Dive Brief:

• The Food and Drug Administration won’t recommend altering the dose regimens of the two coronavirus vaccines currently authorized for emergency use in the U.S. without new clinical data, the agency said late Monday.

• In an unusual statement, commissioner Stephen Hahn and top agency official Peter Marks warned that changing the way the vaccines are used could put “public health at risk” because those immunized may falsely think they’re protected from COVID-19. The vaccines from Moderna as well as partners Pfizer and BioNTech were proven to be 95% protective against symptomatic COVID-19 after two shots given a few weeks apart.

• Hahn and Marks’ opposition comes after the leader of the White House’s Operation Warp Speed effort, Moncef Slaoui, suggested using half-dosesof Moderna’s vaccine, citing study results indicating the immune response generated appeared equally strong in adults 18 to 55 years of age given doses half as strong as the one authorized.

Dive Insight:

The 10-month sprint from discovery to authorization for Moderna’s and Pfizer’s vaccines was the fastest effort of its kind, a scientific milestone that put the pandemic’s end in sight. But having an effective vaccine is only a prelude to having people vaccinated, a task that’s quickly proving difficult in the U.S. and elsewhere.

In the U.S., early rollout of the two shots has fallen well short of early projections from the Trump administration, which had hoped to vaccinate 20 million people by the end of December. More than two-thirds of the roughly 15 million doses that have been distributed across the country remain unused, according to data from the Centers for Disease Control and Prevention.

Hospitalizations from COVID-19, meanwhile, continue to set new highs. And a new, apparently more contagious coronavirus variant that quickly spread in the U.K. has been found in the U.S. and dozens of other countries.

The worrisome news has heightened the pressure to maximize the currently limited supply of vaccines by stretching dosing regimens beyond what was tested in clinical trials. Several prominent health leaders have argued the dosing schedule for each shot could be changed in an effort to vaccinate the public faster. Ideas floated include delaying a second dose, relying primarily on one dose or mixing and matching vaccines.

The U.K., which entered into another national lockdown Monday, adopted such a strategy last week when it approved a vaccine developed by AstraZeneca and the University of Oxford. The country’s drug regulator said the second shot of both that vaccine and Pfizer’s would be administered closer to 12 weeks after the first, an effort to maximize the number of people inoculated right away.

The decision is, essentially, an experiment. Neither of those vaccines were explictly tested that way, although some participants in AstraZeneca’s trials were given shots spaced as many as 26 weeks apart. Pfizer has cautioned that it has no data supporting the protection of a single shot after 21 days.

The U.K.’s decision, though, should mean more people in the country are vaccinated sooner, compared to countries like the U.S. that are holding half of available doses in reserve for the second shot. But some experts worry protection may not be as high as the dosing regimens tested, while others have warned a partial immune response could help new vaccine-resistant coronavirus variants to emerge.

In a statement Monday, Hahn and Marks called any suggested changes to FDA-authorized dosing or schedules “premature and not rooted solidly in the available evidence.”

Few trial participants received the two vaccines off schedule, they said, and those participants that did were generally only followed for a short period of time, making it impossible to “conclude anything definitive about the depth or duration of protection.”

“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,” wrote Hahn and Marks.

The regulator won’t recommend any changes until vaccine manufacturers “have data and science supporting a change,” they said.

Neither Moderna nor Pfizer and BioNTech have done any rigorous testing comparing one- and two-dose regimens.