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 (1379)

Yes. Vaccination should be offered regardless of history of prior SARS-CoV-2 infection.

Viral testing to assess for current SARS-CoV-2 infection or serologic testing to assess for prior infection for the purposes of vaccine decision-making is not recommended. However, as with all vaccines, vaccination should be deferred until after recovery from moderate to severe illness. In addition, to minimize the risk of exposing others to SARS-CoV-2 virus, vaccination of a person diagnosed with COVID-19 generally should be deferred until the person does not pose a risk of infection to others.

Although not required, people who recently had SARS-CoV-2 infection may consider delaying a 2024–2025 COVID-19 vaccine dose by up to 3 months from symptom onset or positive test (if infection was asymptomatic). Among vaccine recipients at increased risk of post-vaccination myocarditis (such as males age 12 through 39 years), an increased interval may reduce the rare risk of myocarditis after vaccination. A recipient’s individual risks for severe disease and current COVID-19 conditions in the community should be taken into account when deciding whether to delay vaccination up to 3 months after infection.

Last reviewed: November 16, 2024

If a child is behind on immunizations, the Advisory Committee on Immunization Practices (ACIP) recommends using the minimum intervals between each dose until the child is caught up. The minimum interval for a vaccine can be used as many times as necessary, until the child is back on schedule. See Table 2 of the CDC Recommended Child and Adolescent Immunization Schedule, available at www.cdc.gov/vaccines/schedules/hcp/index.html.

Last reviewed: June 6, 2023

Fainting is most commonly reported in adolescents, according to the Vaccine Adverse Event Reporting System. One study showed that 62% of all fainting reports to VAERS were among adolescents 11 to 18 years old; however, due to the limitations of VAERS, we cannot use this system to determine how frequently fainting occurs after vaccination.

Last reviewed: August 31, 2022

The ACIP statement (“Human Rabies Prevention-United States, 2008, Recommendations of the Advisory Committee on Immunization Practices”) was published in MMWR on May 23, 2008. This document updated the status of rabies and anti-rabies biologics in the United States. To view this document, go to www.cdc.gov/mmwr/PDF/rr/rr5703.pdf.

In March 2010, ACIP eliminated the fifth dose of vaccine given post-exposure to previously unvaccinated persons with no immunosuppression. To view these recommendations, go to www.cdc.gov/mmwr/pdf/rr/rr5902.pdf.

In May 2022, ACIP eliminated the third dose of vaccine given in the primary series for pre-exposure prophylaxis and introduced updated risk categories with customized recommendations for long-term follow up after completion of the 2-dose primary series. To view this document, visit www.cdc.gov/mmwr/volumes/71/wr/pdfs/mm7118a2-H.pdf.

Last reviewed: May 14, 2023

It is common practice in many developing countries to administer oral polio vaccine (OPV) to children during both routine visits and periodic vaccination campaigns, so a child’s record may indicate more than 4 doses. Some of these doses may not be valid according to the U.S. immunization schedule.

Doses are considered valid if written documentation indicates that doses of polio vaccine were given after 6 weeks of age and the vaccine received was listed as IPV or trivalent OPV (tOPV). A record simply indicating “OPV” also is acceptable if the OPV was administered before April 1, 2016, and it was not noted as being administered during a vaccination campaign.

There specific criteria for OPV documentation because only trivalent polio vaccine doses count as valid for the U.S. polio vaccination schedule. Trivalent OPV ceased to be used globally in April 2016. OPV administered before April 1, 2016, generally was tOPV. However, “OPV” doses recorded as given during a mass vaccination campaign before April 2016 do not count as valid because such campaigns may have used monovalent or bivalent OPV.

If the history is of a complete series of IPV or tOPV in any combination, at least one dose should be administered on or after 4 years of age and at least 6 months after the previous dose. If a complete series cannot be identified that meets these criteria, then the child should receive as many doses of IPV as needed to complete the U.S. recommended schedule.

Last reviewed: July 23, 2023

All adolescents should receive a dose of MenACWY at 11 or 12 years of age. A second (booster) dose is recommended at 16 years of age. Adolescents who receive their first dose at age 13 through 15 years should receive a booster dose at age 16 years. The minimum interval between MenACWY doses is 8 weeks. Adolescents who receive a first dose after their 16th birthday do not need a booster dose unless they become at increased risk for meningococcal disease. Colleges may not consider a second dose given even a few days before age 16 years as valid, so keep that in mind when scheduling patients. People 19 through 21 years of age are not recommended routinely to receive MenACWY. However, MenACWY may be administered to people age 19 through 21 years as catch-up vaccination for those who have not received a dose after their 16th birthday.

Last reviewed: November 15, 2024

ACIP recommends annual vaccination for all people ages 6 months and older who do not have a contraindication to influenza vaccination.

Last reviewed: August 11, 2024

The Advisory Committee on Immunization Practices (ACIP) considers evidence of immunity to varicella to be:

  • Written documentation of 2 doses of varicella vaccine given no earlier than age 12 months with at least 4 weeks between doses.
  • U.S.-born before 1980*
  • A healthcare provider’s diagnosis of varicella or verification of history of varicella disease
  • History of herpes zoster, based on healthcare provider diagnosis or verification of disease history
  • Laboratory evidence of immunity or laboratory confirmation of disease

    *Note: Although there is only a very small chance of susceptibility, due to the potential for severe consequences from varicella infection, year of birth is not accepted as evidence of varicella immunity for healthcare personnel, immunosuppressed people, and pregnant people.

