Jan. 5, 2021 -- Both doses of the Pfizer-BioNTech and Moderna COVID-19 vaccines are needed to reach the proper level of protection against the coronavirus, FDA officials said on Monday.

Although some federal officials have discussed the possibility of stretching the vaccine supply by giving the first dose to more people, the FDA officials emphasized that people who get a vaccine must receive two doses to build immunity to COVID-19 and to make sure the shots work as well as they did in clinical trials.

“We have been following the discussions and news reports about reducing the number of doses, extending the length of time between doses, changing the dose (half-dose), or mixing and matching vaccines in order to immunize more people against COVID-19,” FDA Commissioner Stephen Hahn, MD, and Peter Marks, MD, the FDA’s vaccine division leader, said in a statement.

“These are all reasonable questions to consider and evaluate in clinical trials,” they wrote. “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.”

British officials have said they will allow more than 21 days between doses for the Pfizer vaccine, according to CNN. The U.S. government is considering cutting doses of the two-shot Moderna coronavirus vaccine in half to try to double the number of vaccinations, Moncef Slaoui, PhD, the scientific head of Operation Warp Speed, said Sunday on CBS’s Face the Nation.

Slaoui said research shows that two 50-microgram doses of the Moderna vaccine have the same result in people 18-55 years old as two 100-microgram doses.

He said giving the smaller doses “means exactly achieving the objective of immunizing double the number of people with the doses we have. We know it induces identical immune response to the 100-microgram dose, and therefore we are in discussion with Moderna and with the FDA.”

But Hahn and Marks said the U.S. should stick to the two-dose plan.

Based on the clinical trial data, the Pfizer-BioNTech vaccine doses should be given 21 days apart, and the Moderna vaccine doses should be given 28 days apart. During the clinical trials, 98% of the Pfizer participants received two doses, and 92% of the Moderna participants received two doses. The participants who didn’t receive two doses were followed for a short period of time, so scientists can’t draw any conclusions about the amount or length of immunity that one dose gives.

“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,” Hahn and Marks said.

“We have committed time and time again to make decisions based on data and science,” they wrote. “Until vaccine manufacturers have data and science supporting a change, we continue to strongly recommend that health care providers follow the FDA-authorized dosing schedule for each COVID-19 vaccine.”

Public health officials who served on the FDA’s independent vaccine advisory committee have also dismissed the idea of extending the dose schedules or cutting them in half.

“There’s no data on efficacy of a half dose. If you use a half dose, you’re just making it up. You’re just hoping that you’re right,” Paul Offit, MD, an infectious disease specialist at the University of Pennsylvania and a member of the advisory committee, told CNN.

“Why would you dare to make up something when you don’t know whether or not it works?”

Show Sources

FDA: “FDA Statement on Following the Authorized Dosing Schedules for COVID-19 Vaccines.”

CNN: “FDA says people need both doses of coronavirus vaccines.”

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