a few months to years of becoming sexually active.

Last reviewed: October 13, 2023

Respiratory syncytial virus (RSV) is a common respiratory virus, usually causing mild, cold-like symptoms, such as cough, runny nose, sore throat, headache, fatigue, and fever. Though most people recover in a few days, for some people, RSV can cause serious lower respiratory tract infection (LRTI), such as bronchiolitis or pneumonia. RSV is the most common cause of hospitalization in infants in the United States. Older adults, especially those with chronic health conditions and those age 75 and older, are also more likely to develop severe RSV disease and need hospitalization. There is no specific treatment for RSV illness, only supportive care.

RSV causes annual outbreaks of respiratory illness in people of all ages. In the continental United States, RSV typically circulates in the fall and winter months, between October and March, although its seasonality can vary locally from year to year.

Last reviewed: January 22, 2024

Mainly because little safety or efficacy data exist on doses given before 6 weeks of age, and the vaccines aren’t licensed for this use. The data that exist suggest that the response to doses given before 6 weeks is poor and, in the case of Haemophilus influenzae type b (Hib) vaccine, the response could be detrimental to the infant by possibly reducing the immune response to subsequent doses of Hib conjugate vaccine. Hepatitis B vaccine is an exception because infants respond adequately to this vaccine as early as the day of birth and receipt of this vaccine at birth is necessary to protect infants born to HBsAg-positive mothers.

Last reviewed: June 6, 2023

Vaccine recommendations are determined after extensive studies in large clinical trials. They include studies on how vaccine recipients respond to multiple vaccines given simultaneously. The overall aim is to provide early protection for infants and children against vaccine-preventable diseases that could endanger their health and life.

No scientific evidence exists to support that delaying vaccinations or separating them into individual antigens is beneficial for children. Rather, this practice prolongs susceptibility to disease, which could result in a greater likelihood of the child becoming sick with a serious or life-threatening disease. There could also be added expense (e.g., multiple office visits), additional time off from work for parents, and increased likelihood that the child will fail to get all necessary vaccinations.

Last reviewed: August 31, 2022

ACIP’s “General Best Practice Guidelines for Immunization” still states that a patient’s undocumented history can generally be accepted as proof of vaccination only for influenza and pneumococcal polysaccharide vaccines. See the Adult Vaccination section at

Last reviewed: June 6, 2023

Globally, about 59,000 human rabies deaths occur each year, and 98% of these deaths are caused by the canine (dog) rabies virus variant. In the United States, the canine rabies virus variant has been eliminated, but wildlife variants (such as bat, raccoon, and skunk rabies virus variants) remain. Hawaii is the only U.S. state that is rabies-free. About 5,000 animal cases of rabies are reported each year in the United States. In the 21-year period between January 2000 and December 2020, 52 cases of human rabies were diagnosed in the United States, and 38 of these were caused by rabies acquired in the United States. No U.S. cases occurred in people who had previously received rabies vaccine as pre-exposure prophylaxis.

Last reviewed: May 14, 2023

Haemophilus influenzae is a bacteria that has encapsulated (typeable) or unencapsulated (nontypeable) strains. Encapsulated strains express one of six antigenically capsular polysaccharides (types a, b, c, d, e, or f). Historically, type b (Hib) was the most common type to cause invasive disease, particularly in young children. H. influenzae colonizes the upper respiratory tract of humans and is transmitted person-to-person by inhalation of respiratory droplets or by direct contact with respiratory tract secretions.

Encapsulated H. influenzae nontype b strains, particularly type a, can cause invasive disease similar to Hib disease. Nontypeable strains also can cause invasive disease but more commonly cause mucosal infections such as otitis media, conjunctivitis, and sinusitis. Vaccines are only available for H. influenzae type b; Hib vaccines do not protect against disease caused by any other H. influenzae strains.

Last reviewed: July 31, 2022

Combination vaccines can be used for children who have fallen behind. Combination vaccines may be used when any of the components are indicated and none are contraindicated. The minimum interval between doses is the greatest interval between any of the individual antigens. For example, the minimum interval between the first and second doses of MMR is 4 weeks and the minimum interval between the first and second doses of varicella vaccine is 12 weeks. When the two vaccines are combined in MMRV (ProQuad, Merck) the minimum interval between MMRV dose #1 and dose #2 is 12 weeks, which is the greatest of the minimum intervals of the two vaccines if given separately.

