At a separate vaccine-focused event on Monday, sponsored by Research!America, Peter Marks, MD, director of the FDA’s Center for Biologics Evaluation and Research, answered questions about the two-dose requirement and anaphylactic reactions with the Pfizer/BioNTech vaccine observed in the U.K.
Regarding the dosing regimen, Marks said the FDA and Pfizer/BioNTech spent a lot of time “carefully evaluating a regimen based on good preclinical science — that’s animal studies — and early studies in humans to come up with this two-dose regimen in the case of the Pfizer vaccine.”
That regimen saw a level of protection that was about 95% effective, he said. While some people assume that there is some efficacy after only one dose, researchers only truly know the efficacy with two doses because that was what was tested in the trial, Marks said. “We can estimate that if you didn’t get a second dose, maybe, is it 50% effective? We don’t know for sure, and we don’t know how long it can be effective for.” It could happen that an immune response could occur and then “dwindle,” he said. “[W]e already know people who get very mild COVID-19 tend to lose their immune responses pretty quickly…. I know it’s very tempting to want to vaccinate more people and somehow spread the wealth, but sometimes you have to do the job right rather than just spread the wealth around.”
As for the anaphylactic reactions seen overseas, Marks said that that particular reaction is one “we don’t yet know what to make of.” Polyethylene glycol is the most likely component that could trigger reactions, he said. However, “we’re not going to say that if you ever had a severe reaction, you shouldn’t get the vaccine, because that would exclude 1.5% or more of the American population; instead, as a precaution, we’re telling all sites that administer the vaccine that they need to be ready to treat an allergic reaction,” Marks said.
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