Johnson & Johnson on Wednesday kicked off a final 60,000-person trial of a single-shot Covid-19 vaccine that potentially would simplify distribution of millions of doses compared with leading rivals using two doses.

The company expects results of the phase 3 trial by year-end or early next year, Dr Paul Stoffels, J&J’s chief scientific officer, said in a joint media conference with officials from the National Institutes of Health and the Donald Trump administration.

Rival vaccines from Moderna, Pfizer and AstraZeneca all require two shots separated by several weeks, which make them much more difficult to administer.

“The benefits of a single-shot vaccine are potentially profound in terms of mass immunisation campaigns and global pandemic control,” Dr Dan Barouch, a Harvard vaccine researcher who helped design J&J’s Covid-19 vaccine, said in a telephone interview.

Stoffels said J&J would publish a detailed study protocol for its phase 3 trial on Wednesday on the company’s website.

Stoffels said J&J started the phase 3 trial after seeing positive results in its phase 1/2 trial in the US and Belgium. The company plans to release those results imminently.

Stoffels said the safety and level of protection in the study were on a par with what was seen in the company’s animal studies, and said the results showed a single dose could offer sufficient protection “for a long time”.

J&J’s late-stage trial will use as many as 215 sites in the US, South Africa, Argentina, Brazil, Chile, Colombia, Mexico and Peru. The company plans to manufacture as many as 1 billion doses in 2021, and more after that, Stoffels said.

The goal of the trial is to test whether the vaccine can prevent moderate to severe Covid-19 after a single dose, but it will also look to see if the vaccine can prevent serious disease requiring medical intervention and milder cases of the virus.

Stoffels predicts it will take six weeks to two months to enroll the trial, and said the company hopes to get an answer on whether the vaccine works “around the end of the year or early next year”. It is not clear how fast the company could get regulatory approval, but J&J plans to manufacture doses before approval, so it could start distribution quickly.

The trial will be overseen by an independent Data and Safety Monitoring Board (DSMB) that will review vaccine safety and effectiveness. In the press conference, Dr Francis Collins, director of the National Institutes of Health, said all three vaccines being organised and supported by the federal government’s Operation Warp Speed — J&J’s, Moderna’s and AstraZeneca’s — share a common DSMB. Pfizer is running its own trial and has a separate DSMB, Collins said.

J&J’s trial is designed to test for a vaccine that is 60 per cent effective. In the study protocol, that could be determined after 154 people became infected with the virus. Stoffels said the company would start counting cases of Covid-19 infections within the study population 15 days after individuals are vaccinated.

The DSMB will take its first look at the vaccine’s efficacy after 20 people have become infected.