For the first time, we have an updated coronavirus vaccine.

The Food and Drug Administration today authorized the first redesign of Covid vaccines since they were rolled out in late 2020. If all goes as expected, millions of Americans will be able to receive new booster doses targeting Omicron subvariants as early as next week.

The agency cleared two options: one made by Pfizer for use in people as young as 12, and the other by Moderna, for those 18 and older. The doses can be given to those who have completed their initial shots, whether or not they have also received boosters, as long as their last shot was at least two months ago.

The booster campaign comes as the U.S. is preparing for its third pandemic winter, and as it averages about 90,000 infections and 475 deaths every day. For more on the shots and the rollout, I turned to my colleague Sharon LaFraniere, an investigative reporter in the Washington bureau who has been covering the government’s response to the pandemic.

How do the boosters work?

The new shots are called bivalent, because one half is the original shot and one half is a reformulation designed specifically for BA.4 and BA.5, which are the subvariants of Omicron that together account for most cases in the U.S. The theory is that this combination will be more effective than the original formulation.

How well do they work?

Human trials are just getting going, but evidence from trials in mice indicates that this shot may be more protective against the Omicron subvariants than the original shot.

Why are we relying on trials in mice?

We’re in a time crunch. The virus is evolving too fast for scientists to do human trials because by the time they get results, we could have a new variant.

There will be some data from human trials later this year, maybe in November or sooner, but the government wants to get going right now because most Americans have not had a Covid vaccine shot this year, and we know that the protection that the vaccines induce wanes over time.

The F.D.A. is emphasizing that they are not relying simply on the data from the trials in mice. They’re looking at the totality of the evidence, which includes the results of earlier clinical trials. Before Moderna and Pfizer started testing a bivalent that targeted these subvariants, they made bivalents against Beta and Delta and the original Omicron variant and tested them in humans.

The F.D.A.’s critics say that’s just not good enough, that we need more evidence that this particular bivalent will work before rolling it out nationwide. The F.D.A.’s commissioner said today that the proof will be in the pudding. In other words, they’ll have to carefully follow what happens.

Are the new shots safe?

There really isn’t any question that the shots are safe. They’ve been given to hundreds of millions of people. And according to federal officials, this kind of redesign is what we do for the flu vaccine every year. You’re not getting the same flu shot every fall. It’s been rejiggered, without conducting human clinical trials, to meet whatever variant is expected to circulate.

When can I get a shot?

Today the F.D.A. issued emergency use authorization for the new boosters. So now, the federal government can start shipping doses to the states. But the states can’t start administering them until the C.D.C. director signs off. The C.D.C.’s advisory panel is meeting tomorrow and Friday and will make recommendations. If Dr. Rochelle Walensky, the director, signs off on Friday, then doses will be available, if not this weekend, then soon thereafter.

What does the future of the government’s response look like?

This is likely to be the last free vaccine shot that Americans get. Future shots will probably have to be covered by insurance plans. The big question is what the uptake will be for the new boosters. Some state officials are saying that they’ve stopped aggressively pushing coronavirus shots. With each additional shot that has been offered, there have been fewer and fewer takers.