What we know about mixing COVID-19 vaccine doses

The FDA is planning to allow Americans to receive a COVID-19 booster made by a different drugmaker than the one that made the vaccine with which they were initially inoculated, despite little peer-reviewed efficacy data on the practice.

Four things to know:

1. The National Institutes of Health conducted a study on mixing COVID-19 vaccine doses that included 458 people. It was published Oct. 13 in preprint server MedRxiv and has not been peer reviewed.
   
   Participants received an extra shot made by either the drugmaker that manufactured their original vaccine or a different drugmaker. Their antibody levels were measured two weeks and four weeks after they received the boosters. All the combinations made participants' antibody levels higher. However, participants who received an extra dose made by Pfizer or Moderna had higher increases in their antibody levels than those who received an extra dose made by Johnson & Johnson.
   
   Participants who originally were vaccinated with a Johnson & Johnson shot and received a Moderna booster saw their antibody levels rise 76 times higher within 15 days. Those who received a Pfizer booster saw their antibody levels rise 35 times higher, and those who were administered another Johnson & Johnson shot saw their antibody levels rise four times higher.
   
   
2. Kirsten Lyke, MD, a researcher at the University of Maryland's medical school, presented the NIH trial's result at an Oct. 15 FDA meeting. She told The New York Times that "maybe these things are going to play well together," but warned against jumping to conclusions.
   
   Dr. Lyke said her team hopes to know how well boosters made by different drugmakers increase T cells — not just antibodies — by November so leaders can get "a more rounded picture" on the efficacy of mixing COVID-19 vaccines.
   
   
3. The Korea Disease Control and Prevention Agency in South Korea conducted a study on mixing COVID-19 vaccine doses that involved 499 medical workers.
   
   The researchers found that antibody levels in people who received one dose of AstraZeneca's COVID-19 vaccine followed by one dose of Pfizer's shot were six times higher than those who received two doses of AstraZeneca's shot. The study could lead to countries combining AstraZeneca's shot with an mRNA vaccine to improve results.
   
   
4. State officials have been pushing for the option to mix and match COVID-19 boosters for weeks. Many say the option allows for a more streamlined booster rollout, as states often have a greater supply of one vaccine over others.