WASHINGTON — The White House has blocked new Food and Drug Administration guidelines on bringing potential vaccines for COVID-19 to market that likely would have prevented their approval before the Nov. 3 election.

At issue was the FDA’s planned requirement that participants in mass clinical trials for vaccines be followed for two months to ensure there are no side effects and the vaccines provide lasting protection. Despite the move by the White House, FDA officials say companies are aware of the standards and are expected to comply with them.

FDA Commissioner Stephen Hahn has been attempting to shore up public confidence in the FDA’s vaccine review for weeks, vowing that career scientists, not politicians, will decide if the shots are safe and effective for mass vaccination.

President Donald Trump has repeatedly insisted that a vaccine could be authorized before Election Day, despite top government scientists saying that timeline is very unlikely.