Last reviewed: May 16, 2023

Yes. Death as a result of fulminant hepatic failure is rare, however, older age (over 40 years) and preexisting chronic liver disease increases the risk of severe disease and death from hepatitis A. The person-to-person U.S. multistate outbreaks that began in 2016 have disproportionately affected adults with chronic liver disease and other health problems related to drug use and unstable housing. From 2016 through June 2023, CDC received reports of nearly 45,000 cases of acute HAV infection. Of these, approximately 61% have been hospitalized and nearly 1% (more than 420 people) have died.

Last reviewed: June 25, 2023

Acceptable presumptive evidence of immunity against measles includes at least one of the following:

  • written documentation of adequate vaccination:
    • one or more doses of a measles-containing vaccine administered on or after the first birthday for preschool-age children and adults not at high risk
    • two doses of measles-containing vaccine for school-age children, adolescents, and adults at high risk, including college students, healthcare personnel, and international travelers
  • laboratory evidence of immunity
  • laboratory confirmation of measles (verbal history of measles does not count)
  • birth before 1957

Although birth before 1957 is considered acceptable evidence of measles immunity, healthcare facilities should consider vaccinating unvaccinated personnel born before 1957 who do not have other evidence of immunity with 2 doses of MMR vaccine (minimum interval 28 days).

During an outbreak of measles, healthcare facilities should recommend 2 doses of MMR vaccine at the appropriate interval for unvaccinated healthcare personnel regardless of birth year if they lack laboratory evidence of measles immunity.

Last reviewed: June 19, 2023

The Inflation Reduction Act of 2022 (IRA) requires “first-dollar” coverage (no out-of-pocket cost to patient) for all recommended vaccines for adults covered by traditional Medicaid or Medicaid expansion plans. This coverage requirement applies to all states; however, enforcement policies and implementation may vary by state. Providers should check with their state Medicaid program directly about vaccine payment coverage and amounts paid by specific state plans.

 

Last reviewed: February 14, 2025

ACIP recommends routine administration of a conjugate Hib vaccine series for all infants beginning at age 2 months. Infants 2 through 6 months of age should receive a 3-dose series of ActHIB, Hiberix, Pentacel, or Vaxelis, or a 2-dose series of PedvaxHIB. The first dose can be administered as early as age 6 weeks, but not earlier. Hib-containing vaccine should not be given before 6 weeks of age. Doses given before 12 months of age should be separated by at least 4 weeks. A booster dose (which will be dose 3 or 4 depending on vaccine type used in primary series) of any Hib-containing vaccine is recommended at age 12 through 15 months and at least 8 weeks after the most recent Hib dose. Vaxelis is recommended only for the primary Hib series and is not recommended for use as a booster (4th) dose. A different Hib-containing vaccine licensed for a booster dose should be used.

Medically stable preterm infants should be vaccinated beginning at age 2 months according to the schedule recommended for other infants, on the basis of chronological age. For special situations in children, refer to the current CDC recommended immunization schedule Hib notes: www.cdc.gov/vaccines/hcp/imz-schedules/child-adolescent-notes.html.

Last reviewed: January 21, 2025

Vaccines must always be dispensed with a prescription or order from a physician or other healthcare provider authorized by the state to prescribe medications. However, there are situations where vaccines can be administered using a standing order or vaccine protocol that is not patient-specific. In these situations, a physician or other healthcare provider does not need to be physically present for the vaccine to be administered. Several studies have shown that the use of standing orders can improve vaccination rates, and ACIP recommends the use of standing orders programs in both outpatient and inpatient settings. A comprehensive set of standing orders for the routine vaccines given to children and adults can be found at www.immunize.org/clinical/topic/standing-orders-templates/.

Last reviewed: January 20, 2025

Yes. Several studies have measured the effect of vaccination on preventing RSV-associated hospitalizations among people vaccinated in the first RSV season after the recommendation for use of the vaccine. Estimates were similar for both the GSK and Pfizer products and showed that they reduced the risk of hospitalization among recipients overall by between 75% and 82%. For additional details, see the CDC MMWR published in August 2024: www.cdc.gov/mmwr/volumes/73/wr/pdfs/mm7332e1-H.pdf .

Last reviewed: August 25, 2024


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Last reviewed: May 23, 2023

Yes. Several studies have measured the effect of vaccination on preventing RSV-associated hospitalizations among people vaccinated in the first RSV season after the recommendation for use of the vaccine. Estimates were similar for both the GSK and Pfizer products and showed that they reduced the risk of hospitalization among recipients overall by between 75% and 82%. For additional details, see the CDC MMWR published in August 2024: www.cdc.gov/mmwr/volumes/73/wr/pdfs/mm7332e1-H.pdf.