Last reviewed: July 15, 2023

The FDA-licensed cholera vaccine CVD 103-HgR (Vaxchora, by Emergent BioSolutions) is currently unavailable in the United States. The manufacturer temporarily halted production in December 2020. CDC published ACIP recommendations for its use in September 2022, available at

CVD 103-HgR is recommended for travelers ages 2–64 years old going to areas of active toxigenic Vibrio cholerae O1 transmission. Criteria for “active” cholera transmission for a given country and a table classifying countries according to transmission levels are found at

Vaccination against cholera is not routinely recommended because cholera is rare in travelers and most travelers do not visit areas of active transmission. However, disease is more likely to occur in travelers who may have limited access to safe food and water in outbreak settings, including outbreak response workers. Clinicians should consider these factors in addition to the level of cholera transmission in the destination country, length of stay, patient’s age and history of pre-existing medical conditions, and the availability of intravenous hydration when assessing a traveler’s risk for cholera infection and the need for vaccination. Other WHO-prequalified cholera vaccines not licensed in the United States may be available in the destination country.

All travelers to cholera-affected areas should follow safe food and water precautions and proper sanitation and personal hygiene measures as primary prevention strategies against cholera infection. Travelers who develop severe diarrhea should promptly seek medical attention for rehydration therapy.

Last reviewed: August 21, 2023

Meningococcal disease is a bacterial infection caused by Neisseria meningitidis. Meningococcal disease usually presents clinically as meningitis (about 50% of cases), bacteremia (30% of cases), or bacteremic pneumonia (15% of cases). N. meningitidis colonizes mucosal surfaces of the nasopharynx and is transmitted through direct contact with large-droplet respiratory tract secretions from patients or asymptomatic carriers. Meningococcal disease can be severe. The overall case-fatality ratio in the U.S. is 15%, and 10%–20% of survivors have long-term sequelae such as neurologic disability, limb or digit loss, and hearing loss.

N. meningitidis is classified into 12 serogroups based on characteristics of the polysaccharide capsule. Most invasive disease (such as meningitis and sepsis) is caused by serogroups A, B, C, W, X and Y. The relative importance of serogroups depends on geographic location and other factors such as age. Between 2011 and 2020 in the United States, serogroup B caused about 60% of cases among children younger than 5 years old, and serogroups C, W, or Y caused about two out of three cases in people age 11 years or older. Serogroup A is rare in the U.S. Historically, serogroup A was common in the meningitis belt of sub-Saharan Africa, but after the implementation of a meningococcal serogroup A conjugate vaccine campaign, serogroup A disease has been nearly eliminated in the meningitis belt.

Nasopharyngeal carriage rates are highest in adolescents and young adults who serve as reservoirs for transmission of N. meningitidis.

Last reviewed: July 15, 2023

An excellent resource for vaccine contraindications and precautions is the CDC online resource, “General Best Practice Guidelines for Immunization”. A table in the Contraindications and Precautions chapter has a listing for all routine vaccines in the United States available at In addition, a table of contraindications and precautions is provided in the appendix of current CDC recommended adult and child/adolescent immunization schedules, available at

Last reviewed: February 19, 2024

Since the Global Polio Eradication Initiative was launched in 1988, the number of polio cases worldwide has declined by more than 99.99%. Among the three wild poliovirus (WPV) serotypes, only type 1 (WPV1) has been detected since 2012. Global eradication of type 2 WPV was declared in 2015; type 3 WPV was declared eradicated in 2019. The number of detected WPV1 cases has reached a historic low (33 cases in 2018 and 176 in 2019) in the last two countries with endemic WPV1 transmission (Afghanistan and Pakistan). However, during 2021–2022, Malawi and Mozambique reported nine WPV1 cases that were genetically linked to Pakistan. These cases highlight the risk for importation and the importance of maintaining enhanced global monitoring and surveillance.

