The US government is backing J&J with $456m (£350m) in funding in the hope of speeding up production of a vaccine to end the pandemic.
The vaccine uses a common cold virus known as adenovirus type 26 or Ad26 to ferry coronavirus proteins into cells in the body, causing the body to mount an immune defence.
Dr Paul Stoffels, J&J’s chief scientific officer, told Reuters: “This gives us confidence that we can test a single-shot vaccine in this epidemic and learn whether it has a protective effect in humans.”
Johnson & Johnson has started human safety trials for its Covid-19 vaccine after releasing details of a study in monkeys that showed its best-performing vaccine candidate offered strong protection in a single dose.
When exposed to the virus, all six animals who got the candidate vaccine were completely protected from lung disease and five out of six were protected from infection as measured by the presence of virus in nasal swabs, according to the study published in the journal Nature.
Dr Stoffels said that previous tests of this type of vaccine in other diseases found that a second shot significantly increases protection. But in a pandemic a single-shot vaccine has a significant advantage, sidestepping a lot of the logistical issues involved in getting people to come back for their second dose.
The company plans to take up the question of one or two doses in its phase 1 trial, which kicked off this week in the US. Depending on those results, J&J plans to start large-scale, phase 3 testing with a single-shot regimen in the second half of September.
Around the same time, the company will start a parallel phase 3 study testing a two-shot regimen of the vaccine, Dr Stoffels said.
In the monkey study, scientists from J&J and Harvard’s Beth Israel Deaconess Medical Centre studied seven different potential vaccines in 32 animals and compared the results to 20 control animals who got placebo shots. Six weeks later, all of the animals were exposed to the Sars-Cov-2 virus. All 20 animals that received the placebo developed high levels of virus in their lungs and nasal swabs.
In the best-performing candidate, which J&J selected for human testing, none of the animals had virus in their lungs and only one showed low levels of virus in nasal swabs.
Lab tests showed they all had developed antibodies capable of neutralising the virus after a single shot.
Dr Dan Barouch, a vaccine researcher at Beth Israel Deaconess who led the research in collaboration with J&J, said of the results: “This study shows that even just a single immunisation with the Ad26 vaccine leads to neutralising antibody responses and robust protection of monkeys against Covid-19”.