The administration has also ordered about 3.1 million courses of a pill from Merck & Co., and has an option in its contract to purchase more than 2 million additional courses.

The pills expand treatment options for people who test positive for the virus, though the medicines have to be taken relatively early in the illness to be effective. Officials have warned that the pills are not a replacement for vaccines.

“Although antivirals are promising, we must be sure to get our population vaccinated. Antivirals, as good as they are, are not our first line of defense against Covid-19, because we all know it’s much, much more important to prevent an infection than it is to treat,” Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, said at a news briefing last week.

The Washington Post earlier reported White House plans to buy the Pfizer pill. Spokespeople for Pfizer and the White House didn’t immediately respond to requests for comment.

A clinical trial of Merck’s pill showed 50% efficacy at preventing hospitalization and death, though it’s not clear if the trials and efficacy data are directly comparable between the two medicines.