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Issue 1,648: August 18, 2022
Ask the Experts: Answers Questions about
Haemophilus influenzae type B (Hib) Vaccination
and COVID-19 Vaccines
As an additional service to IZ Express readers, we periodically publish special editions such as this one, providing you with new and updated Ask the Experts questions and answers from’s experts. This issue includes 4 Q&As about Hib vaccination and 7 questions from’s Ask the Experts COVID-19 vaccination Q&As, last updated on August 3, 2022. Because COVID-19 vaccination recommendations and clinical guidance are updated frequently, reminds our readers to visit CDC’s web page Use of COVID-19 Vaccines in the United States regularly.

You can find all of these Q&As and more than a thousand other Q&As about vaccines and vaccine administration on our "Ask the Experts" main page at's team of experts includes Kelly L. Moore, MD, MPH (team lead), Carolyn B. Bridges, MD, FACP, and Iyabode Beysolow, MD, MPH.
Haemophilus influenzae type b (Hib) vaccination questions

Haemophilus influenzae type b (Hib) vaccination questions

Q: What Hib vaccines are available in the United States?

A: Three monovalent Hib vaccines are available in the United States: PedvaxHIB (PRP-OMP, Merck), ActHIB (PRP-T, Sanofi) and Hiberix (PRP-T, GSK). These vaccines are composed of Hib purified polyribosylribitol phosphate (PRP) capsular polysaccharide chemically bound (conjugated) to a protein to enhance the quality of the immune response to PRP. All three vaccines are approved for infants in a 3- or 4-dose series (depending on brand).
Two combination vaccines containing Hib are currently available in the United States: Pentacel (DTaP-IPV/Hib, Sanofi) and Vaxelis (DTaP-IPV-Hib-HepB, MSP Company). Pentacel is licensed for use in children younger than age 5 years and contains Hib conjugate, DTaP, and inactivated polio vaccines; it is approved as a 4-dose series for infants at age 2, 4, 6, and 15 through 18 months, but it is not approved for use as the DTaP/IPV booster dose recommended at age 4 to 6 years. Vaxelis (DTaP-IPV-Hib-HepB, MSP Company) is licensed for use in children younger than age 5 years and is FDA-approved and recommended by CDC as a 3-dose primary series of Hib for infants at age 2, 4, and 6 months. Vaxelis is not approved for use as a Hib booster (4th) dose. Vaxelis contains the same PRP-OMP Hib antigen as PedvaxHIB, but in a reduced amount.

Q: What is the Hib vaccine schedule for children who have fallen behind or are completely unvaccinated?

A: Healthcare providers should refer to the catch-up schedule which is approved and published each year by the ACIP, AAP, and AAFP (available at The catch-up schedule will help determine the number of additional doses needed and the minimum intervals between doses. However, if a healthy child receives a dose of Hib vaccine at age 15 months or older, he or she does not need any further doses regardless of the number of doses received before age 15 months. Some high-risk children between the age of 15 months and 59 months will be recommended for two doses of Hib vaccine based on previous history of incomplete vaccination.

Q: Can all of the licensed Hib-containing vaccines be used interchangeably?

A: Yes. If either ActHIB, Vaxelis, Pentacel, or Hiberix is used for a routine primary series dose, a complete routine primary series consists of three doses. 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 (DTaP-IPV-Hib-HepB, MSP Company) 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.

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Q: Which adults should receive Hib vaccine?

A: Hib vaccine is not routinely recommended for healthy adults 19 years and older, even if the person did not receive Hib vaccine as a child. However, ACIP recommends that one dose of Hib vaccine should be administered to persons who have anatomical or functional asplenia or sickle cell disease or are undergoing elective splenectomy if they have not previously received Hib vaccine. Hib vaccine should be administered 14 or more days before splenectomy if possible. Recipients of a hematopoietic stem cell transplant should be vaccinated with a 3-dose series of Hib vaccine 6 to 12 months after a successful transplant, regardless of vaccination history; at least 4 weeks should separate doses. Hib vaccine is not recommended for adults with HIV infection since their risk for Hib disease is low.

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COVID-19 Vaccination Questions

Q: Who is eligible to receive a booster dose of a COVID-19 vaccine?

A: ACIP recommends that all people age 5 years and older who have completed primary vaccination against COVID-19 with an mRNA COVID-19 vaccine or the Janssen COVID-19 Vaccine (Johnson & Johnson) should receive at least one age-appropriate COVID-19 vaccine booster dose at the appropriate interval. Novavax COVID-19 Vaccine primary series recipients are not yet recommended to receive a booster dose and the Novavax vaccine is not yet authorized for use as a booster dose for individuals who received a different primary series vaccine.
The mRNA COVID-19 vaccines should be used when booster doses are indicated in almost all circumstances. However, offering the Janssen COVID-19 Vaccine to an individual age 18 years or older under the limited terms of its current FDA emergency use authorization ( is preferable to not providing any COVID-19 vaccine.
Please refer to CDC’s At-A-Glance COVID-19 Vaccination Schedules document for the most current recommended booster dose schedules by age and product type:
For further details, please review the recommendations information available through CDC’s web page, “Use of COVID-19 Vaccines in the United States” at:

Q: What is the Novavax COVID-19 Vaccine?

