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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 (1256)

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

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

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

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

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

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

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

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

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

An excellent resource for vaccine contraindications and precautions is the ACIP “General Best Practice Guidelines for Immunization”, available at A table in the Contraindications and Precautions chapter has a listing for all routine vaccines available in the United States. also has summary tables of contraindications and precautions for people of all ages ( and a summary for vaccines given to adults (

Contraindications and precautions to non-routine vaccines of special interest are not listed in the “General Best Practice Guidelines for Immunization” but may be accessed directly on the CDC website:

Last reviewed: August 29, 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

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

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