Undercutting Scientists, Trump Says Tightening Covid-19 Vaccine Guidelines ‘Sounds Like a Political Move’

And its vaccine has potentially big advantages over some competitors. It uses a technology that has a long safety record in vaccines against other diseases. Its vaccine could require just one shot instead of two. And unlike other vaccine candidates, 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,” Mr. Trump tweeted about the trial on Wednesday morning. “@FDA must move quickly!”

The president has repeatedly claimed that a vaccine will be ready before Election Day, and has 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 taken the rare step of releasing the detailed blueprints of their trials, which are typically considered proprietary. And the F.D.A. is expected this week to release stricter guidelines outlining the criteria it will use to vet clinical trial data.

“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 seven 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 on 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.

Johnson & Johnson has begun manufacturing the vaccine on an industrial scale to build up a supply that can be released immediately if the vaccine is authorized, Dr. Stoffels said in an interview on Wednesday. He expected to have tens of millions of doses ready by the end of the year. “Then we can ramp up to many more batches,” he said.

Johnson & Johnson’s vaccine uses an adenovirus to carry a gene from the coronavirus into human cells. The cell then produces coronavirus proteins, but not the coronavirus itself. These proteins can potentially prime the immune system to fight off a later infection by the virus.