“I would lean toward thinking BA.4, BA.5 is a good choice unless it dramatically extends the timeline,” said Jesse Bloom, a virologist at the Fred Hutchinson Cancer Center in Seattle, voicing support for the subvariant vaccine. “If using BA.4, BA.5 only modestly extends the timeline, I think it’s a good choice.”

The updated shots will test the public’s openness to an accelerated vaccine program that is reminiscent of the way annual flu shots are formulated, but that is entirely new when it comes to the coronavirus.

The original Covid vaccines had to withstand slow and laborious testing: Volunteers took the shots and then went about their lives while researchers tracked who got sick. But there is now ample evidence that the shots are safe. And any tweaks to the recipe could be wasted if scientists were to spend the better part of a year testing them.

Instead, vaccine manufacturers have been studying volunteers’ blood samples in the lab to gauge their immune responses to a booster that is tailored to the first version of Omicron.