When Johnson & Johnson’s coronavirus vaccine won emergency use authorization on Saturday from the Food and Drug Administration, the move augmented the nation’s vaccination effort with a third major tool — one that differs markedly from the first two authorized vaccines, made by Pfizer-BioNTech and Moderna.

Most notably, it is administered in a single dose instead of two, and can be kept unfrozen in an ordinary refrigerator for up to three months — features that promise greater flexibility as public health officials try to immunize Americans as quickly as possible.

Much is still to be determined about how this new tool will be used. Here is what we know so far.

When will people start getting the new vaccine?

Within the next few days. Johnson & Johnson started shipping out doses on Monday, and they can be used as soon as they reach vaccination sites starting on Tuesday.

Will adding the new supply speed up vaccination efforts?

At first, the increase in availability will be limited. The company had about 3.9 million doses on hand to ship right away, but after that, deliveries could be patchy for a few weeks. (For comparison, the nation is using up that many doses of the Pfizer-BioNTech and Moderna vaccines in a little more than two days.)

By the end of March, Johnson & Johnson says it will ship roughly 16 million more doses. Even so, the Pfizer-BioNTech and Moderna vaccines will continue to make up the majority of the nation’s supply.

How is the new vaccine being allocated?

The same way the two earlier vaccines are: in proportion to each state or territory’s population.

Who will get the new vaccine?

That’s still under discussion. The Centers for Disease Control and Prevention has said that the vaccine can be given to people 18 and over, and state officials are working out what their policies will be.

Because the new vaccine is given in a single shot and doesn’t require cold storage, some experts and officials have suggested directing it toward hard-to-reach segments of the population (like rural residents or homeless people), or to people who might not keep an appointment for a second shot (like college students or those with mobility issues).

But there is concern about appearing to favor or disfavor some groups, and the Biden administration has said it will insist that the new vaccine be distributed equitably.

Will I be able to choose which vaccine I receive?

That’s not clear. Right now, people are getting whichever vaccine the site has on hand when their turn comes, and appointment scheduling systems generally don’t tell users beforehand which it will be. Depending on how states decide to deploy the Johnson & Johnson vaccine, though, it may be possible to effectively choose what you get by choosing where you sign up to get it.

Which vaccine should I prefer?

Health experts say the best shot is the one you can get the soonest, whichever one it turns out to be. All three authorized vaccines are highly protective, and the differences among them pale in comparison, they say, with the risk you would run by being picky and passing up a chance to get a shot because it was not your top choice.