What if coronavirus could be prevented with a pill? Pfizer said on Monday it has started testing an oral antiviral drug for people exposed to COVID-19. In a mid-to-late stage study, Pfizer will test a pill in up to 2,660 healthy adults aged 18 or older. Participants in the trial must live in the same household as a person with a confirmed case of coronavirus. The antiviral pill is designed to block the activity of a key enzyme needed for the coronavirus to multiply. Pfizer and its rivals, including U.S.-based Merck & Co Inc., have been racing to develop an easy-to-administer pill for COVID-19. To date, Gilead Sciences’ intravenous drug remdesivir is the only approved antiviral treatment for COVID-19 in the U.S. While effective, it is time-consuming, costly and requires medical assistance to administer the treatment.