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FAQ: COVID-19 Booster Guidance


As more data become available, COVID-19 booster guidance will continue to evolve. This guidance was last updated on Oct. 29, 2021.

General Booster Guidance

Q: Am I currently eligible for a COVID-19 booster shot?

A: You are currently eligible for a booster shot if:

You completed your one-dose initial vaccine series of the Johnson and Johnson (J&J) vaccine at least two months ago.


If you completed your two-dose initial vaccine series of the Pfizer or Moderna vaccine at least six months ago, and you fall into one of the following categories:

  • 65 years of age or older.
  • Residents in long-term care settings.
  • 18-64 with underlying medical conditions.
  • 18-64 and at increased risk of exposure and transmission because of occupational (this includes healthcare workers) or institutional setting.


Q: Why is a booster shot recommended?

A: COVID-19 vaccination remains effective in preventing severe disease, although recent data suggest that this protection may wane or decrease over time.

This decreasing effectiveness is likely due to waning (decreasing) immunity over time as well as the greater infectiousness and disease-causing ability of the Delta variant which causes virtually all COVID‑19 at the current time.


Q: I’m eligible for a booster or an additional dose, where can I get one?

A: Visit Vaccines.gov to find a vaccine location near you for a full list of sites where you can get vaccinated for COVID‑19. All Carilion Clinic retail pharmacy locations offer COVID-19 vaccines and boosters (appointments are preferred and can be scheduled by calling the pharmacy location nearest you).


Q: Is the booster shot different from the initial vaccine series?

A: The contents of the booster doses are identical to the initial doses for all three vaccines. The only difference is that the Moderna booster contains half the dose of the initial vaccine.


Mixing and Matching Booster Doses

Q: Does my booster shot need to be the same type of vaccine as my initial vaccine series?

A: The FDA and CDC have said that you are able to mix and match the brand of booster that you receive. They have not recommended one type of vaccine over another; rather, they suggest that the decision should be made at the individual level based on personal benefits and risks and after discussion with the individual’s physician or pharmacist.


Q: What’s the thought behind making the mixing and matching approach available?

A: Preliminary data show that the antibody response to booster in a mix and match approach differ depending on the initial vaccine used and the booster used. Based on these data, and side effect profiles, there may be reasons to choose to boost using a mix-and-match approach based on your personal benefits and risks.

The chart below shows the percent of patients with an increase in antibody production (Ab titers) based on their pairing of initial vaccine brand with their booster brand. Some additional considerations are mentioned below in answers to other questions.

Initial Vaccine Series


% Patients w/ 4-fold rise in Ab titers

at Day 15 post booster























Q: Is there any increased risk from mixing and matching vaccine doses?

A: There does not appear to be increased risk from a mix-and-match approach; for certain genders and age groups, there may be options to further decrease rare side effects. Speak with your physician or pharmacist about what is right for you.


Q: If my first vaccine series was Pfizer or Moderna, why might I consider the J&J for my booster?

A: There is a theoretical possibility that the J&J vaccine might be better at supporting what’s called cell‑mediated immunity – the other arm of the immune response that works in tandem with one’s antibody response. As an example, for young males, using the J&J vaccine to boost for those who received the mRNA vaccines initially might reduce the rare risk of inflammation of the heart (myocarditis).


Q: If my first vaccine was J&J, why might I consider Pfizer or Moderna?

A: For those who initially received the J&J vaccine, it appears that the mRNA vaccines might provide benefit by inducing a greater antibody response, strengthening one’s protection against COVID-19. It might also be useful for younger women to decrease the rare risk of blood clots. Others believe that sticking with the J&J vaccine for the booster makes sense since it might always have been better at strengthening immunity as a 2‑dose series of vaccinations rather than the current single dose.


Additional Doses for the Immunocompromised

Q: What about additional doses for immunocompromised individuals?

A: An additional third dose of the mRNA vaccines (Pfizer and Moderna) for immunocompromised individuals are classified separately from booster vaccinations. They are intended to improve immunocompromised people’s response to their initial two-dose vaccine series because their immune systems may not be able to build enough protection after just the two doses. In contrast to this additional dose for immunocompromised persons, boosters are designed to be administered after a person has completed their vaccine series and protection against the virus has decreased over time.

That said, moderately and severely immunocompromised people 18 years and older who have received their 2-dose initial vaccine series with either Pfizer or Moderna, along with the third dose, may receive a single COVID-19 booster dose (Pfizer, Moderna, or J&J) at least 6 months after their third mRNA vaccine dose—this would be a fourth dose of vaccine.

Immunocompromised persons who received the J&J vaccine initially should receive a booster dose with any vaccine (J&J, Pfizer, or Moderna) two months after their initial dose; there are no current further recommendations for additional vaccinations, even if you are immunocompromised, for the group that initially received the J&J vaccine.