The United States is poised to get a third vaccine for the coronavirus disease 2019 (Covid-19), but because at first glance the Johnson & Johnson shot may not be seen as equal to other options, health officials are girding for the question: Which one is best?

If cleared for emergency use, the J&J vaccine would offer a one-dose option that could help speed vaccinations, tamp down a pandemic that has killed more than 500,000 people in the US and stay ahead of a mutating virus.

“I think it’s going to be huge,” said Dr. Virginia Caine, director of the public health department in Marion County, Indiana, which includes Indianapolis. She expects the easier-to-use vaccine will give local officials more flexibility for mobile vaccination clinics or pop-up events.

The challenge will be explaining how protective the J&J shot is after the astounding success of the first US vaccines.

Two doses of the Pfizer and Moderna shots were found to be about 95-percent effective against symptomatic Covid-19. The numbers from J&J’s study are not that high, but it’s not an apples-to-apples comparison. One dose of the J&J vaccine was 85-percent protective against the most severe Covid-19. After adding in moderate cases, the total effectiveness dropped to about 66 percent.

“I don’t think it’s a second-tier vaccine, but we’ve got to avoid that perception,” said Dr. Thomas Balcezak of Yale New Haven Health System.

The J&J shot was tested in the US, Latin America and South Africa at a time when more contagious mutated versions of the virus were spreading. That wasn’t the case last fall, when Pfizer and Moderna were wrapping up testing, and it’s not clear if their numbers would hold against the most worrisome of those variants.

Importantly, the Food and Drug Administration reported this week that, just like its predecessors, the J&J shot offers strong protection against the worst outcomes. By 28 days after the injection, there were no hospitalizations or deaths in study volunteers given the J&J shot, compared with 16 hospitalizations and seven deaths in those given a dummy shot.