The one-dose jab from Johnson & Johnson is less effective than other rivals.
Johnson & Johnson's coronavirus vaccine, of which the EU plans to buy up to 400 million doses, is around 66% effective in the global trial, the company has said.
The company said the vaccine, which only requires one jab, was 85% effective overall in preventing severe disease.
It also demonstrated complete protection against hospitalisation and death, the company said.
The efficacy results of the vaccine, which was developed at J&J's Janssen Pharmaceutical Companies, were based on 43,783 trial participants.
Most of the vaccines in use worldwide at present require two doses, usually administered 4-6 weeks apart.
But single-dose vaccines are significantly easier to roll out to populations, particularly in territories where it can be a challenge to trace people who have not had their second vaccine.
There were 468 symptomatic cases of COVID-19 amid participants in the J&J study.
The shot uses a cold virus like a Trojan horse to carry the spike gene into the body, where cells make harmless copies of the protein to prime the immune system in case the real virus comes along.
The vaccine developed by AstraZeneca and Oxford University works in similar fashion, but requires two doses.
Both the AstraZeneca and J&J vaccines can be stored in a regular refrigerator, making them easier to transport and to use in developing countries.
The frozen kind made by Pfizer has to be stored at temperatures as low as -70C.
There was a difference of protection depending on the location: The jab was 72% effective in the United States, compared with 66% in Latin America and 57% in South Africa.
“Gambling on one dose was certainly worthwhile,” Dr Mathai Mammen, global research chief for J&J’s Janssen Pharmaceutical unit, told The Associated Press.
It came after Novavax said its vaccine was shown to be 89.3% effective in preventing coronavirus in its UK trial.