This story has been updated.

Saturday’s authorization of a third coronavirus vaccine has raised a new question for many Americans: Which shot should I get?

The answer, experts say, is whichever one you can.

The new Johnson & Johnson vaccine joins the Pfizer-BioNTech and Moderna vaccines, which are already being distributed nationwide. Food and Drug Administration reviews have found all three vaccines to be highly effective at preventing hospitalizations and deaths from covid-19. One of the main differences between them is that the Pfizer-BioNTech and Moderna vaccines require two shots given several weeks apart, while the Johnson & Johnson vaccine is a single shot, which might be more appealing to some people.

The FDA’s review of Johnson & Johnson’s vaccine found that it was more than 80 percent effective at preventing severe illness in a large international clinical trial. But the review noted that the vaccine had lower efficacy among higher-risk older adults in the trial, a finding that experts hope will not discourage some people from taking it.

At this point in the vaccines’ rollout, it’s difficult in many parts of the country to get any vaccine, much less a particular brand. And even if people could pick and choose now, experts say, that would probably hinder the goal of mass vaccination — which is to protect as many lives as quickly as possible.

“What we really are hoping for and what we want out of these vaccines is that we keep people out of the hospital and we keep people from dying,” said Joshua Barocas, an infectious-disease physician at Boston Medical Center. And, he added, of the Johnson & Johnson trial, “when we look at that data, that data is still robust.”

Although the Johnson & Johnson vaccine was 42 percent effective at preventing moderate to severe illness among the group of older adults with comorbidities, the FDA emphasized that the statistical significance was uncertain and that none of those older adults died or needed to be hospitalized in the month after they were vaccinated. In fact, the vaccine completely prevented hospitalization and deaths among all trial participants who received the shot, providing a level of protection against serious illness that experts say is comparable to the Pfizer-BioNTech and Moderna vaccines, which were shown to be more than 90 percent effective in trials.

The lower efficacy rate of the Johnson & Johnson vaccine in the older subgroup “requires further evaluation going forward,” said William Moss, executive director of the International Vaccine Access Center at the Johns Hopkins Bloomberg School of Public Health. But he, too, urged people to get whichever vaccine is available to them.

“Although cases, hospitalizations and deaths are coming down, there’s still a lot of community transmission in this country,” Moss said. “Until we really get that down, it’s really important that people get protected as early as possible.”

The more people who get vaccinated, “the lower everybody’s risk becomes, regardless of which vaccine you get,” Barocas said.

If many Americans opted to wait for a preferred vaccine, it would not only leave more people at risk of developing a serious case of covid, but it would also probably slow down the country’s vaccination program, which only recently started gaining steam after a tepid start.

“Most vaccine sites administer one [type of] vaccine on a given day,” Manisha Juthani, an infectious-disease specialist at Yale Medicine, wrote in an email. “If you go to a site and they are not administering a vaccine that you want, you will miss your opportunity and have to reschedule while the same thing could happen on another day.” Furthermore, you could be denying another person the opportunity to be vaccinated.

Given the current rates of vaccine development and production, experts estimate that most adults who want to be vaccinated should be able to do so by the summer, said Marc Siegel, an associate professor of medicine in the Division of Infectious Diseases at George Washington University.

And as more vaccines emerge and more research is done, recommendations regarding which vaccines are best for certain populations may be released. With time, Siegel said, there will probably be clear public guidance from the Advisory Committee on Immunization Practices, an independent panel that advises the Centers for Disease Control and Prevention on the use of vaccines.

But until then, Barocas urged people to focus not on which vaccination you want, but on why you want one. And the answer, he said, should be that you want a vaccination because you don’t want to be hospitalized or die.

“Regardless of your risk group, how you slice it, that’s what these vaccines are doing,” he said. “They’re keeping you out of the hospital and keeping you alive.”