Last reviewed: August 25, 2024

No vaccine component, including the diluent used to reconstitute a vaccine, should be stored in vegetable bins, nor in the space occupied by vegetable bins of a household-style refrigerator. CDC recommends that you position vaccines and diluents 2 to 3 inches from the unit walls, ceiling, floor, and door. If using a household-style unit, avoid storing vaccines and diluents in any part of the unit that may not provide stable temperatures or sufficient air flow, such as directly under cooling vents; in deli, fruit, or vegetable drawers; or on refrigerator door shelves. The instability of temperatures and air flow in these areas may expose vaccines to inappropriate storage temperatures. For more information, refer to page 14 of the CDC Vaccine Storage and Handling Toolkit available at www.cdc.gov/vaccines/hcp/admin/storage/toolkit/storage-handling-toolkit.pdf.

Last reviewed: July 26, 2023

People who are at increased risk for acquiring HAV infection include the following:

  • Travelers to countries that have high or intermediate endemicity of HAV infection
  • Men who have sex with men (MSM)
  • Users of injection and non-injection drugs (in other words, all who use illegal drugs)
  • People with occupational risk of exposure (those who work with HAV-infected non-human primates or researchers handling hepatitis A virus)
  • People who anticipate close contact with an international adoptee coming from a country with high or intermediate endemicity of HAV infection
  • People living with HIV infection
  • People experiencing homelessness, including temporary shelters and other unstable living arrangements
  • People living in group settings for those with developmental disabilities and other settings where hygiene is difficult to maintain
  • People who are incarcerated
Last reviewed: June 25, 2023

No, do not use the same unit. Frequent opening of the refrigerator door to retrieve food items can adversely affect the internal temperature of the unit and potentially damage the vaccines.

Last reviewed: July 26, 2023

ACIP recommends that the rotavirus vaccine series be completed with the same product whenever possible. However, vaccination should not be deferred because the product used for a previous dose is not available or is unknown. In these situations, the provider should continue or complete the series with the product available. If any dose in the series was RotaTeq, or the vaccine product is unknown for any dose in the series, a total of 3 doses of rotavirus vaccine should be administered. The minimum interval between doses of rotavirus vaccine is 4 weeks. All doses should be administered by age 8 months and 0 days.

Last reviewed: June 7, 2023

Since DTaP and pneumococcal conjugate (PCV) are the vaccines most likely to cause a local reaction, it is prudent to give DTaP and PCV in separate limbs (if possible), so there is no confusion about which vaccine caused the reaction.

Last reviewed: December 28, 2022

Yes. In clinical trials among immunocompetent adults age 50 years or older, Shingrix reduced the risk of PHN by 91%. One study among hematopoietic cell transplant recipients reported that vaccination reduced the risk of PHN by 89%.

Last reviewed: March 9, 2022

Yes. As with any combination vaccine, it may be used when any of the components are indicated and none are contraindicated. Providers must observe spacing intervals such that the minimum interval between doses is equal to the greatest interval of any of the individual antigens. Pediarix may only be used in children younger than age 7 years.

Last reviewed: January 27, 2025

In general, neither exposure to or recovery from an infectious disease is a contraindication or precaution to vaccination. In particular, recovery from varicella (chickenpox) is not a reason to withhold a live vaccine, such as MMR.

COVID-19 is the exception to this general rule. CDC recommends that routine vaccination should be deferred for persons with suspected or confirmed COVID-19, regardless of symptoms, until criteria have been met for them to discontinue isolation. The reason for this exception is that vaccination visits for these individuals should be postponed to avoid exposing healthcare personnel and other patients to the virus that causes COVID-19.

Last reviewed: August 29, 2022

In February 2022, ACIP voted on the following recommendation or TBE vaccination: TBE vaccine is recommended for people who are moving or traveling to a TBE-endemic area and will have extensive exposure to ticks based on their planned outdoor activities and itinerary.

In addition, TBE vaccine may be considered for persons traveling or moving to a TBE-endemic area who might engage in outdoor activities in areas ticks are likely to be found. The decision to vaccinate should be based on an assessment of their planned activities and itinerary, risk factors for a poorer medical outcome, and personal perception and tolerance of risk. For more information about TBE vaccine from CDC, visit www.cdc.gov/tick-borne-encephalitis/hcp/vaccine/index.html.

Last reviewed: August 21, 2023

Gardasil 9 (9vHPV, Merck) is the only HPV vaccine being distributed in the United States. Bivalent Cervarix (2vHPV, GSK) and quadrivalent Gardasil (4vHPV, Merck) are no longer available in the United States.

9vHPV is an inactivated 9-valent vaccine licensed by the FDA in 2014. It contains 7 oncogenic (cancer-causing) HPV types (16, 18, 31, 33, 45, 52, and 58) and two HPV types that cause most genital warts (6 and 11). The 9vHPV vaccine is licensed for people age 9 through 45 years.

Last reviewed: March 2, 2024

Multiple manufacturers are producing inactivated, recombinant, and live attenuated influenza vaccines for the U.S. market for the 2024-25 season. All vaccines are trivalent (containing two influenza A and one influenza B virus vaccine antigen).

Immunize.org offers a 1-page printable document that summarizes each of the products available for the current influenza vaccination season: www.immunize.org/catg.d/p4072.pdf.