A: Novavax COVID-19 Vaccine, Adjuvanted contains the SARS-CoV-2 spike protein and Matrix-M adjuvant. The saponin-based adjuvant is made from extracts of the bark of the Soapbark tree native to Chile. It is added to enhance the immune response of the vaccine recipient. The spike protein is produced in insect cells. It was authorized in July 2022 for emergency use in adults age 18 years or older as a two-dose primary series, with the doses given at least 3 weeks apart. It is not currently authorized for use as a booster dose in people who have already received a primary series.
The initial ACIP recommendations for use of Novavax COVID-19 Vaccine are available here:

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Q: How well does the Novavax COVID-19 Vaccine work?

A: The effectiveness of Novavax COVID-19 Vaccine against currently circulating variants of SARS-CoV-2 is not precisely known. Clinical trials of this vaccine were conducted against earlier variants no longer in circulation; however, the vaccine showed very good effectiveness against moderate to severe disease during clinical trials. Testing conducted on antibodies in the serum of Novavax vaccine recipients suggests that vaccination provides substantial protection against severe disease and death caused by currently circulating variants, consistent with other available COVID-19 vaccines.

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Q: What are the recommendations for COVID-19 vaccination of children younger than age 12 years?

A: All children age 6 months and older should be vaccinated against COVID-19. Both mRNA vaccines (Pfizer-BioNTech and Moderna) are authorized for use in children down to age 6 months. The number of doses and recommended formulations vary by age and by immunocompromised health status.
Refer to the CDC’s current recommended immunization schedule for age groups beginning at age 6 months in this PDF document:

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Q: My patient turns 12 years old later this month. Should they get the lower-dose Moderna or Pfizer-BioNTech vaccine formulation approved for age 11 years now or wait and get the higher dose products authorized at age 12?

A: Do not wait. The simple rule is that children should receive a vaccine dosage and formulation authorized specifically for their age on the day of vaccination with each dose. However, the FDA authorization provides flexibility for product selection when managing children who transition from a younger to an older age group during the primary series. CDC has prepared reference documents outlining recommended and acceptable options for each mRNA vaccine when children transition from a younger to an older age group while receiving the primary series.

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Q: A 3-year-old child in my practice who received a first dose of Moderna COVID-19 vaccine at an outside clinic just received a dose of Pfizer-BioNTech COVID-19 vaccine as dose 2 instead of the second dose of Moderna vaccine. Do we need to give another dose?

A: Children who are age 6 months through 4 years and receive different mRNA products for the first two doses need to follow a 3-dose schedule. The third dose of either mRNA vaccine should be administered 8 weeks after the second dose to complete the 3-dose primary series.

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Q: Why has CDC provided the option to increase the interval between dose 1 and dose 2 of mRNA and Novavax COVID-19 vaccines up to 8 weeks?

A: CDC recommends the option of an 8-week interval between doses 1 and 2 of mRNA and Novavax vaccines for most people. Large observational studies in other countries have provided evidence that an 8-week interval may result in higher antibody levels and a reduced risk of vaccine-associated myocarditis, which is most likely to occur after dose 2. For this reason, an 8-week interval of an age-appropriate product may be optimal for some people age 6 months through 64 years, especially males age 12 through 39 years who are at the greatest risk of myocarditis after dose 2.
Shorter intervals (3 weeks for Pfizer-BioNTech and Novavax; 4 weeks for Moderna) protect the recipient more quickly. Therefore, a shorter interval remains the recommended interval for the following groups:

  • All people who are moderately to severely immunocompromised
  • All adults age 65 years and older
  • Others who need rapid protection due to increased risk of exposure or risk of severe disease

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IZ Express Disclaimer
ISSN 2771-8085

About IZ Express

IZ Express is supported in part by Grant No. 1NH23IP922654 from CDC’s National Center for Immunization and Respiratory Diseases. Its contents are solely the responsibility of and do not necessarily represent the official views of CDC.

IZ Express Disclaimer
ISSN 2771-8085

Editorial Information

  • Editor-in-Chief
    Kelly L. Moore, MD, MPH
  • Managing Editor
    John D. Grabenstein, RPh, PhD
  • Associate Editor
    Sharon G. Humiston, MD, MPH
  • Writer/Publication Coordinator
    Taryn Chapman, MS
    Courtnay Londo, MA
  • Style and Copy Editor
    Marian Deegan, JD
  • Web Edition Managers
    Arkady Shakhnovich
    Jermaine Royes
  • Contributing Writer
    Laurel H. Wood, MPA
  • Technical Reviewer
    Kayla Ohlde

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