What Happens If You Mix COVID Vaccine Doses?

Now that a Pfizer booster dose has been approved for Americans 65 and up, people with underlying health conditions and people who work in high-risk settings, many people are wondering: Can I mix and match vaccines? 

“Things change so quickly as far as the science, epidemiology, the virus, people’s behavior — it’s hard to keep up,” said Dr. Kami Kim, director of infectious disease and international medicine at the University of South Florida Health Morsani College of Medicine. 

That means that what experts know about mixing and matching doses is evolving, but here’s what you need to know now:

Official recommendations say to avoid mixing and matching.

According to official guidance from the Centers for Disease Control and Prevention, whenever possible, people who received the Pfizer vaccine should get two doses of that vaccine as well as booster dose if they’re eligible.

Likewise, people who receive the Moderna vaccine should receive two doses from that manufacturer. Federal regulators haven’t yet weighed in on a booster dose for Moderna, but they have approved a third dose of the vaccine for people with weakened immune systems. 

And Johnson & Johnson folks should stick with one dose of their vaccine — for now. 

“It’s an area of active research, and there’s no specific guidance for it,” said Dr. Angela Branche, an infectious disease expert with the University of Rochester’s School of Medicine.

But experts do now say its OK to mix the mRNA vaccines in certain circumstances.

Last winter, the CDC somewhat quietly changed its official stance on mixing and matching vaccines, from a “don’t do it ever” approach to new guidelines that say it’s OK to mix and match the two mRNA vaccines (Pfizer and Moderna) in “exceptional circumstances.” Like if someone doesn’t know what they got for their first dose. Or if a second dose of the same vaccine is unavailable for some reason. 

When the CDC OK’d a third dose of the two mRNA vaccines for people with weakened immune systems back in August, they also left the door open to mixing and matching. “If the mRNA vaccine product given for the first two doses is not available or is unknown, either mRNA COVID-19 vaccine product may be administered,” the agency said. 

Johnson & Johnson is a question mark in all of this.

Nearly 15 million people in the United States got the one-dose Johnson & Johnson vaccine, which uses a different mechanism to produce antibodies than the two mRNA vaccines. And many have been feeling like they’ve been left out in the cold in conversations around boosters and mixing and matching. 

The CDC does say that people who received one dose of either Pfizer or Moderna, but who are unable to get a second dose, can get a Johnson & Johnson shot — and they’re considered to just be fully vaccinated with that shot after two weeks. The prior shot basically doesn’t count.

Johnson & Johnson has recently released findings suggesting a second booster dose of its vaccine produces a strong immune response. It says that two doses were 94% effective at protecting against symptomatic infection, compared to the single shot being about 72% effective against moderate to severe disease in the U.S.

But people who received the Johnson & Johnson vaccine aren’t eligible for boosters at this point. Nor are immunocompromised people who received the Johnson & Johnson vaccine. 

Some experts think we’re moving toward mixing and matching vaccines.

Some countries in Europe — which has different vaccines approved than here in the U.S. — have recommended a mixed-dose approach during the pandemic. And experts like Kim said we could well be headed that way here in the U.S. 

“There are studies in which people are looking at all of this. They’re looking at all sorts of different combinations,” Kim said. 

“It looks like mixing and matching is safe. So what a lot of people have been doing — and informally advising people — is that it shouldn’t harm you to go and get another dose,” she added, saying that’s really only for people who are at high risk of serious outcomes, and that’s not formal policy. Everyone else should ideally sit tight.

Also, it’s really important to remember that vaccines work well and are continuing to keep people out of the hospital. Also keep in mind that the U.S. has plenty of vaccines for boosters and enough for children, if and when they get approved. There’s no need to feel like you won’t be able to get your same shot.

 “I understand people’s anxiety and concern and really wanting to know what to do to be as safe as possible and be protected,” Branche said. “But you know the first rule of medicine is to not give people what they don’t need.”

Experts are still learning about COVID-19. The information in this story is what was known or available as of publication, but guidance can change as scientists discover more about the virus. Please check the Centers for Disease Control and Prevention for the most updated recommendations.