Pfizer, which is developing the vaccine with German partner BioNTech, said that it may say if the vaccine is effective as soon as this month based on its 40,000 person clinical trial but that it also needs safety data that will not be available until November at the earliest.
Trump has said repeatedly that there would be a vaccine available before the election.
The president's rush to a vaccine has also raised concerns that the U.S. Food and Drug Administration, acting in haste, might not conduct an adequate review of a vaccine.
U.S. health officials have sought to assuage those concerns out of fear that not enough Americans would take a vaccine early on.
Earlier this month, the FDA formalized a requirement that the vaccine makers collect two months of safety data on one-half of trial participants.
The Pfizer update was most welcomed after a week of setbacks.
Clinical trials for a vaccine developed by Johnson & Johnson and an antibody therapy from Eli Lilly were both put on pause this week.
A vaccine is seen as key to controlling the spread of the virus.
The encouraging news not only sent shares of Pfizer higher - it helped lift stock across Wall Street - snapping a three-day losing streak.
