Johnson & Johnson says it will enroll 60,000 participants in the next stage of its COVID-19 vaccine trial, making it the largest in the U.S.

The feverish race for a coronavirus vaccine got an infusion of energy Wednesday as Johnson & Johnson announced that it has begun the final stage of its clinical trials, the fourth company to do so in the United States as the country hits a grim milestone of 200,000 deaths from the pandemic.

Johnson & Johnson is a couple of months behind the leaders, but its advanced vaccine trial will be by far the largest, enrolling 60,000 participants. The company said it could know by the end of this year if its vaccine works.

Its vaccine has potentially consequential advantages over some competitors and uses a technology that has a long safety record in other vaccines. Their vaccine could require just one shot instead of two — important considering that the entire population of the world needs vaccination. And it does not have to be kept frozen as it is delivered to hospitals and other places where it will be given to patients, simplifying the logistics of hundreds of millions of doses.

“Big news,” President Donald Trump tweeted about the trial Wednesday morning. “@FDA must move quickly!” he added, referring to the Food and Drug Administration, which oversees vaccine approval.

The president has repeatedly claimed that a vaccine will be ready before Election Day and urged federal regulators to act quickly to approve one, raising fears that they will bow to the pressure and rush their vetting process. The federal government’s Operation Warp Speed program has invested more than $10 billion in private companies’ coronavirus vaccines to date, including about $1.5 billion to Johnson & Johnson.

Facing criticism over secrecy, several companies — including Johnson & Johnson on Wednesday — have released the detailed blueprints of their trials, which are typically considered proprietary. The FDA is expected this week to release stricter guidelines outlining the criteria it will use to vet clinical trial data.

Never in history has a vaccine been tested and manufactured so quickly — in months instead of years. Right behind Johnson & Johnson are Sanofi and Novavax, which may prove just as good or better than the leading contenders.

“We need multiple vaccines to work,” said Dr. Dan Barouch, a virologist at Beth Israel Deaconess Medical Center who led the development of the technology used in Johnson & Johnson’s trial. “There are 7 billion people in the world, and no single vaccine supplier will be able to manufacture at that scale.”

Johnson & Johnson’s advanced trial, known as a Phase 3 trial, started Monday. At a news conference, Dr. Paul Stoffels, the company’s chief scientific officer, said the company might be able to determine by the end of the year if the vaccine is safe and effective. The company will soon be posting a manuscript online with data from the earlier phases of its trials, he said.