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Third dose vs. booster: Doctors explain vaccine recommendations

Monitor staff
Published: 8/26/2021 1:27:01 PM

Last week, the CDC announced that many Americans will soon be eligible for COVID-19 booster shots. Immunocompromised Americans are eligible for a third dose even sooner.

Two experts, Dr. Michael McLeod from Concord Hospital and Dr. Michael Calderwood from Dartmouth-Hitchcock have waded through guidance from numerous state and federal recommendations to bring you answers about the booster and third dose of the COVID-19 vaccine. The following has been edited for brevity and clarity:

What is the difference between the booster shot and the third dose of the COVID-19 vaccine?

McLeod: The two mRNA COVID-19 vaccines, Pfizer and Moderna, are now recommended to have a booster after 8 months from the second dose of the original two-dose series. There is no difference in the ingredients or amount of the booster vaccine for these two. It is important to point out the booster is only indicated for individuals who have already received the two dose series.

The latest guidelines for severely immunocompromised individuals is to have them receive a third dose of the vaccine. Again, there is no difference in the ingredients or amount and dose. However, the timing is different. A third dose may be administered after 28 days for the second dose while a booster is recommended after 8 months from the second dose. Third doses are for severely immunocompromised individuals whereas a booster is recommended for individuals 16 or older for Pfizer and 18 or older for Moderna.

Calderwood: The FDA has authorized the use of a third dose of the Pfizer and Moderna COVID-19 vaccines for immunocompromised people, with the third dose to be administered at least 28 days following the first two doses. This is different than a booster dose that is being discussed for the general population. Instead, this third dose is recommended as part of the primary series for people with medical conditions or receiving treatments associated with moderate-to-severe immunocompromise.

Why are booster shots necessary?

McLeod: We are starting to see the immunity toward mild to moderate disease is starting to wane after eight months. A booster will stimulate your immune system once again to make more antibodies and a stronger immune response to COVID-19.

Calderwood: A booster is given when the protection from a vaccine begins to wane. This may happen because the body’s immune response decreases over time or the virus causing disease has changed over time and the original vaccination is not as effective. The CDC have presented data on decreasing vaccine effectiveness against infection, but stable vaccine effectiveness against hospitalization and death. It is unknown whether the waning effectiveness has to do with the Delta variant or the time that has elapsed since people received their full doses. Others have raised concern that the data are confounded by mask use and its subsequent discontinuation. Finally, some are concerned that we are moving to provide a third dose of the original vaccine instead of developing a vaccine modified for improved coverage of Delta.

What about third doses?

McLeod: The goals are the same. There is a significant number of severely immunocompromised patients who will not have detectable antibodies after a two dose series.

Calderwood: We have data showing that people who are immunocompromised are more likely to develop breakthrough infections resulting in hospitalization, that the vaccine effectiveness and antibody response is lower in people who are immunocompromised after two doses of a mRNA vaccine, and that a third dose of a mRNA vaccine can boost the prevalence of protective antibodies in people who are immunocompromised. As for side effects, these have been reported as similar to prior doses and mostly mild or moderate.

For immunocompromised Granite Staters who qualify for a third dose of the COVID-19 vaccine, will proof of a medical condition be required in order to get the shot?

McLeod: Patients seeking vaccination outside of their primary care providers can self attest to their immunocompromised status.

Which mRNA shot should people get as their booster? Are there any advantages or disadvantages of mixing and matching mRNA shots?

McLeod: It is recommended that your booster dose be the same mRNA vaccine as your original series. So, if you received two doses of the Pfizer vaccine as your original series, your booster should be Pfizer as well. While there are studies ongoing to see if alternative combinations of vaccines are more effective, it is not recommended at this time.

What about Johnson & Johnson recipients? Are they at a disadvantage?

McLeod: We have to remember that the J&J vaccine received Emergency Use Authorization in March 2020, whereas Pfizer received the first EUA in the U.S. in December 2020 and Moderna a week later. There is three more months of data on Pfizer and Moderna in comparison to J&J. The FDA has said they are reviewing the data on J&J and a booster is likely but they want to see more data before making a final decision.

Calderwood: It is my hope that we will have data soon on booster doses, potentially with mRNA vaccines, for those who received the J&J vaccine. The benefit of the J&J vaccine is that it provides great protection after a single dose (compared with a single dose of mRNA vaccine), but the mRNA vaccines provide greater protection for those who complete the vaccine series.


Teddy Rosenbluth bio photo

Teddy Rosenbluth is a Report for America corps member covering health care issues for the Concord Monitor since spring 2020. She has covered science and health care for Los Angeles Magazine, the Santa Monica Daily Press and UCLA's Daily Bruin, where she was a health editor and later magazine director. Her investigative reporting has brought her everywhere from the streets of Los Angeles to the hospitals of New Delhi. Her work garnered first place for Best Enterprise News Story from the California Journalism Awards, and she was a national finalist for the Society of Professional Journalists Best Magazine Article. She graduated from UCLA with a bachelor’s degree in psychobiology.



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