Last reviewed: August 11, 2024

Currently, protection from serious lower respiratory tract disease (LRTD) after RSV vaccination is expected to last for about two RSV seasons, but exactly how well it protects over a longer period of time is not yet known. In clinical trials, the ability of the vaccine to reduce the risk of serious lower respiratory tract disease was tested in some people over 2 RSV seasons, and all RSV vaccines provided meaningful protection in the second season.

RSV vaccination is currently recommended as a single dose: revaccination is not yet recommended. ACIP will update its recommendations concerning whether to revaccinate, and when, as more data become available over time concerning how long protection from a single dose lasts and how effective revaccination is in boosting protection. A small study by GSK did not show a meaningful benefit of revaccination with Arexvy just one year after dose 1; however, revaccinating after two or three years, or longer, might be helpful. Studies are underway to evaluate the benefit of revaccination at different intervals.

Last reviewed: August 25, 2024

FDA licensed the first pneumococcal conjugate vaccine against seven serotypes (PCV7, Prevnar7, Pfizer) in 2000. A large clinical trial showed PCV7 reduced invasive disease caused by vaccine serotypes by 97%. Compared to unvaccinated children, children who received PCV7:

  • Had 20% fewer episodes of chest X-ray confirmed pneumonia
  • Had 7% fewer episodes of acute otitis media
  • Underwent 20% fewer tympanostomy tube placements

FDA licensed PCV13 based on studies comparing the serologic response of children who received PCV13 to those who received PCV7. Substantial evidence demonstrates that routine infant PCV7 and PCV13 vaccination reduces the carriage and transmission of vaccine serotypes.

Researchers conducted a randomized placebo-controlled trial (CAPiTA trial) in the Netherlands among approximately 85,000 adults 65 years or older from 2008 through 2013. This trial evaluated the clinical benefit of PCV13 in the prevention of pneumococcal pneumonia. The results of the CAPiTA trial demonstrated:

  • 46% efficacy against vaccine-type pneumococcal pneumonia
  • 45% efficacy against vaccine-type non-bacteremic pneumococcal pneumonia
  • 75% efficacy against vaccine-type invasive pneumococcal disease (IPD, i.e., bacteremia or meningitis)

FDA licensed PCV15 and PCV20 in 2021 based on studies comparing the serologic response of adults who received either PCV15 or PCV20 to those who received PCV13. These studies showed PCV15 and PCV20 induced antibody levels comparable to those induced by PCV13 and shown to be protective against invasive disease. FDA subsequently expanded the indication for use of PCV15 and PCV20 to include children starting at age 6 weeks in 2022 and 2023, respectively, based on serologic studies. PCV21 was licensed in 2024 based on a similar evaluation of serologic response to vaccination.

Last reviewed: November 13, 2024

Two or more injectable or nasally administered live vaccines not administered on the same day should be separated by at least 4 weeks to minimize the potential risk for interference. If two such vaccines are separated by less than 4 weeks, the second vaccine administered should not be counted and the dose should be repeated at least 4 weeks later. Alternatively, one can perform serologic testing to check for immunity, but this option may be more costly, may not be practical if multiple antigens are involved (such as measles, mumps and rubella), and may provide results that are difficult to interpret.

In cases where the vaccine doses given less than 28 days apart are two doses of the same live vaccine in a series (e.g., 2 doses of MMR vaccine), not different vaccines, you do not need to repeat the second dose if it was inadvertently administered within the 4-day “grace period” before day 28. If given more than 4 days earlier than day 28, the second dose should be repeated after the recommended minimum interval from the invalid dose.

The oral vaccines Ty21a typhoid, cholera and rotavirus vaccines can be administered on the same day with or at any interval before or after other live vaccines (injectable or intranasal). However, ACIP recommends that oral cholera vaccine should be administered before Ty21a vaccine, and at least 8 hours should separate the oral cholera vaccine and the first dose of Ty21a in order to minimize the risk that the oral cholera vaccine buffer might interfere with the enteric coating of the oral Ty21a vaccine.

Last reviewed: June 6, 2023

Not that we know. Nearly all vaccines have been reported to be associated with the fainting. Post-vaccination fainting has been most frequently reported after three vaccines commonly given to adolescents (HPV, MenACWY, and Tdap). However, the association with these vaccines may simply reflect the fact that adolescents are generally more likely to experience fainting in response to pain or anxiety.

Last reviewed: August 31, 2022

Treatment after an exposure (PEP) in a previously unvaccinated person requires receiving a dose of human rabies immune globulin (HRIG) and four (or five if the person’s immune system is suppressed) doses of vaccine. Pre-exposure prophylaxis (PrEP) requires only two doses of vaccine and no immune globulin. If a person who is up to date with the recommended PrEP schedule is exposed to rabies, the person’s PEP treatment is completed with two doses of vaccine (on day 0 and day 3).

Last reviewed: May 14, 2023

Use the date of administration to make a presumptive determination of what type of OPV was received. Only trivalent doses count as valid for the U.S. polio vaccination schedule.

Trivalent OPV was used throughout the world before April 2016. In April 2016, all countries using tOPV switched to bivalent OPV (bOPV). In addition, some countries also use monovalent OPV (mOPV) during special vaccination campaigns. Doses recorded as bOPV or mOPV, and unspecified OPV doses noted on an immunization record as given during a vaccination campaign, do not count as valid doses for the U.S. polio vaccination schedule.

