COVID-19 Vaccine Booster QA

1. What is a booster?

How does this differ from an additional dose for immunocompromised individuals? Booster doses of a vaccine are very common in the field of vaccines. Boosting means getting another dose of vaccine at a different time after the initial dose(s) of a vaccine, which scientists call the “priming” stage. It’s like wall paint: priming coat, maybe a second priming coat, followed by another boosting coat. For vaccines, the boost is weeks, months, or years later, rather than a day or two for wall paint. Like a second coat of paint, the boosting improves upon the priming. Most vaccines we get have boosting components. For hepatitis B, babies get the vaccine at 1, 2 and 6 months. The 6-month vaccine is the boost. For the measles vaccine, children get it at about 12 or 15 months and then the boost 5 years later when they enter elementary school. For tetanus, you get a booster every 10 or so years. The additional doses given to immunocompromised individuals are to improve their immune response to the initial vaccine series. They often do not build enough, or any, protection against the virus from the original vaccination.

2. Who needs to get a booster?

Who is eligible to get a booster? For mRNA vaccines (Pfizer and Moderna), the FDA and ACIP both recommend boosting for all people 65 years of age and older, for people from 18-64 with comorbidities for severe COVID-19, and for those whose occupational exposure puts them at high risk of COVID-19. For the J&J vaccine, they recommend a second dose of vaccine for all people over age 18 who received the initial dose. People who had any complications from the initial J&J vaccination such as venous thrombosis should get a second dose of an mRNA vaccine rather than a second dose of the J&J vaccine.

3. When do you need to get a booster? (how long after you finished your initial vaccine series?)

The FDA recommends getting a booster at least 6 months after completion of the initial series of the Moderna or Pfizer vaccines. The J&J booster is recommended at least 2 months after the initial J&J vaccine.

4. Why are boosters important?

Boosters are important for 2 main reasons. (1) The Delta variant is more infectious and therefore vaccinated people are becoming infected. (2) The booster dose increases the antibodies and T cells that the body makes as a result of vaccination. The levels are higher than after the initial vaccination. These responses are what protect people from infection and disease.

5. If I get a booster, am I preventing other people around the world from being vaccinated?

No. The US government is using the booster doses from our national stockpile. The doses have been bought already and these doses are restricted by contract for use in the US. The US has contracted out with Pfizer, Moderna and J&J to buy doses for donation to other countries. The US government has donated more than 1 billion doses of vaccines so far.

6. Should I get a booster from the same vaccine maker as my original COVID shot?

The best data on efficacy and safety are from using the same booster as the vaccine you received. There are small studies in which other manufacturers’ vaccines have been used as a boost: the “mix and match” studies. Those data show the antibody responses are increased by mix and matching. So, the FDA has authorized mix and match booster doses. This means that your booster vaccine does not have to be made by the same company as your initial vaccine(s).

7. Is it safe to get a booster from a different vaccine maker?

As long as the vaccine has been approved by the FDA, it is safe. This includes the vaccines from Pfizer, Moderna and J&J.

8. Is there benefit to getting a booster from a different vaccine maker?

Right now, there is no evidence that a “mix and match” booster dose will provide more benefit than getting a booster that is the same as the initial vaccination.

9. I heard that the Moderna booster will only be a half dose. Why not a full dose?

The half dose still enhances the levels of T cells and antibodies to much higher levels than after the 2 initial doses at full dosage. Several other vaccines have boosted well with “fractional dosing.” This is not a new concept in vaccinology. 

10. How long will the booster last?

Will we have to get boosters every year, like the flu shot? We don’t know how long it will last. Much of this relates to how much the virus changes. We need to continue to study people’s responses to see how long they last.

11. Are there side effects for the booster?

Your side effects will most likely be like those you had when you got your original vaccination. There is no evidence so far that the side effects will be worse with the boosters.

12. Are we supposed to register again for the booster?

Booster doses are done in the same system as the initial vaccinations.

13. Are we going to receive any proof that we have received the booster?

You should bring your CDC COVID-19 Vaccination Record card when you get your booster, and it will be documented there. If you did not receive a CDC COVID-19 Vaccination Record card at your first appointment, contact the vaccination site where you got your first shot or your state health department to find out how you can get a card. Learn more about what to do if you need a copy of your CDC COVID-19 Vaccination Record card.

14. Is the booster free?


15. If I need a booster shot, does that mean that the vaccines aren’t working?

No. COVID-19 vaccines are working well to prevent severe illness, hospitalization, and death, even against the Delta variant. However, public health experts are starting to see reduced protection against mild and moderate disease, especially among certain populations.

16. When can I get a COVID-19 vaccine booster if I am NOT in one of the recommended groups?

Additional groups of people may be recommended to receive a booster shot as more data become available.

17. Do the boosters address the new variants that have surfaced? How effective are they against the Delta variant, for example?

The booster vaccines are not different from the original vaccinations. They have all shown protection against the variants, including Delta. 18. What is the difference in efficacy between the different boosters? (Moderna, Pfizer, J&J and any other international boosters) Booster doses of all three EUA vaccines (Moderna, J&J and Pfizer) increase immune responses and provide additional efficacy compared to not getting a booster.