The Food and Drug Administration appears to have rejected any alternative strategy for the distribution of covid-19 vaccines—for now, at least. In a statement released Tuesday, FDA chief Stephen Hahn called the possibility of any dosing or scheduling changes “premature” and not supported by the current evidence. At the same time, he did welcome clinical trials to test out these plans, which include giving people a smaller amount of vaccine per dose to stretch out the limited supply.
There have been several proposed strategies for making the most of the limited supply of vaccine available. These include giving only one dose of the Moderna and Pfizer/BioNTech vaccines (which both involve two doses, given about a month apart) or delaying the second dose to allow more people to get a first dose. Another plan calls for mixing and matching the similar Moderna and Pfizer vaccines if need be, such that one person could receive a first dose of Pfizer and a second dose of Moderna or vice versa. And the latest idea—floated by U.S. health officials this past weekend—would be to give a half-dose of the Moderna vaccine for both shots (so people would receive 50 milligrams per dose instead of 100 milligrams).
The UK’s health regulators have now allowed doctors to give a delayed second dose of the Pfizer vaccine, up to 12 weeks after the first dose. It’s also allowed for patients to receive mixed doses, though this would likely be used only in rare cases where people simply don’t remember which vaccine they received first. The U.S. won’t be following suit anytime soon, it seems.
In his statement released Tuesday, Hahn agreed that these ideas are reasonably worth exploring in clinical trials, as many experts have argued—but stopped short of endorsing them right now.
“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,” he said. “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.”
In discussing the strategies, Hahn reiterated the lack of evidence for their viability. The possibility that half-doses might work nearly as well as a full dose, for instance, is based on clinical trial results from around 200 people. And giving people a single full dose might provide as much protection from illness initially, but scientists have worried that without the second dose, immunity could drop off much sooner. Hahn also argued that introducing these changes mid-steam would likely only undermine public health if they turn out to be misguided.
Pfizer, for its part, has not supported the use of delayed doses in the UK, and with the FDA’s statement, it’s unlikely that Moderna would support a half-dose strategy either. There may yet be small clinical trials testing out these ideas, but Americans will presumably be getting full doses of both for the foreseeable future. The bigger problem right now is getting all the doses we do have to as many people as possible.