This decline in polio cases worldwide is attributable primarily to use of the live, attenuated oral poliovirus vaccine (OPV) in national routine immunization schedules and mass vaccination campaigns. The success and safety record of OPV use is offset by the rare emergence of genetically divergent vaccine-derived polioviruses (VDPVs), whose genetic drift from the parental OPV strains indicates prolonged replication or circulation. Circulating VDPVs (cVDPVs) can emerge in areas with low immunization coverage and can cause outbreaks of paralytic polio. In addition, immunodeficiency-associated VDPVs (iVDPVs) can emerge in people with primary immunodeficiencies and can replicate and be excreted for years. During January 2020–April 2022, a total of 1,856 cVDPV cases were identified in 33 countries. In July 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.

In April 2016, all OPV-using countries switched from using trivalent OPV (tOPV; Sabin types 1, 2, and 3) to bivalent OPV (bOPV; Sabin types 1 and 3). To control and prevent cVDPV2 outbreaks, traditional monovalent type 2 OPV (mOPV2) has been distributed in affected countries; in addition, a novel monovalent type 2 OPV (nOPV2) that is less likely to produce cVPDV disease was listed under an Emergency Use Authorization (EUL) by the World Health Organization (WHO) in November 2020, (Bio Farma, Indonesia) to address the rising cases of cVDPV2. As of March 2023, close to 600 million doses of the nOPV2 have been used across 28 countries in outbreak response.

Additional information about the polio eradication program is available on the CDC website at

Last reviewed: July 23, 2023

In 2022, a provisional total of 121 cases of measles from 6 states were reported to CDC. The sharply limited global travel and social interactions precipitated by the COVID-19 pandemic, which began in early 2020, also reduced the opportunities for introduction and transmission of measles, mumps, and rubella viruses in the United States. Current CDC measles surveillance updates can be found at

Since the pre-vaccine era, there has been a more than 99% decrease in mumps cases in the United States. However, outbreaks still occasionally occur. In 2006, there was an outbreak affecting more than 6,584 people in the United States, with many cases occurring on college campuses. In 2009, an outbreak started in close-knit religious communities and schools in the Northeast, resulting in more than 3,000 cases. Since 2015, numerous outbreaks have been reported across the US, in college campuses, prisons, and close-knit communities, including a large outbreak in northwest Arkansas where almost 3,000 cases were reported in 2016. These outbreaks have shown that when people with mumps have close contact with a lot of other people (such as among residential college students and families in close-knit communities) mumps can spread, even among vaccinated people. However, outbreaks are much larger in areas where vaccine coverage rates are lower. A provisional total of 322 cases of mumps were reported to CDC in 2022.

Rubella was declared eliminated (the absence of endemic transmission for 12 months or more) from the United States in 2004. Fewer than 10 cases (primarily import-related) have been reported annually in the United States since elimination was declared. Rubella incidence in the United States has decreased by more than 99% from the pre-vaccine era. A provisional total of 3 cases of rubella, and no cases of congenital rubella syndrome, were reported in 2022.

Last reviewed: June 19, 2023

Influenza is the second most frequent cause of death from a vaccine-preventable disease in the United States after COVID-19. Rates of infection from seasonal influenza are highest among children, but the risks for complications, hospitalizations, and deaths are higher among adults age 65 years and older, children younger than 5 years, pregnant individuals, and people of any age who have medical conditions that place them at increased risk for complications from influenza.

From the 2010–11 through the 2022–23 seasons (excluding 2020–2021, when COVID-19 control measures resulted in almost no influenza activity), the annual influenza-related disease burden has varied from approximately 9 to 41 million illnesses, 4 to 21 million medical visits, 140,000 to 810,000 hospitalizations and 12,000 to 61,000 deaths per year, including an average of 129 pediatric deaths reported to CDC (range 37–199) each year. While the 2020–21 and 2021–22 seasons’ disease burden was substantially limited as a result of measures taken by many people to reduce the transmission of COVID-19, such as wearing face masks in public and limiting interactions with other people, influenza activity returned to pre-pandemic levels in 2022–23. For additional information about disease burden from CDC, see

Last reviewed: September 10, 2023

Vaccine recommendations in the U.S. are issued primarily by two national bodies—the CDC Advisory Committee on Immunization Practices (ACIP) and the American Academy of Pediatrics (AAP) Committee on Infectious Diseases. To access the most current ACIP recommendations, visit ACIP Vaccine Recommendations and Guidelines (CDC) for statements in alphabetical order or ACIP Vaccine Recommendations ( for statements in chronological order (can also be sorted by vaccine). AAP vaccine recommendations are published in the AAP Red Book, and are generally available on the AAP website.

