WASHINGTON (BLOOMBERG) - Johnson & Johnson has begun dosing up to 60,000 volunteers in a study of its Covid-19 vaccine, marking the first big US trial of an inoculation that may work after just one shot.

J&J is the fourth vaccine maker to move its candidate into late-stage human studies in the US.

If enrollment goes as expected, the trial could yield results as soon as year-end, allowing the company to seek emergency authorisation early next year, should it prove effective, Johnson & Johnson Chief Scientific Officer Paul Stoffels said Tuesday (Sept 22).

"We are convinced that the single dose could be very efficacious," he said on a call with the press, citing promising results from earlier research.

The study will compare the vaccine with a placebo injection, with a goal of showing whether it reduces cases of moderate to severe Covid-19.

J&J will publish detailed trial plans Wednesday, Mr Stoffels said. Frontrunners Pfizer Inc, Moderna Inc and AstraZeneca Plc have already done the same.

"It is likely that multiple Covid-19 vaccine regimens will be required to meet the global need," said Dr Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, in a statement.

J&J's vaccine "may be especially useful in controlling the pandemic if shown to be protective after a single dose."

The J&J study is nearly two months behind those of Moderna, working with NIAID, and Pfizer, partnered with BioNTech SE, whose final-stage trials started in late July. Pfizer has said it could get efficacy results by the end of October. Those vaccines use two-dose regimens.

J&J's vaccine could offer an advantage in distribution over two-shot inoculations that will require vaccination sites to ensure recipients return for their second dose. The company said its vaccine can be stored at refrigerator temperatures for three months, far longer than the Pfizer vaccine that requires deep freezing for long-term storage.

The product is made from a cold virus, called an adenovirus, that's modified to make copies of the coronavirus's spike protein, which the pathogen uses to enter cells.

The altered virus can't replicate in humans, but it induces an immune response that prepares the body for an actual Covid-19 infection. The vaccine was developed with researchers at Harvard University who have spent years working on the adenovirus vaccine platform, which is also used in J&J's Ebola vaccine.

The healthcare behemoth is running the study in conjunction with NIAID and the Biomedical Advanced Research and Development Authority at sites in the US, Brazil, Mexico, South Africa and other countries.

It will include significant representation among those over the age of 60, as well as minorities at disproportionate risk of becoming infected, including Black, Hispanic, American Indian and Alaskan Native peoples, according to a statement.

J&J has also agreed to collaborate with the UK on a separate phase 3 clinical trial that will test a two-dose regimen of the vaccine in multiple countries, according to Mr Stoffels.

The decision to begin the final-stage trial was based on data from an earlier human study that showed a single shot was safe and stimulated a strong immune response, Mr Stoffels said.

Animal studies showed that a single shot protected five out of six macaque monkeys from infection after exposure, according to a study published in Nature in July.

Like other final-stage vaccine trials, J&J's study is monitored by an independent board of doctors and statisticians who wait for a certain number of coronavirus cases to accumulate before looking at the data.

The trial aims to accumulate 154 Covid-19 cases for a final efficacy analysis. If the vaccine turns out to be more than 90 per cent effective, the trial could produce results after just 20 cases, Mr Stoffels said. He said that scenario is unlikely, though.

The single-shot trial may also take less time to complete than it would with two doses. Scientists will start counting cases just 15 days after patients get their inoculations or placebo shots.

Despite accelerated timelines, Operation Warp Speed won't cut corners in confirming vaccine safety or efficacy, National Institutes of Health director Francis Collins said on the call.

"That absolutely will not be allowed to happen," he said.