The Janssen-developed Covid-19 vaccine is 66 percent successful, the Belgian company reported.
The international trial looked at delivering just one dose of the vaccine, making it much simpler to roll out than those needing two, reports the BBC
The UK has already pre-ordered 30m doses.
The news comes shortly after Novavax reported that their jab was productive at 89 percent. Before they can be included, both would need to be updated by regulators.
Janssen, a Johnson & Johnson pharmaceutical corporation, is also researching whether giving two doses would provide either better or longer-lasting safety.
The company said its initial findings showed one dose prevented 85% of severe cases.
Dr Paul Stoffels, the chief scientific officer at Johnson & Johnson, said that would "potentially protect hundreds of millions of people from serious and fatal outcomes of Covid-19".
The Janssen vaccine uses a common cold virus that has been engineered to make it harmless.
It then safely carries part of the coronavirus's genetic code into the body. This is enough for the body to recognise the threat and then learn to fight coronavirus.
This trains the body's immune system to fight coronavirus when it encounters the virus for real.
This is similar to the approach used by the University of Oxford and AstraZeneca.