Last reviewed: August 22, 2020

Other oral cholera vaccines have been prequalified by the World Health Organization (WHO). These vaccines are not available in the United States and ACIP has not provided recommendations on use of these vaccines. To learn more about these vaccines, visit the World Health Organization Cholera Vaccines page:

For further information from CDC about cholera vaccines and preventing cholera, visit and

Last reviewed: August 21, 2023

The varicella zoster virus (VZV) spreads from person to person by direct contact or through the air by coughing or sneezing. It is highly contagious. It can also be spread through direct contact with fluid from a blister of a person infected with varicella, or from direct contact with a skin lesion from a person with zoster (shingles). People with varicella are infectious 1 to 2 days before skin lesions appear until all lesions have crusted over, usually 4 to 7 days after the appearance of skin lesions.

Last reviewed: March 16, 2023

Reinfection appears to be uncommon but does occur. Reinfection may present as a persistent cough rather than typical pertussis.

Last reviewed: March 31, 2022

Rotavirus is contagious and the infection is usually spread from person to person, through the fecal-oral route. Fecal-oral transmission occurs when bacteria or viruses found in the stool of one person are swallowed by another person. This can occur when small amounts of fecal matter may be found on surfaces such as toys, books, clothing, etc. and on the hands of parents or child-care providers; but are usually invisible. Rotavirus may also be transmitted through intake of fecally-contaminated water or food or by respiratory droplets that people sneeze, cough, drip, or exhale. Rates of the illness among children in developed and less developed countries are similar.

Last reviewed: June 7, 2023

Dengue is common throughout tropical and subtropical regions of the world. About half of the world’s population lives in areas suitable for DENV transmission and the main vector of dengue, the Aedes aegypti mosquito, is difficult to control and continues to expand its geographic range. Each year, up to 400 million people get infected with dengue viruses. Approximately 100 million people get sick from infection, and about 40,000 die from severe dengue.

Dengue is endemic in the United States territories and freely associated states of Puerto Rico, American Samoa, the U.S. Virgin Islands, the Federated States of Micronesia, the Republic of Marshall Islands, and the Republic of Palau. CDC classifies an area as endemic for dengue if the area has frequent or continuous dengue transmission, with evidence of more than 10 dengue cases in at least three of the previous 10 years. Dengue epidemics occur in a cyclical pattern every 3–7 years, with all four DENV serotypes reported in the Pacific Islands and the Caribbean. Limited surveillance data are available from the Federated States of Micronesia, the Republic of Marshall Islands, and the Republic of Palau.

Dengue can cause outbreaks with high numbers of cases in some years. In 2021, only 435 cases were reported in US territories, and 63 cases were reported in U.S. states. About 90% of the population at risk for dengue in US territories live in Puerto Rico. From 2010-20, approximately 95% of locally acquired dengue cases in the US occurred in Puerto Rico (29,779 cases), with the most cases and hospitalizations in Puerto Rico occurring among adolescents age 10 through 19 years (11,000 cases leading to 4,000 hospitalizations). Small convenience studies of children ages 9 through 16 conducted during vaccine trials estimated that 50% to 56% of children in this age group showed serologic evidence of past DENV infection. Dengue is not endemic in the continental United States or Hawaii; however, outbreaks and travel-associated cases have occurred in Texas, Florida, and Hawaii in recent years.

Last reviewed: February 16, 2022

Illness caused by HAV infection cannot be distinguished from other types of acute viral hepatitis, but it typically has an abrupt onset that can include fever, malaise, anorexia, nausea, abdominal discomfort, dark urine, and jaundice. The likelihood of having symptoms with HAV infection is related to age. In children younger than age 6 years, 70% of infections are asymptomatic. When illness does occur in young children, it is typically not accompanied by jaundice. In older children and adults, infection typically is symptomatic, with jaundice occurring in more than 70% of patients.

