Should You Mix COVID-19 Vaccines?

Several countries are recommending a “mix-and-match” approach in COVID-19 vaccinations. 

Germany’s government encourages people who get a first dose of the AstraZeneca vaccine to get an mRNA vaccine, like Pfizer or Moderna, for their second dose, according to the Associated Press. Canada supports the same approach, The Washington Post reported.

The AstraZeneca vaccine is not yet authorized in the United States. Current guidance from the Centers for Disease Control and Prevention (CDC) states that COVID-19 vaccines are not interchangeable except in “exceptional situations,” although some overseas residents have already tried mixing vaccines. 

Matt Collins, an author and marketing specialist who lives in Chile, initially received two shots of Sinovac-CoronaVac. He got a booster Pfizer shot when visiting family in Michigan over the July 4 weekend.

Collins’ decision to get a booster was influenced by his Type 1 diabetes, which makes him more vulnerable to viruses like COVID-19, and his skepticism of the efficacy of Sinovac, which is Chile’s dominant vaccine.

“As a Type 1 diabetic, similar to a variety of folks who have chronic illnesses, our immune systems are weakened,” Collins tells Verywell. “If I can be more protected than I would have been, by just having a single vaccine, I'm all for that.”

Nearly 56% of the population in Chile is fully vaccinated, yet the country is seeing a surge in case numbers.

“As I started talking to a lot of people in Chile, what I learned is that they had received the Sinovac vaccine, but were also getting sick,” Collins says. “That was a big factor in me getting a booster.”

Current studies present a range of efficacy rates for Sinovac, from about 50% to 90%, according to The New York Times. 

In contrast to the varying data on the Sinovac vaccine, the Pfizer, Moderna, and Johnson & Johnson vaccines available in the U.S. currently reveal more consistent efficacy rates when studied. According to CDC data, two doses of Pfizer or Moderna are about 94% effective at preventing COVID-19 and one dose of Johnson & Johnson is 66.3% effective.

In the U.S., the CDC currently does not recommend additional doses. Recent studies showed that Pfizer’s and Moderna’s COVID-19 vaccines could offer long-lasting immunity, which means a booster shot may not be needed anytime soon.

Virologist Andrew Pekosz, PhD, tells Verywell that for those seeking a booster shot in the U.S., any of the FDA-authorized vaccines should offer extra immunity.

“Most data about vaccinations suggests that it probably doesn't matter what you get as a booster,” he adds. “You can still stimulate those immune responses a little bit more efficiently and get to a good level of protection.”

Each initial vaccination gives the body a set of responses to fight off the virus, and a booster shot strengthens these immune responses rather than changing them, he adds. 

As the Delta variant is now the dominant variant in the U.S., scientists may also need to consider whether it is more effective to develop a variant-specific booster, he says.

“That's the critical question that needs to be addressed, whether or not we go back to the original formulation that we had and try to boost with that, or whether we change it to a spike protein that's present in one of the variants that's currently circulating,” he adds.

Pekosz says health authorities may decide on the need for a booster by this fall, after more data is released. 

Tom Kenyon, MD, MPH, chief health officer at Project HOPE and former director of global health at the CDC, writes to Verywell that more research is needed to determine whether mixing vaccines will work. 

“There is a theoretical possibility that using different vaccines could actually strengthen the immune response to different parts of the spike protein,” he says. “But we need to await further study results before allowing mixing vaccine doses."

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.