WASHINGTON – Pfizer said Monday that it is now testing a pill that could help people ward off COVID-19 if a close contact, such as family member, gets the virus. The drugmaker said it is looking at the efficacy of the pill as used 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 has COVID-19. 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. Pfizer in March began an early-stage clinical trial of the new antiviral therapy for the coronavirus, with the goal of rolling it out by year-end. Part of a group of medicines called protease inhibitors, which are used to treat HIV and Hepatitis C, the drug hampers production of enzymes needed for the virus to multiply in human cells. The oral antiviral therapy could potentially be prescribed "at the first sign of infection, without requiring that patients are hospitalized or in critical care," Pfizer Chief Scientific Officer Dr. Mikael Dolsten stated in March. Researchers expect that using ritonavir in combination with an oral antirviral medication 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," Dolsten said in a statement from the drugmaker. Pfizer is also studying its potential treatment in people who are already infected with the virus. It is 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. – Agencies