The Covid-19 vaccine developed by Janssen is 66% effective, the Belgian company has announced.
But nobody needed hospital treatment or died from coronavirus after the vaccine took effect in the international trial.
Crucially, it looked at giving just one dose of the vaccine, which makes it significantly easier to roll out than those requiring two.
Although there are also signs it is less effective against the new variant that is spreading in South Africa.
The news comes shortly after Novavax announced their jab was 89% effective– and both will need to be reviewed by regulators before they can be used.
Janssen, a pharmaceutical company owned by Johnson & Johnson, is also investigating whether giving two doses will give either stronger or longer-lasting protection.
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
The company is aiming to make one billion doses this year.
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.
Dr Mathai Mammen, from Janssen, said: “A single-dose regimen with fast onset of protection and ease of delivery and storage provides a potential solution to reaching as many people as possible.
The ability to avoid hospitalisations and deaths would change the game in combating the pandemic.”
However the vaccine was just 57% effective in the South African part of the trial, where a new version of the coronavirus is spreading.
The US has ordered 100m doses of the Janssen vaccine and Canada 38m, while the UK has already pre-ordered 30m doses.
The health secretary, Matt Hancock, said: “This is yet more good news from Janssen on vaccines.
“If this jab is approved this could significantly bolster our vaccination programme, especially as a single-dose vaccine.
“Once the full data has been submitted [to the Medicines & Healthcare products Regulatory Agency], they will consider the evidence to determine whether the vaccine meets robust standards of safety, effectiveness & quality.”