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COVID questions: Booster shots, mixing vaccines

July 20, 2021

Editor’s note: We will be publishing answers to questions about COVID-19 and the pandemic each week in this COVID questions column. If you have a question, please email it to

Q: I had my second Moderna shot in late January. Is it ok for me to get a booster?

A: There is still so much to learn — how long immunity persists after vaccines, what kinds of reactions people might have with additional doses, and do the vaccines need to be updated for new strains of SARS-CoV2 — that it’s just too soon to know whether we will all need to have booster shots like we do for the flu.

Currently, the Centers for Disease Control and Prevention does not recommend boosters for any COVID-19 vaccine, indicating that the need for and timing of COVID-19 booster doses have not been established.

There are scenarios in the future where booster shots could make sense to employ. Examples could include boosters for select groups who have immune-weakening diseases such as HIV, organ transplants or cancer and who may not receive full protection from the initial COVID-19 immunization, or more widespread boosters if they are needed to respond to emerging variants.

For now, the priority needs to be continuing to provide vaccines to the unvaccinated – both locally and globally. Once more countries around the world, and more communities in the United States, have been vaccinated with the available supply, it may be conceivable for high-risk populations, like people 65 years old and older, organ transplant recipients and perhaps healthcare workers to receive booster shots eventually

— James Conway, Pediatric Infectious Diseases Fellowship Program Director and School of Medicine and Public Health Office of Global Health Director, from UW Health News

Q: Can I take two different types of covid 19 vaccines, with two doses each?

A: Once an individual has been fully immunized with one of the currently authorized vaccines — either two doses of Pfizer or Moderna, or a single dose of Janssen (J&J) — there is currently no indication for receiving additional COVID-19 vaccines. Doing so would likely result in more pronounced side effects than were experienced with the original two doses of Moderna vaccine, and would not offer much additional immunity while using up valuable vaccine that should be directed to someone who is unimmunized.

Individuals who have received one dose of Moderna or Pfizer vaccine should complete the vaccine series with the same vaccine. There is no data available regarding safety nor immune protection when people switch between vaccines, and this is not recommended.

— James Conway, Pediatric Infectious Diseases Fellowship Program Director and School of Medicine and Public Health Office of Global Health Director

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