Hepatitis A signs and symptoms usually resolve in 2-3 months, although 10% to 15% of symptomatic people have prolonged illness (usually referred to as relapsing hepatitis A) lasting up to 6 months and should be considered infectious during that time.

Last reviewed: June 25, 2023

A “dormitory-style” refrigerator is a small combination refrigerator/freezer unit that is outfitted with one exterior door and an evaporator plate (cooling coil), which is usually located inside an icemaker compartment (freezer) within the refrigerator. This type of unit has severe temperature control and stability issues. However, there are compact refrigerators or freezers noted as “pharmaceutical grade” or “purpose-built for vaccine storage” that have been engineered to maintain even temperatures throughout the unit, and these may meet the needs of a small office. In general, the unit you select must be large enough to store the year’s largest vaccine inventory without crowding and to store water bottles (in a refrigerator) and frozen coolant packs (in a freezer) to minimize temperature fluctuations. One way to assure that the unit you purchase will reliably maintain proper vaccine storage temperatures is to look for a unit labeled as meeting the NSF/ANSI 456 certification standard for vaccine storage. This voluntary certification indicates that the model has been tested and certified to maintain proper storage conditions under a range of normal clinic conditions.

Last reviewed: July 26, 2023

Yes. Vaccines can be administered in a patient care area. The recommendation from CDC’s safe injection practices experts is that storing and preparing vaccines should not be done in the same area where patient care is conducted. These activities should be done in a separate area.

Last reviewed: December 28, 2022

ACIP has not made recommendation related to revaccination or booster doses with the FDA-licensed oral cholera vaccine, CVD 103-HgR. As more information becomes available, CDC will update its recommendations accordingly.

Last reviewed: August 21, 2023

During their lifetime about 30% of Americans will develop herpes zoster, which translates into an estimated 1 million cases each year in this country. The risk of zoster increases with increasing age; about half of all cases occur among people age 60 years or older. People who are immunosuppressed, including those with leukemia, lymphoma, and human immunodeficiency virus (HIV) infection, and people who receive immunosuppressive drugs, such as steroids and cancer chemotherapy, also are at greater risk of zoster. Most people have only one episode of shingles. The risk of recurrence is low in people who are not immunosuppressed, but the precise incidence is unknown.

Last reviewed: March 9, 2022

The disease is spread from person to person by droplets in the air. The pneumococci bacteria are common inhabitants of the human respiratory tract. They may be isolated from the nasopharynx of 5%–90% of healthy people.

Last reviewed: July 26, 2022

Measles can lead to serious complications and death, even with modern medical care. The 1989–1991 measles outbreak in the U.S. resulted in more than 55,000 cases and more than 100 deaths. In the United States, from 1987 to 2000, the most commonly reported complications associated with measles infection were pneumonia (6%), otitis media (7%), and diarrhea (8%). For every 1,000 reported measles cases in the United States, approximately one case of encephalitis and two to three deaths resulted. The risk for death from measles or its complications is greater for infants, young children, and adults than for older children and adolescents.

Mumps most commonly causes fever and parotitis. Up to 25% of persons with mumps have few or no symptoms. Complications of mumps include orchitis (inflammation of the testicle) and oophoritis (inflammation of the ovary). Other complications of mumps include pancreatitis, deafness, aseptic meningitis, and encephalitis. Mumps illness is typically milder, with fewer complications, in fully vaccinated case patients.

Rubella is generally a mild illness with low-grade fever, lymphadenopathy, and malaise. Up to 50% of rubella virus infections are subclinical. Complications can include thrombocytopenic purpura and encephalitis. Rubella virus is teratogenic and infection in a pregnant woman, especially during the first trimester can result in miscarriage, stillbirth, and birth defects including cataracts, hearing loss, mental retardation, and congenital heart defects.

Last reviewed: June 19, 2023

If signs or symptoms of illness occur, they begin an average of 90 days (range: 60–150 days) after exposure to HBV.

