WASHINGTON >> The Food and Drug Administration released updated safety standards Tuesday for makers of COVID-19 vaccines despite efforts by the White House to block them, clearing the way for requirements that are widely expected to prevent the introduction of a vaccine before Election Day.

In the new guidelines posted on its website, the FDA said vaccine makers should follow trial participants for at least two months to rule out any major side effects before seeking emergency approval. That standard had been a sticking point between the FDA and White House officials, who said it could unreasonably delay the availability of COVID-19 vaccines.

President Donald Trump has repeatedly insisted a vaccine could be authorized before Nov. 3, even though top government scientists working on the effort have said that timeline is very unlikely. On Monday Trump said vaccines are coming “momentarily,” in a video recorded after he returned to the White House.

Former FDA officials have warned that public perception that a vaccine was being rushed out for political reasons could derail efforts to vaccinate millions of Americans.

A senior administration official confirmed Monday that the White House had blocked FDA’s plans to formally publish the safety guidelines based on the two-month data requirement, arguing there was “no clinical or medical reason” for it.

But on Tuesday, the FDA posted the guidance on its website, making clear that regulators plan to impose the safety standards for any vaccine seeking an expedited path to market.

FDA Commissioner Stephen Hahn said in a statement that he hoped the guidelines would help “the public understand our science-based decision-making process that assures vaccine quality, safety and efficacy.”

The requirements are aimed at companies seeking rapid approval through the FDA’s emergency authorization pathway. That accelerated process, reserved for health emergencies, allows medical products onto the market based on a lower bar than traditional FDA approval. But FDA has made clear only vaccines that are shown to be safe and effective will be authorized for coronavirus.

Former FDA acting commissioner Dr. Stephen Ostroff said the requirements seem reasonable given the agency is in largely “uncharted territory” in terms of considering emergency use of a vaccine. The agency has only previously cleared one vaccine through the method — a decades-old shot that was authorized to prevent anthrax poisoning in 2005.

“There really is no margin for error here,” Ostroff said. “Even when you’re talking about limited use of a vaccine there has to be some level of assurance that there isn’t a risk here that would far outweigh the benefit.”

Dr. Peter Marks, the head of FDA’s vaccine division, said Tuesday that the two-month follow-up requirement was chosen to be “something reasonably aggressive, but not too conservative — right in the middle.” He spoke at a symposium organized by Johns Hopkins University.

Initial doses of vaccines for emergency use would likely be reserved for medical workers and people with health conditions that make them particularly vulnerable to coronavirus. Full FDA approval for the general population will require significantly more data and is not expected until mid-2021.

The White House attempt to block the guidance followed a string of instances in which the Trump administration has undercut its own medical experts working to combat the pandemic. FDA’s Hahn has been attempting to shore up public confidence in the vaccine review process for weeks, vowing that career scientists, not politicians, will decide if the shots are safe and effective.

Pfizer CEO Albert Bourla has stoked excitement by saying that he expects data on whether the company’s candidate works to be ready in late October. But a number of variables would still have to align for the company to submit, and the FDA to review and greenlight, a vaccine application before Nov. 3. Pfizer’s competitors Moderna, AstraZeneca and Johnson & Johnson are working on longer research timelines.

Vaccine development typically takes years, but the U.S. government has invested billions in efforts to accelerate the process and help multiple drugmakers prepare multiple candidates. All the doses will be purchased by the federal government for use vaccinating the U.S. population.

Beyond exposing the rift between the White House and FDA, the delay in releasing the guidelines may have had limited practical effect.

FDA scientists have been discussing the guidelines publicly for weeks and have made clear that the recommendations have already been shared with each of the vaccine developers.

Former FDA Commissioner Scott Gottlieb said Tuesday that drugmakers depend on the FDA’s science-based endorsement to vouch for the safety and effectiveness of their products.

“I can’t imagine a circumstance where a sponsor would challenge or seek to undermine the FDA’s role here,” Gottlieb said at the COVID-19 symposium. “This is precisely the moment when we need an objective, neutral arbiter.”

Last week, Gottlieb and six other former FDA commissioners blasted the Trump administration for “undermining the credibility” of the agency in a op-ed calling for the release of the then-stalled vaccine guidelines.

“Scientists should make decisions based on data, unfettered by political pressure or the intrusions of ideology or vested interests,” the officials wrote.