Johnson & Johnson’s single-dose vaccine protects against the coronavirus disease 2019 (Covid-19), according to an analysis by US regulators Wednesday that sets the stage for a final decision on a new and easier-to-use shot to help tame the pandemic.

The Food and Drug Administration’s scientists confirmed that overall the vaccine is about 66-percent effective at preventing moderate to severe Covid-19. The agency also said J&J’s shot — one that could help speed vaccinations by requiring just one dose instead of two — is safe to use.

That’s just one step in the FDA’s evaluation of a third vaccine option for the US. On Friday, the agency’s independent advisers will debate if the evidence is strong enough to recommend the long-anticipated shot. Armed with that advice, FDA is expected to make a final decision within days.

The vaccination drive has been slower than hoped, hampered by logistical issues and weather delays even as the country mourns more than 500,000 virus-related deaths. So far, about 44.5 million Americans have received at least one dose of vaccine made by Pfizer or Moderna, and nearly 20 million have received the second dose required for full protection.

J&J tested its single-dose option in 44,000 people in the US, Latin America and South Africa. Different mutated versions of the virus are circulating in different countries, and the FDA analysis cautioned that it’s not clear how well the vaccine works against each variant. But J&J previously announced the vaccine worked better in the U.S. – 72-percent effective against moderate to severe Covid-19, compared with 66 percent in Latin America and 57 percent in South Africa.

Still, in every country it was highly effective against the most serious symptoms, and early study results showed no hospitalizations or deaths starting 28 days after vaccination.

While the overall effectiveness numbers may suggest the J&J candidate isn’t quite as strong as two-dose competitors, all of the world’s Covid-19 vaccines have been tested differently, making comparisons nearly impossible. While it wouldn’t be surprising if one dose turns out to be a little weaker than two doses, policymakers will decide if that’s an acceptable trade-off to get more people vaccinated faster.

J&J was on track to become the world’s first one-dose option until earlier this month, Mexico announced it would use a one-dose version from China’s CanSino. That vaccine is made with similar technology as J&J’s but initially was developed as a two-dose option until beginning a one-dose test in the fall.

The rival Pfizer and Moderna vaccines being used in the US and numerous other countries must be kept frozen, while the J&J shot can last three months in the refrigerator, making it easier to handle. AstraZeneca’s vaccine, widely used in Europe, Britain and Israel, is made similarly and also requires refrigeration but takes two doses.