“We know this is likely to happen at some point, so we need to beat it to the punch and nip it in the bud before it gets out of hand and starts to take over,” said Katherine Seley-Radtke, a medicinal-chemistry professor at the University of Maryland, Baltimore County, whose lab is studying antiviral combination therapies.

Some researchers and the drugmakers, however, say the risk is low that resistance can develop to the new Covid-19 pills because they are taken over just five days, too short a time for the virus to change meaningfully.

HIV is more likely to develop resistance against treatment consisting of a single drug, the researchers say, because it is more prone to mutations when it replicates than the pandemic coronavirus. Another factor: the course of treatment is much longer for a chronic disease than an acute infection.

Pfizer and Merck researchers said they didn’t see resistance develop during the clinical trials evaluating the pills. The company researchers also said each of the pills has characteristics that cut the risk of resistance, though they are looking for any signs.