Last reviewed: July 21, 2023

Most HPV infections are asymptomatic and go away completely on their own within 2 years (usually in the first 6 months) after infection without causing clinical disease. Some infections are persistent and can lead to precancerous lesions or cancer. HPV infections caused by certain HPV types cause almost all cases of anogenital warts in women and men and recurrent respiratory papillomatosis.

According to CDC surveillance data from 2015 to 2019, every year in the United States, about 47,199 new cases of cancer (26,177 among women and 21,022 among men) are found in parts of the body where human papillomavirus (HPV) is often found (referred to as HPV-associated cancers). About 79% of these cancers are probably caused by HPV (referred to as HPV-attributable cancers).

Each year, between 2015 and 2019, an average of 12,293 cases of cervical cancer, the most widely known HPV-associated cancer, occurred in the United States. HPV is also associated with vulvar, and vaginal cancer in females, penile cancer in males, and anal and oropharyngeal cancer in both females and males. Between 2015 and 2019, oropharyngeal cancers were the most commonly occurring HPV-associated cancers, with an average of 20,839 reported cases each year (17,238 among men and 3664 among women). See and for more information on trends in HPV-associated cancer.

Last reviewed: October 13, 2023

For routinely administered vaccines, there is no vaccine series that needs to be restarted because of an interval that is longer than recommended. In certain circumstances, oral typhoid vaccine (which may be given for international travel) needs to be restarted if the vaccine series isn’t completed within the recommended time frame.

Last reviewed: June 6, 2023

Paul Offit, MD, and Charlotte Moser, BS, of the Vaccine Education Center (VEC) at the Children’s Hospital of Philadelphia, published an article, “The Problem with Dr. Bob’s Alternative Vaccine Schedule,” in the January 2009 issue of Pediatrics. It includes a section about aluminum. You can read it in its entirety at

Here are some additional sources of related information:

“Aluminum in Vaccines: What you should know” is available from VEC at

“Vaccine Ingredients: What you should know” is available from VEC at

Last reviewed: August 31, 2022

Most adults experience a mild upper respiratory tract infection with RSV, with symptoms lasting only a few days. Some adults with RSV develop lower respiratory tract disease, including pneumonia. Because public health surveillance and testing for RSV in adults been limited in the past, our estimates of the burden of RSV disease are not precise and may underestimate the true burden among adults. Among U.S. adults age 65 and older, RSV is responsible for approximately 60,000 to 160,000 hospitalizations and 6,000 to 10,000 deaths each year.

Older adults at highest risk for severe RSV infection generally have one or more chronic medical conditions, including:

  • lung diseases (including chronic obstructive pulmonary disease [COPD] and asthma)
  • cardiovascular diseases (such as congestive heart failure and coronary artery disease)
  • moderate or severe immune compromise (whether caused by disease or by treatment with an immunosuppressive medication)
  • diabetes mellitus
  • neurologic or neuromuscular conditions
  • kidney disorders
  • liver disorders
  • hematologic disorders

Other factors associated with increased risk include:

  • frailty
  • advanced age (hospitalization rates are highest among those age 75 or older)
  • residence in a nursing home or other long-term care facility

In addition to these listed factors, individual patients may have other underlying factors that a health-care provider determines might increase the risk for severe respiratory disease.

Last reviewed: January 22, 2024

The Food and Drug Administration (FDA) website has a list of all vaccines licensed for use in the United States at Note that some licensed vaccines may not currently be distributed in the United States (for example the Cervarix brand of HPV vaccine and the Zostavax brand of shingles vaccine).

Last reviewed: June 6, 2023

Two rabies vaccines are available in the United States. Both vaccines contain inactivated rabies virus. HDCV vaccine (Imovax, Sanofi) is produced in human diploid cell culture. PCECV vaccine (RabAvert, Bavarian Nordic) is produced in chick embryo cell culture. Both types are considered equally safe and effective. The products are interchangeable: it is acceptable to administer a mixed-brand series, if needed.

Last reviewed: May 14, 2023

Before 1985, Hib was the leading cause of bacterial meningitis and a common cause of other invasive diseases (such as epiglottitis, pneumonia, septic arthritis, cellulitis, purulent pericarditis, and bacteremia) among U.S. children younger than 5 years of age. An estimated 20,000 cases of invasive Hib disease occurred in this age group each year. Meningitis occurred in approximately two-thirds of children with invasive Hib disease; 15%–30% of survivors had hearing impairment or severe permanent neurologic sequelae. Approximately 4% of all cases were fatal.

