The top contender is a medication from Merck & Co. and Ridgeback Biotherapeutics called molnupiravir, Dieffenbach said. That is the product being tested in the Kellys’ Seattle trial. Two others are a candidate from Pfizer, known as PF-07321332, and AT-527, an antiviral produced by Roche and Atea Pharmaceuticals.

They work by interfering with the virus’s ability to replicate in human cells. In the case of molnupiravir, the enzyme that copies the viral genetic material is forced to make so many mistakes that the virus can’t reproduce. That, in turn, reduces a patient’s viral load, shortening infection time and preventing the kind of dangerous immune response that can cause serious illness or death…

If the antiviral pills prove effective, the next challenge will be to ramp up a distribution system that can rush them to people as soon as they test positive. Griffin said it will take something akin to the program set up last year by UnitedHealthcare, which sped Tamiflu kits to 200,000 at-risk patients enrolled in the insurer’s Medicare Advantage plans.

Merck officials predicted that the company could produce more than 10 million courses of therapy by the end of the year. Atea and Pfizer haven’t released similar estimates.

Even more promising? Studies evaluating whether antivirals can prevent infection after exposure.