Data released Tuesday from a late-stage clinical trial showed that study participants in 10 countries including the U.S. who received a second dose of the company’s vaccine two months after the first had 75% protection against symptomatic Covid-19. Participants in the U.S. had 94% protection against the illness. J&J didn’t explain the reason for the difference in efficacy rates.

A double dose of the vaccine provided participants with 100% protection against severe or critical Covid-19 at least two weeks after the second shot, J&J said.

The company earlier this year said a large clinical trial showed that a single dose of its vaccine was 66% effective at protecting people from moderate to severe Covid-19.

“We now have generated evidence that a booster shot further increases protection against Covid-19 and is expected to extend the duration of protection significantly,” said Paul Stoffels, chief scientific officer at J&J. The company said it has shared available data with the Food and Drug Administration.

J&J Says Covid-19 Vaccine Booster Two Months After First Shot Increases Protection – WSJ