The Food and Drug Administration said Tuesday that manufacturers of Covid-19 vaccines will need to follow tens of thousands of study participants for at least two months to look for any possible safety issues before the agency would consider authorization.
Given the timeline of when clinical trials began, the new guidance indicates that the earliest a Covid-19 vaccine might get an emergency use authorization would be the end of November — not Election Day, as President Donald Trump has previously touted.
Full coverage of the coronavirus outbreak
The FDA will require two months of follow-up for at least half of the study participants after they receive their last doses of vaccine. The vaccine candidates furthest ahead in phase 3 clinical trials, from Moderna and Pfizer, each require two doses, given about one month apart.
The Moderna trial, for example, aims to enroll 30,000 participants. For the FDA to consider granting an emergency use authorization, two months will have to pass since at least 15,000 of the participants received their second doses of the vaccine. The trial began at the end of July.
Even so, for a vaccine to receive an emergency use authorization within months of development is unprecedented. Clinical trials have historically taken years to determine safety and efficacy.
Trump took to Twitter on Tuesday evening to blast the new guidelines as being politicized.
"New FDA Rules make it more difficult for them to speed up vaccines for approval before Election Day," Trump tweeted. "Just another political hit job!
The new guidance is a "balance" of speed and safety, Dr. Peter Marks, director for the FDA's Center for Biologics Evaluation and Research, said Tuesday during a virtual symposium hosted by Johns Hopkins University and the University of Washington.
"With close to 1,000 or more people dying a day," Marks said, "there has to be balance."
He said he "can't deny" that the agency will most certainly have less safety data than usual, but he added the FDA will also rely on large claims-based databases linked to medical records to watch for adverse side effects.
Two months of follow-up seems to be a "reasonably aggressive" amount of time for any potential adverse events, such as Guillain-Barré syndrome or rare spinal cord injuries, to show themselves in a large group of people taking a vaccine, Marks said.
The FDA posted the guidance on its website Tuesday as part of a packet of briefing materials ahead of a meeting of its vaccine advisory committee, scheduled for Oct. 22.
The head of the Pharmaceutical Research and Manufacturers of America, Stephen J. Ubl, said in a statement the group welcomes the FDA's guidance, adding that "science is guiding every aspect" of the work at drug companies.
Download the NBC News app for full coverage of the coronavirus outbreak
Dr. Thomas File, president of the Infectious Diseases Society of America, said in an emailed statement: "It is imperative that a vaccine be approved on the basis of FDA's quality standards and that its safety and efficacy are established before it is authorized. A vaccine that has been approved with speed, rather than safety and efficacy, at the forefront will compound the challenges posed by this pandemic."
Three drugmakers are testing their Covid-19 vaccines in phase 3 trials in the U.S. They are Pfizer, Moderna and Johnson & Johnson. AstraZeneca started its phase 3 study in September but paused it shortly afterward because of a safety issue in a study participant in the U.K. That trial remains halted in the U.S.
Follow NBC HEALTH on Twitter & Facebook.