You may count a record of an “OPV” dose as valid if the dose was administered before April 1, 2016, and was not noted as being administered as part of a mass vaccination campaign. OPV doses administered on or after April 1, 2016, should not be counted as a valid dose for the U.S. polio vaccination schedule.

Last reviewed: July 23, 2023

No. ACIP recommends giving a dose of MMR to infants age 6 through 11 months before international travel, but not varicella vaccine. Varicella vaccine is neither approved nor recommended for children younger than age 12 months in any situation.

Last reviewed: May 16, 2023

Zero, one, or two doses of MMR vaccine are needed for the adults described below.

Zero doses:

  • adults born before 1957 except healthcare personnel*
  • adults born 1957 or later who are at low risk (i.e., not an international traveler or healthcare worker, or person attending college or other post-high school educational institution) and who have already received one or more documented doses of live measles vaccine
  • adults with laboratory evidence of immunity or laboratory confirmation of measles

One dose of MMR vaccine:

  • adults born in 1957 or later who are at low risk (i.e., not an international traveler, healthcare worker, or person attending college or other post-high school educational institution) and have no documented vaccination with live measles vaccine and no laboratory evidence of immunity or prior measles infection

Two doses of MMR vaccine:

  • high-risk adults without any prior documented live measles vaccination and no laboratory evidence of immunity or prior measles infection, including:
    • healthcare personnel*
    • international travelers born in 1957 or later
    • people attending colleges and other post-high school educational institutions

People who previously received a dose of measles vaccine in 1963–1967 and are unsure which type of vaccine it was, or are sure it was inactivated measles vaccine, should be revaccinated with either one (if low-risk) or two (if high-risk) doses of MMR vaccine.

* Healthcare personnel born before 1957 should be considered for MMR vaccination in the absence of an outbreak, but are recommended for MMR vaccination during outbreaks.

Last reviewed: June 19, 2023

All children should receive a series of DTaP at ages 2, 4, and 6 months, with boosters at ages 15–18 months and at 4–6 years. The fourth dose may be given as early as age 12 months if at least 6 months have elapsed since the third dose.

Last reviewed: March 31, 2022

Yes. Dengvaxia is included in the VFC program. The most current VFC resolution is available at www.cdc.gov/vaccines-for-children/about/index.html.

The cost of the pre-vaccination screening test will be covered by Medicaid for those who are eligible.

Last reviewed: January 17, 2025

There is no federal law requiring a refusal form. Several major medical organizations, including the American Academy of Pediatrics, have stated that healthcare providers may decide it is in their best interest to formally document a parent’s refusal to accept vaccination for their (minor) child. To read a discussion on this topic and to access a prototype refusal form, see “Record of Vaccine Declination” that can be accessed at www.immunize.org/catg.d/p4059.pdf.

Last reviewed: February 16, 2025

To assist with the shared clinical decision-making around the option to vaccinate against meningococcal serogroup B disease and the timing of vaccination, CDC has provided some specific considerations about the disease and the vaccine that the patient and provider may weigh:

  • Serious nature of invasive meningococcal serogroup B infection, with a high risk of death and permanent complications
  • Low level of serogroup B disease in the United States, with an average of 34 cases each year among people age 16 through 23 years between 2015 and 2018, declining to 17 cases in 2022.
  • Increased risk among college students, especially those who are freshmen, attending a 4-year university, living in on-campus housing, or participating in sorority and fraternity life
  • Protection of MenB vaccine against most strains of meningococcal serogroup B bacteria
  • Estimated relatively short duration of MenB vaccine protection, with antibody levels waning within 1–2 years of completing the primary series; however, if a booster is indicated (e.g., during an outbreak) antibody titers rise in one to two weeks after booster dose administration
  • Evidence to date suggests no impact of MenB vaccine on meningococcal B carriage (may protect an individual from invasive disease but is unlikely to impact transmission of the bacteria to others)
Last reviewed: November 15, 2024

CDC defines an “additional primary dose” as a subsequent dose of vaccine administered to people who are less likely to develop a protective immune response after initial vaccination because of moderate or severe immunocompromise. All previously unvaccinated people with moderate or severe immunocompromise are recommended to receive an initial 3-dose primary mRNA COVID-19 series or 2-dose primary Novavax protein vaccine series. 

CDC defines a “booster dose” as a subsequent dose of vaccine administered to enhance or restore protection which might have declined over time after primary series vaccination.

Last reviewed: November 16, 2024

No. ACIP considers a dose of MenACWY given to a 10-year-old child to be valid for the first dose in the adolescent series. Doses given before age 10 years should not be counted. The child should receive the second (booster) dose at age 16 years as usual.

