Is it safe to mix and match COVID vaccines after your first dose? Experts weigh in

Katie Camero
·3 min read

Both coronavirus vaccines authorized for emergency use in the U.S. require two doses to offer maximum protection against COVID-19, but that doesn’t mean you can mix and match which ones you get, health experts say.

Like past pandemics have shown, one of the fastest ways to slow the spread of a highly contagious virus is to inoculate as many people as possible as quickly as the manufacturing progress allows.

But in the U.S. and United Kingdom, where vaccination efforts are stumbling over limited supplies, hesitancy among potential recipients and disorganization among hospital systems, local and state officials and federal leaders, desperation has fueled debates over new tactics that might protect more people in a shorter amount of time.

Meanwhile, health experts say it’s not known if mixing the two vaccines is safe or effective.

“I understand that thinking as a sort of desperation measure, but it’s not the same thing as evidence-based dosing,” Dr. Greg Poland, director of Mayo Clinic’s Vaccine Research Group, told Minnesota’s KARE 11.

“The way we decide whether a vaccine is safe and effective is to subject it to clinical trials. Those clinical trials in the case of both the Moderna and the Pfizer vaccine suggested that we need two doses... those trials only included two doses of the same vaccine. Once you depart from that, then we can no longer be confident about the safety, the efficacy or even the durability,” Poland said.

And such a move during the ongoing pandemic would likely delay the administration of second doses — another unknown regarding how plays on timing might affect a COVID-19 vaccine’s ability to offer full protection.

The Centers for Disease Control and Prevention says both the authorized COVID-19 vaccines from Pfizer-BioNTech and Moderna “are not interchangeable with each other or with other [coronavirus] vaccine products.”

That’s because, as Poland explained, the mixing of vaccines has not been tested in clinical trials, although experts say those studies are under way.

The agency does list “exceptional situations” that could warrant a vaccine cocktail, however.

In cases where the vaccine product used for someone’s first dose is no longer available due to supply shortages or cannot be determined because the product is unknown, “any available mRNA COVID-19 vaccine may be administered,” the CDC says.

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Health officials in the U.K. mentioned similar exceptions, saying “it is reasonable to offer one dose of the locally available product to complete the schedule” in the event the first-dose vaccine is not available or is unknown.

They said the option is justifiable if a person faces an immediate high risk of coronavirus infection, given a second shot is likely to “help boost the response to the first dose.”

But exceptions are a last resort, according to the head of immunizations at Public Health England Mary Ramsay.

“Every effort should be made to give [patients] the same vaccine, but where this is not possible it is better to give a second dose of another vaccine than not at all,” Ramsay told the British Medical Journal.

Yet, the U.S. Food and Drug Administration, the group responsible for determining if the COVID-19 vaccine candidates are safe, stands on firmer ground.

On Jan. 4, the former FDA Commissioner of Food and Drugs Dr. Stephen Hahn issued a statement citing the lack of scientific evidence behind vaccination tactics that have not been tested in clinical trials.

“These are all reasonable questions to consider and evaluate in clinical trials. However, at this time, suggesting changes to the FDA-authorized dosing or schedules of these vaccines is premature and not rooted solidly in the available evidence,” Hahn wrote.

“Without appropriate data supporting such changes in vaccine administration, we run a significant risk of placing public health at risk, undermining the historic vaccination efforts to protect the population from COVID-19,” he added.