Johnson & Johnson's long-awaited vaccine appears to protect against Covid-19 with just one shot - but not as strong as some two-shot rivals.
J&J said on Friday that in the US and seven other countries, the single-shot vaccine was 66% effective overall at preventing moderate to severe illness, and much more protective - 85% - against the most serious symptoms.
There was some geographic variation.
The vaccine worked better in the US - 72% effective against moderate to severe Covid-19 - compared with 57% in South Africa, where it was up against an easier-to-spread mutated virus.
"Gambling on one dose was certainly worthwhile," Dr Mathai Mammen, global research chief for J&J's Janssen Pharmaceutical unit, told The Associated Press.
The company said within a week, it will file an application for emergency use in the US, and then abroad.
It expects to supply 100 million doses to the US by June, and expects to have some ready to ship as soon as authorities give the green light.
These are preliminary findings from a study of 44,000 volunteers that is not completed yet.
Researchers tracked illnesses starting 28 days after vaccination - about the time when, if participants were getting a two-dose variety instead, they would have needed another shot.
After day 28, no-one who got vaccinated needed to be admitted to hospital or died regardless of whether they were exposed to "regular Covid or these particularly nasty variants," Dr Mammen said. When the vaccinated did become infected, they had a milder illness.
Defeating the scourge that has killed more than two million people worldwide will require vaccinating billions, and the shots being rolled out in different countries so far all require two doses a few weeks apart for full protection.
Early data is mixed on exactly how well all the different kinds work, but shots made by Pfizer and Moderna appear to be about 95% protective after the second dose.
But amid shortages, some countries have advised delaying the second dose of certain vaccines with little data on how that would affect protection.