Last reviewed: November 15, 2024
Table 1: Hepatitis B laboratory nomenclature
HBsAg: Hepatitis B surface antigen is a marker of infectivity. Its presence indicates either acute or chronic HBV infection.
anti-HBs: Antibody to hepatitis B surface antigen is a marker of immunity. Its presence indicates an immune response to HBV infection, an immune response to vaccination, or the presence of passively acquired antibody. (It is also known as HBsAb, but this abbreviation is best avoided since it is often confused with abbreviations such as HBsAg.)
anti-HBc (total): Antibody to hepatitis B core antigen is a nonspecific marker of acute, chronic, or resolved HBV infection. It is not a marker of vaccine-induced immunity. It may be used in prevaccination testing to determine previous exposure to HBV infection. (It is also known as HBcAb, but this abbreviation is best avoided since it is often confused with other abbreviations.)
IgM anti-HBc: IgM antibody subclass of anti-HBc. Positivity indicates recent infection with HBV (<6 mos). Its presence indicates acute infection.
HBeAg: Hepatitis B “e” antigen is a marker of a high degree of HBV infectivity, and it correlates with a high level of HBV replication. It is primarily used to help determine the clinical management of patients with chronic HBV infection.
Anti-HBe: Antibody to hepatitis B “e” antigen may be present in an infected or immune person. In persons with chronic HBV infection, its presence suggests a low viral titer and a low degree of infectivity.
HBV-DNA: HBV Deoxyribonucleic acid is a measure of viral load and reflects viral replication. It correlates well with infectivity. It is used to assess and monitor the treatment of patients with chronic HBV infection.

 

Last reviewed: January 17, 2025

Yes, for the primary series. If either ActHIB (PRP-T), Hiberix (PRP-T), Vaxelis (DTaP-IPV-Hib-HepB), or Pentacel (DTaP-IPV/Hib) is used for a routine primary series dose, a complete routine primary series consists of three doses. PedvaxHIB (PRP-OMP) requires a 2-dose primary series, but if administering a mixed-product primary series including only one dose of PedvaxHIB, a total of 3 doses is needed to complete the primary series.

Vaxelis is not recommended for use as a Hib booster (4th) dose. A different Hib-containing vaccine licensed for the booster dose should be used. If Vaxelis is inadvertently given as the booster dose, the dose does not need to be repeated with another Hib-containing vaccine, if the proper spacing of prior doses is maintained.

Last reviewed: January 21, 2025

No. According to ACIP, vaccines administered outside the U.S. generally can be accepted as valid if the schedule (i.e., minimum ages and intervals) is similar to that recommended in the U.S. However, with the exception of the influenza and pneumococcal polysaccharide vaccines, only written documentation should be accepted as evidence of previous vaccination. In general, if records cannot be located or will definitely not be available anywhere because of the patient’s circumstances, children 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. More information is available in the relevant subsection of CDC’s General Best Practices for Immunization, available at  www.cdc.gov/vaccines/hcp/imz-best-practices/special-situations.html#cdc_report_pub_study_section_7-persons-vaccinated-outside-the-united-states.

Last reviewed: January 20, 2025


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Last reviewed: May 24, 2023

ACIP has recommended that all adults age 65 years and older receive a second dose of the 2024–2025 Formula COVID-19 vaccine 6 months after their first 2024–2025 Formula COVID-19 vaccine dose (minimum interval of 2 months). However, ACIP has not made any standing COVID-19 vaccination recommendations that go beyond the 2024–2025 Formula COVID-19 vaccines. 

Last reviewed: November 16, 2024

In general, CDC recommends waiting a minimum of 8 weeks (2 months) since the last 2023–2024 Formula COVID-19 vaccine to receive an age-appropriate 2024–2025 Formula COVID-19 vaccine.

Any person who initiated a COVID-19 vaccination primary series (e.g., a child 6 months through 4 years or a person with moderate or severe immunocompromise) with a previous formulation should follow the age-appropriate recommended schedule for completion of their primary series with the 2024–2025 formula of the same brand, if feasible. 

Last reviewed: November 16, 2024

ACIP recommends that infants who have had rotavirus gastroenteritis before receiving the full series of rotavirus vaccination should still start or complete the schedule according to the age and interval recommendations because the initial rotavirus infection might provide only partial protection against subsequent rotavirus disease.

Last reviewed: June 7, 2023

According to CDC, more than 80% of healthy adults who receive PPSV23 develop antibodies against the serotypes contained in the vaccine that persist for at least 5 years. Older adults and people with some chronic illnesses or immunodeficiency may not respond as well and their antibody levels may decline more quickly.

Overall, the vaccine is 60% to 70% effective in preventing invasive pneumococcal disease caused by serotypes in the vaccine. PPSV23 shows less effectiveness among immunocompromised people; however, because of their increased risk of invasive pneumococcal disease, CDC recommends PPSV23 for people in these groups who receive PCV15. There has not been consensus regarding the ability of PPSV23 to prevent non-bacteremic pneumococcal pneumonia; however, recent observational studies reported 21%–46% effectiveness against PPSV23-type pneumococcal pneumonia when PPSV23 was given less than 5 years before illness onset.

Unlike conjugate vaccines, PPSV23 vaccination has not been shown to decrease nasal carriage of pneumococcal bacteria among those vaccinated.

Last reviewed: November 13, 2024

Shingrix is recommended for the prevention of herpes zoster and related complications for immunocompetent adults 50 years of age and older, including those who previously received Zostavax. On October 20, 2021, ACIP recommended 2 doses of RZV for the prevention of herpes zoster and related complications in adults age 19 years or older who are or will be immunodeficient or immunosuppressed because of disease or therapy.

