One shot protects individuals from the virus, according to analysis by US regulators 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 (FDA) scientists confirmed that overall the vaccine is about 66% 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 is 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 this advice, the FDA is expected to make a final decision within days.
The vaccination drive in America 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 65 million Americans have received at least one dose of vaccine made by Pfizer or Moderna, shots that require two doses several weeks apart for full protection.
J&J previously announced the vaccine worked better in the US - 72% effective against moderate to severe Covid-19, compared with 66% in Latin America and 57% in South Africa.
Still, in every country it was highly effective against the most serious symptoms, and early study results showed no deaths starting 28 days after vaccination.