The US regulator FDA has published new guidelines for pharmaceutical companies seeking emergency approval to market a corona vaccine.
For safety reasons, data should be collected for at least two months on volunteers vaccinated with drugs in development.
According to American media, this means that it can be challenging to come up with a vaccine before the presidential election.
Pharmaceuticals must keep track of the side effects of new drugs against the coronavirus during those two months. The regulator wants to ensure that vaccines are safe before they are distributed to millions of Americans.
The decision is a setback for President Donald Trump, who has suggested that a vaccine will be available before election day.
The New York Times had previously reported based on anonymous sources that the White House was obstructing the publication of the new guidelines.
The information eventually appeared on the site of the regulator without further explanation.