ACIP published its zoster vaccination recommendations for immunocompetent adults age 50 years and older in January 2018: www.cdc.gov/mmwr/volumes/67/wr/pdfs/mm6703a5-H.pdf.

ACIP published its recommendations for the use of recombinant zoster vaccine in adults age 19 years or older who are or will be immunocompromised in January 2022: www.cdc.gov/mmwr/volumes/71/wr/pdfs/mm7103a2-H.pdf.

Last reviewed: March 9, 2022

Although birth before 1957 is considered acceptable evidence of measles immunity for routine vaccination, healthcare facilities should consider vaccinating unvaccinated healthcare personnel (HCP) born before 1957 who do not have laboratory evidence of measles immunity, laboratory confirmation of disease, or vaccination with 2 appropriately spaced doses of MMR vaccine.

However, during a local outbreak of measles, all healthcare personnel, including those born before 1957, are recommended to have 2 doses of MMR vaccine at the appropriate interval if they lack laboratory evidence of measles.

Healthcare facilities should check with their state or local health department’s immunization program for guidance. Access contact information here: www.immunize.org/coordinators.

Last reviewed: June 19, 2023

CDC published a revised ACIP recommendation for IPV on August 6, 2009, to include a dose given at age 4 through 6 years (and at least 6 months after the preceding dose), regardless of the number of doses given before the fourth birthday. In general, you do not need to administer an additional IPV dose to older teens (or adults) who had already completed a routine 4-dose schedule of polio vaccination before their fourth birthday before the change was published in August 2009; however, if a dose given after the fourth birthday is needed for compliance with state requirements, you may want to check with your state immunization program or immunization registry manager to see what they accept/expect in this case. Contact information for state immunization managers can be found at www.immunize.org/coordinators.

Last reviewed: July 23, 2023

No. Pediarix is licensed for doses 1, 2, and 3 of the DTaP primary series through age 6 years.

Last reviewed: January 27, 2025

No. Pregnancy tests prior to vaccination are not routinely recommended. However, patients of childbearing age should be asked about the possibility of their being pregnant prior to being given any vaccine for which pregnancy is a contraindication or precaution. The patient’s answer should be documented in the medical record. If the patient responds that they believe they may be pregnant, a test should be performed before administering vaccines not recommended or contraindicated in pregnancy.

Last reviewed: August 29, 2022

CDC recommends using separate refrigerator and freezer units for vaccine storage, but still allows use of a combination refrigerator/freezer if you only use the refrigerator portion for storing vaccines (as you are doing). CDC also recommends that you store food and beverages in a separate storage unit from vaccines, which you are technically doing, but there may still be an impact on the refrigerator temperature by the opening and closing of the freezer door by staff. (In most two-compartment units, cold air from the freezer is circulated for cooling the refrigerator.)

The best situation would be to get a stand-alone pharmaceutical/purpose-built refrigerator unit for your vaccines (consider one that meets the voluntary NSF/ANSI 456 certification for vaccine storage), and use your refrigerator/freezer combination unit for your food and drinks. For more information about storage unit features and recommendations, refer to pages 9 and 31 of the CDC Vaccine Storage and Handling Toolkit available at www.cdc.gov/vaccines/hcp/admin/storage/toolkit/storage-handling-toolkit.pdf.

Last reviewed: July 26, 2023

The ACIP recommends that routine HPV vaccination be initiated for all children at age 11 or 12 years. ACIP notes that vaccination may be started at age 9 years, if preferred, and should start at age 9 for any child that the provider at risk of exposure to HPV due to suspected abuse. There is no downside to beginning the series at age 9, and this option is often easier for families and clinics because it gives more time to complete the 2-dose series before the 13th birthday. Vaccination is also recommended for all people age 13 through 26 years who have not been vaccinated previously or who have not completed the vaccination series.

Last reviewed: March 2, 2024

The 2024-25 vaccines are all trivalent (containing two influenza A and one influenza B strains). The B/Yamagata virus vaccine antigens are no longer included in influenza vaccines because B/Yamagata viruses have not been detected globally since March 2020.

The 2024-25 vaccines include a new influenza A(H3N2) component.

Egg-based influenza vaccines include:

  • an A/Victoria/4897/2022 (H1N1)pdm09-like virus;
  • an A/Thailand/8/2022 (H3N2)-like virus; and,
  • a B/Austria/1359417/2021 (B/Victoria lineage)-like virus.

Cell culture-based or recombinant vaccines include:

  • an A/Wisconsin/67/2022 (H1N1)pdm09-like virus;
  • an A/Massachusetts/18/2022 (H3N2)-like virus; and,
  • a B/Austria/1359417/2021 (B/Victoria lineage)-like virus.
Last reviewed: August 11, 2024

All three RSV vaccines are currently licensed and recommended as a one-time dose for any person. Data are not available at this time to make recommendations for revaccination. At this time, a pregnant person who receives Abrysvo (Pfizer) during one pregnancy is not recommended to receive Abrysvo during a subsequent pregnancy: in subsequent pregnancies, the baby should receive nirsevimab (Beyfortus, Sanofi) after delivery for RSV protection. ACIP will make decisions concerning RSV revaccination as more data become available over time.

