Not so fast: FDA warns of 'premature' changes to COVID-19 vaccine dosing in clash with Slaoui

The FDA warns against changing the dosing levels and schedules of COVID-19 vaccines by Moderna and Pfizer-BioNTech. (FDA)

Amid concerns over limited COVID-19 vaccine supplies, some have proposed tweaking the shots’ dosing to immunize more people. One suggestion came from none other than U.S. vaccine czar Moncef Slaoui, Ph.D. But the FDA’s stepping forward to dismiss the idea—at least for now.

Any changes to currently authorized vaccine dosing regimens pose a “significant risk of placing public health at risk” and undermine “the historic vaccination efforts to protect the population from COVID-19,” FDA Commissioner Stephen Hahn, M.D., and Peter Marks, M.D., Ph.D., head of the agency’s biologics department, said in a statement Monday.

The comment came on the heels of Operation Warp Speed chief Slaoui saying the vaccine task force is working with the FDA and Moderna to potentially reduce the company’s mRNA-1273 dose in half to stretch the supply.

In an interview with CBS’ "Face the Nation," Slaoui said earlier clinical data had shown the Moderna mRNA vaccine could elicit “identical” effective immune response in people aged 18 to 55 at either the authorized dose or half its level. But the half-dose regimen was not tested in the shot's phase 3 efficacy trial.

The FDA’s statement also came as the U.K. government has extended dosing intervals for Pfizer and BioNTech’s COVID-19 shot and AstraZeneca’s AZD1222 from the trial-validated three or four weeks to up to 12 weeks. The country’s chief medical officers argued that doing so can enable more vaccinations during the early days of the rollout so that more people can quickly get some initial protection.

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But the FDA said available data at this point don’t support those changes. Tweaking the dosing now is “premature and not rooted solidly in the available evidence,” Hahn and Marks said in their statement.

FDA briefing documents prepared for the evaluation of the Moderna and Pfizer-BioNTech shots noted “some protection against COVID-19 disease following one dose.” For example, Pfizer-BioNTech’s BNT162b2 posted an estimated efficacy of 52.4% between the two doses in its phase 3 trial. The findings have led to some suggesting just one dose would be enough.

But the FDA said the first-dose data are “commonly being misinterpreted.” The data were generated from a very short follow-up period, “such that we cannot conclude anything definitive about the depth or duration of protection,” Hahn and Marks said.

In fact, the FDA already made that point clear in its briefing documents. For BNT162b2, for example, it stressed that the post hoc analysis “cannot support a conclusion on the efficacy of a single dose of the vaccine, because the time of observation is limited by the fact that most of the participants received a second dose after three weeks.” Translation: No information is available about protection beyond the three-week interval for the single-dose idea.

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As for AstraZeneca’s AZD1222, which hasn’t yet been authorized in the U.S., the vaccine was tested in two doses given 28 days apart in its phase 3 trial. But the British drugmaker said preliminary data showed an increased immune response for up to 12 weeks after the first dose. The company intends to publish updated data from the ongoing trials.

While the FDA insists on administering two full doses of the vaccines at their authorized dosing window at this point, the agency seems open to persuasion by data and science.

“Until vaccine manufacturers have data and science supporting a change, we continue to strongly recommend that healthcare providers follow the FDA-authorized dosing schedule for each COVID-19 vaccine,” Hahn and Marks said.