Following the licensure of conjugate Hib vaccines in 1987 and 1989, the incidence of Hib disease fell dramatically. During 1989 – 2000, the annual incidence of invasive Hib disease among children younger than 5 years of age decreased by 99%, to less than one case per 100,000 children. Since 2000, the average annual incidence rate of invasive Hib disease among children younger than 5 years of age in the United States remained below the Healthy People 2020 goal of 0.27/100,000 children.

Only 18 confirmed cases of invasive Hib disease among children younger than 5 years were reported in 2019. The majority of Hib disease in the United States occurs among unimmunized and underimmunized infants and children (those who have an incomplete primary series or are lacking a booster dose) and among infants too young to have completed the primary immunization series.

Last reviewed: July 31, 2022

If the child has no health insurance on the day he or she presents at a medical practice or health department for immunization, the child is VFC eligible because he or she is uninsured. A child must be screened for VFC eligibility at each visit, even though the eligibility form needs to be updated only when the child’s eligibility status changes.

Last reviewed: August 26, 2022

Switching between combination and single-antigen vaccines poses no problem if you maintain the recommended minimum intervals for all vaccines and the vaccines are licensed for the age of the patient.

Last reviewed: July 15, 2023

A contraindication is a condition in a recipient that increases the risk for a serious adverse reaction to vaccination and is a condition under which vaccines should not be administered. In addition to contraindications found in prescribing information, ACIP may recommend against the use of a vaccine under certain conditions (e.g., a lack of data).

A precaution is a condition in a recipient that might increase the risk for a serious adverse reaction, might cause diagnostic confusion, or might compromise the ability of the vaccine to produce immunity. For example, not vaccinating a person who is moderately or severely acutely ill avoids diagnostic confusion between the underlying illness and side effects of vaccination. Vaccination may be deferred if a precaution is present, although a vaccination might be indicated in the presence of a precaution if the benefit of protection from the vaccine outweighs the risk for an adverse reaction.

Last reviewed: August 29, 2022

In the U.S., all infants and children should receive 4 doses of IPV at ages 2, 4, 6–18 months, and 4–6 years. The first dose may be given as early as 6 weeks of age. The final dose should be administered at 4 years of age or older, regardless of the number of previous doses, and should be given 6 months or more after the previous dose. A fourth dose in the routine IPV series is not necessary if the third dose was given at 4 years of age or older and 6 months or more after the previous dose.

Infants and children traveling to areas where there has been wild or vaccine-derived poliovirus (cVDPV) circulation in the last 12 months should be vaccinated according to the routine schedule. If the routine series cannot be administered within the recommended intervals before protection is needed, an accelerated schedule can be used: 1) the first dose should be given to infants 6 weeks of age and older, 2) the second and third doses should be administered at 4 weeks or more after the previous doses, and 3) the minimum interval between the third and fourth doses is 6 months. If the age-appropriate series is not completed before departure, the remaining IPV doses to complete a full series should be administered when feasible, at the intervals recommended for the accelerated schedule. If doses are needed while residing in the affected country, the polio vaccine that is available (IPV or oral polio vaccine [OPV]) may be administered.

In addition to single-antigen IPV, five pediatric combination vaccines that contain IPV vaccine are licensed and recommended for use in the United States: DTaP-HepB-IPV (Pediarix, GSK), DTaP-IPV/Hib (Pentacel, Sanofi), DTaP-IPV (Kinrix, GSK), DTaP-IPV (Quadracel, Sanofi), and DTaP-IPV-Hib-HepB (Vaxelis, MSP Vaccine Company).

Last reviewed: July 23, 2023

The incidence of meningococcal disease has declined steadily in the U.S. since a peak of reported disease in the late 1990s. Even before routine use of a meningococcal conjugate vaccine against serogroups A, C, W, and Y (MenACWY) was recommended for adolescents in 2005, the overall annual incidence of meningococcal disease had decreased 64%, from 1.1 cases per 100,000 population in 1996 to 0.4 cases per 100,000 population in 2005. In 2020, the rate of meningococcal disease in the United States reached a historic low of 0.07 cases per 100,000 population.