Last reviewed: August 25, 2024

If vaccines are given too close together, it can result in a less than optimal immune response. However, in most instances, a difference of a few days is unlikely to have a negative effect on immune response. With the exception of rabies vaccine, ACIP allows a grace period of 4 days (i.e., vaccine doses administered up to 4 days before the recommended minimum interval or age can be counted as valid). However, if a dose was administered 5 or more days earlier than the recommended minimum interval between doses, it is not valid and must be repeated. The repeat dose should be spaced after the invalid dose by the recommended minimum interval. Note that for hepatitis A vaccination, if the second dose is administered too early and must be repeated, the recommended interval between the invalid dose and the repeat dose is 6 months; however, if the repeat dose is administered earlier than 6 months no further doses are recommended as long as the interval between the first and final dose is at least 6 months.

If the first dose in a series is given 5 days or more before the recommended minimum age, the dose should be repeated on or after the date when the child reaches at least the minimum age. If the vaccine is a live vaccine, ensuring that a minimum interval of 28 days has elapsed from the invalid dose is recommended. Avoid such errors by knowing the minimum intervals and ages for routinely given vaccines. You can look up such information in the ACIP “General Best Practices Guidelines for Immunization”, available at www.cdc.gov/vaccines/hcp/imz-best-practices/timing-spacing-immunobiologics.html, Table 3-2.

The 4-day “grace period” should not be used when scheduling future vaccination visits, and should not be applied to the 28-day interval between different live parenteral vaccines not administered at the same visit. It should be used primarily when reviewing vaccination records (for example, when evaluating a vaccination record prior to entry to daycare or school).

Last reviewed: June 6, 2023

Falls that occur due to fainting after vaccination can be prevented by assuring that the vaccinated person is sitting in a chair or lying down and is observed for 15 minutes following vaccination.

Last reviewed: August 31, 2022

Human rabies immune globulin (HRIG) is the IgG fraction of plasma from human donors who have received multiple doses of rabies vaccine and have high levels of anti-rabies antibody. HRIG is administered once to previously unvaccinated individuals exposed to a rabid animal to provide rabies virus neutralizing antibody coverage until the patient responds to vaccination by actively producing virus-neutralizing antibodies. HRIG is administered once on day 0 at the time postexposure prophylaxis (PEP) is initiated, in conjunction with human rabies vaccine. If HRIG was not administered when vaccination was begun on day 0, it can be administered up to and including day 7 of the PEP series. If anatomically feasible, the full dose of HRIG is infiltrated around and into any wounds. Any remaining volume is injected intramuscularly at a site distant from vaccine administration. HRIG should not be administered in the same syringe or at the same anatomic site as the first vaccine dose. However, subsequent doses (i.e., on days 3, 7, and 14) of vaccine in the 4-dose PEP vaccine series can be administered in the same anatomic location in which HRIG was administered.

Last reviewed: May 14, 2023

First-year college students living in residence halls should be vaccinated against meningococcal ACWY disease. Before enrollment, administer a dose of MenACWY vaccine to those previously unvaccinated, to those who have not had a dose of MenACWY since turning 16, and to those whose most recent MenACWY dose (given after turning 16) was not given within the past 5 years. Some schools, colleges, and universities have policies requiring vaccination against meningococcal disease as a condition of enrollment.

Last reviewed: November 15, 2024

Varicella vaccine is neither approved nor recommended for children younger than age 12 months. Assuming that the child is not immunocompromised, varicella zoster immune globulin (VariZIG) is also not recommended.

ACIP does not have a recommendation for acyclovir for varicella postexposure prophylaxis. The American Academy of Pediatrics provide some guidance on this issue in the current edition of the Red Book.

Last reviewed: May 16, 2023

The most current ACIP recommendations for Tdap can be accessed here at www.cdc.gov/mmwr/volumes/69/wr/pdfs/mm6903a5-H.pdf.

A listing of the recommendations follows:

  • Tdap can be given regardless of the interval since the last Td was given. There is NO need to wait 2–5 years to administer Tdap following a dose of Td.
  • Adolescents should receive a single dose of Tdap (instead of Td) at the 11–12-year-old visit.
  • Adolescents and adults who have not received a dose of Tdap, or for whom vaccine status is unknown, should receive a single dose of Tdap as soon as feasible. As stated above, Tdap can be administered regardless of interval since the previous Td dose.
  • Children age 7–10 years who are not fully immunized against pertussis (i.e., did not complete a series of pertussis-containing vaccine before their seventh birthday) should receive a single dose of Tdap. If needed, they should complete their series with Td or Tdap. If a Tdap dose is administered at age 10 years or older, the Tdap dose may count as the adolescent Tdap dose.
  • All healthcare personnel, regardless of age, should receive a single dose of Tdap as soon as feasible if they have not previously received Tdap and regardless of the time since the last dose of Td.
  • Pregnant teens and adults should receive Tdap during each pregnancy, preferably between 27- and 36-weeks’ gestation. Mothers who have never received Tdap and who do not receive it during pregnancy should receive it immediately postpartum.
  • Tdap may be administered in any situations where Td only was previously recommended.
  • After receiving an initial dose of Tdap, either Tdap or Td can be used to fulfill the decennial (every 10 years) Td booster dose recommendation.
Last reviewed: March 31, 2022

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