The recommendations, which call for small trials more like those required for annual flu vaccines, would greatly accelerate the review process at a time when scientists are increasingly anxious about how the variants might slow or reverse progress made against the virus…

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.