In 2020, of U.S. cases with known serogroup, 55 cases were serogroup B (incidence rate of 0.02 cases per 100,000 population) and 119 cases were serogroups C, Y, or W. The incidence of disease is extremely low in all age groups, but is highest in infants under 1 year (0.43 cases per 100,000), children age 1 through 4 years (0.12 cases per 100,000), and adolescents age 16–23 (0.09 cases per 100,000 years).

Last reviewed: July 15, 2023

The CDC has assembled clinical information about COVID-19 at this site:

Last reviewed: October 26, 2023

While even healthy children and adults get severe influenza or die from influenza and its complications, the risk of severe influenza is higher for children younger than 5 years, adults 50 years and older, pregnant people, Alaska Natives and American Indians, and residents of nursing homes or other long-term care facilities. Medical conditions that increase a person’s risk of severe influenza include chronic pulmonary (including asthma), cardiovascular (excluding isolated hypertension), renal, hepatic, neurologic, hematologic, or metabolic disorders (including diabetes mellitus); immunocompromising conditions due to any cause (including but not limited to immune suppression caused by medications or HIV); extreme obesity (body mass index of 40 or greater for adults); and the chronic use of aspirin- or salicylate-containing medications in children through age 18 (due to the risk of Reye syndrome after influenza infection).

More information about risk factors for severe influenza infection can be found at:

Last reviewed: September 10, 2023

The ACIP-recommended child and adolescent schedules are and approved by the CDC (, American Academy of Pediatrics (, American Academy of Family Physicians (AAFP,, American College of Obstetricians and Gynecologists (ACOG,, American College of Nurse-Midwives (, American Academy of Physician Associates (AAPA,, and National Association of Pediatric Nurse Practitioners (

The ACIP-recommended adult schedules are approved by CDC and American College of Physicians (, AAFP, ACOG, American College of Nurse-Midwives, AAPA, American Pharmacists Association (, and Society for Healthcare Epidemiology of America (

Last reviewed: April 20, 2023

Yes. Rotavirus infection of adults is usually asymptomatic but may cause diarrheal illness. Outbreaks of diarrheal illness caused by rotavirus have been reported, especially among elderly persons living in retirement communities. For more information on this issue see, page 1456.

Last reviewed: June 7, 2023

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 In addition, CDC has developed simple illustrations and descriptions to explain this phenomenon at

Last reviewed: February 16, 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

No. Hospitalization should be used as an opportunity to provide recommended vaccinations. Hospitalized patients who are not moderately or severely acutely ill may be vaccinated during hospitalization or at discharge. Current, recent or upcoming anesthesia, surgery or hospitalization are not contraindications to vaccination, although certain factors may lead a healthcare provider to consider these situations a precaution to vaccination for a specific patient.

Last reviewed: August 29, 2022

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

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 the most common disease caused by pneumococcal infection. An estimated 400,000 hospitalizations from pneumococcal pneumonia occur in the United States annually.

Pneumococcal pneumonia can occur in combination with bacteremia and/or meningitis (invasive pneumococcal pneumonia), or it can occur alone (non-invasive pneumococcal pneumonia). Non-invasive pneumococcal pneumonia can be severe. Symptoms include abrupt onset of fever, shaking chills or rigors, chest pain, cough, shortness of breath, rapid breathing and heart rate, and weakness. The fatality rate is 5%–7% and may be much higher in older adults. Pneumococcal bacteremia occurs in about 25%–30% of patients with pneumococcal pneumonia.

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.

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: July 26, 2022

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

In the United States, approximately 64% of HPV-related cancers are attributable to HPV 16 or 18, two types included in all HPV vaccines. Approximately 10% are attributable to HPV types 31, 33, 45, 52, and 58, which are included in the 9-valent HPV vaccine.

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

Last reviewed: October 13, 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 handouts on administering intramuscular and subcutaneous vaccines to children and adults at and to adults only at

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

Last reviewed: December 28, 2022

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