Shares of Merck & Co. Inc. $(MRK)$ and Ridgeback Biotherapeutics said Wednesday that they have started a Phase 3 trial of molnupiravir, and oral antiviral therapeutics for the prevention of COVID-19 infection. Merck's stock gained 0.4% in premarket trading. The drug makers said the study is enrolling people at least 18 years old who live in the same house as someone who tested positive for SARS-CoV-2 and had symptoms. Primary endpoints include percentage of participants with COVID-19 through day 14, the percentage of those with an adverse event and the percentage of those who had to discontinue to the study because of an adverse event. ""As the pandemic continues to evolve and surges are being reported in many places around the world, it is important that we investigate new ways to protect individuals exposed to the virus from becoming infected with symptomatic disease," said Dr. Nick Kartsonis, senior vice president, vaccines and infectious diseases, clinical research, Merck Research Laboratories. "If successful, molnupiravir could provide an important additional option towards reducing the burden of COVID-19 on our communities." Merck's stock has slipped 2.2% year to date through Tuesday, while the SPDR Health Care Select Sector ETF $(XLV)$ has rallied 19.2% and the Dow Jones Industrial Average has advanced 15.5%.