NEW YORK — Pfizer has started testing its potential COVID-19 treatment as a preventive medicine aimed at warding off the virus if a close contact gets it.

The drugmaker said Monday that it will study the pill it is developing in combination with a low dose of the HIV drug ritonavir in people who are at least 18 years old and live in the same household with someone who is infected.

Pfizer plans to enroll 2,660 people in the late-stage study. Those participating will get either the treatment combination or a fake drug orally twice a day for five to 10 days.

Researchers expect that the use of ritonavir will help slow the breakdown of the potential treatment so it remains active longer to help fight the virus.

“If successful, we believe this therapy could help stop the virus early – before it has had a chance to replicate extensively,” Pfizer Chief Scientific Officer Dr.Mikael Dolsten said in a statement from the drugmaker.

Pfizer Inc. also is studying its potential treatment in people who are already infected with the virus. It’s designed to be prescribed at the first sign of infection without requiring patients to be hospitalized. The drugmaker expects to see results from those studies by the end of the year.

The pill aims to block a key enzyme that the virus needs to replicate.