The FDA urges healthcare providers not to mix and match vaccines, halve doses, or extend time between shots

elderly woman vaccine
A medical worker gives an elderly woman an influenza vaccine in Zoetermeer, Netherlands.

  • As the coronavirus vaccine rollout stalls in the US, some public health and scientific leaders are suggesting altering the shots’ administration in order to protect more people faster. 
  • But the FDA warned healthcare providers not to halve doses, mix and match shots, or widen the window between the doses’ delivery. 
  • Until we know how protective a single shot is and how long that protection might last, “making such changes that are not supported by adequate scientific evidence may ultimately be counterproductive to public health,” the agency said. 
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By the morning of January 4, only 2.8 million Americans had received their first dose of the coronavirus vaccine, despite the Trump administration’s goal of 20 million vaccinated in 2020. Meanwhile, an American dies of COVID-19 every 33 seconds

The dire situation has led some healthcare leaders to consider bending the vaccine administration protocol in order to, in theory, protect more people quicker. 

But on Monday, the US Food and Drug Administration tried to quash such attempts in the US, issuing a statement urging healthcare providers to administer coronavirus vaccines as they’ve been studied – that is, in two complete doses, made by the same company, and given three weeks apart for the Pfizer vaccine or four weeks apart for Moderna’s. 

The chief of Operation Warp Speed mulled giving out half doses, and the UK has discussed mixing vaccines

Moncef Slaoui, chief of Operation Warp Speed, said on Sunday that officials may give out half doses of Moderna’s COVID-19 shot in order to double the number of people immunized. 

There have also been contested discussions in the UK about mixing vaccines – that is, giving someone a first dose of the Pfizer vaccine and a second dose of Moderna’s, or vice versa, depending on availability. And, the UK’s health service has widened the window between recommended doses in an effort to maximize the number of people with some protection. 

In its statement, the FDA said whether such tweaks work are “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. 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.” 

The Infectious Diseases Society of America concurred, issuing a statement Tuesday in support of FDA’s stance. 

Changes to the vaccination schedule “carry the risk of both undermining the effectiveness of vaccines and eroding the public trust necessary for successful vaccine uptake,” it says. “As always, our approach against this pandemic must be founded in science, leadership, funding, collaboration and cooperation.” 

We don’t know for sure how protective a single dose is or how long protection from it lasts 

Some scientists have looked at the evidence and concluded that a single dose should provide some protection for at least a month beginning two weeks after vaccination.

However, experts say it would be a mistake to hope for full protection after one shot, when the vaccines currently available globally (from Pfizer, Moderna, and AstraZeneca) are all designed to be given in two doses.

The FDA said we can’t know much about how long a single dose is protective, or how strong that protection might be, since those questions weren’t studied and their answers may be related.

Until we better understand those questions, dolling out single or meager doses “is concerning,” the FDA said. 

“If people do not truly know how protective a vaccine is, there is the potential for harm because they may assume that they are fully protected when they are not, and accordingly, alter their behavior to take unnecessary risks.” 

The agency acknowledged that suggested tweaks to the vaccines’ delivery are well-intentioned, but said, “making such changes that are not supported by adequate scientific evidence may ultimately be counterproductive to public health.” 

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