Asked at a news briefing on Monday afternoon how much the variants would need to spread before updated vaccines were necessary, Dr. Woodcock did not give any specific criteria. “We need to anticipate this and work on it so that we have something in our back pocket before the threshold is upon us,” she said.

An updated Covid-19 vaccine can skip the monthslong process of a randomized clinical trial that would compare it with a placebo, the agency said. But a tweaked vaccine will still need to go undergo some testing. In trials proposed by the F.D.A., researchers will draw blood from a relatively small group of volunteers who have been given the adapted vaccine. Scientists will then observe what percentage of volunteers’ samples produce an immune response to the variants in the lab, and how large that response is. The vaccines will be judged acceptable if they produce an immune response that is relatively close to the one prompted by the original vaccines.

Dr. Peter Marks, the top vaccine regulator at the F.D.A., said at the news briefing that studies would include a “few hundred” people and last several months.

Volunteers will also be monitored carefully for side effects. The agency said the testing could be done in a single age group and then extrapolated to other age groups.

The guidance also encouraged the use of animal studies to support the case for modified vaccines, in case immune response studies come up with ambiguous conclusions.

The F.D.A. acknowledged that many questions remained unanswered, such as what type of data would trigger the need for an adapted vaccine and who would make that decision. The agency also noted that scientists had not yet determined the minimum level of antibodies in a vaccinated person’s blood that would protect someone from the virus.

Some other vaccines are regularly updated in a similar way. Because the influenza virus evolves rapidly from one year to the next, vaccine developers have to come up with new recipes annually.



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