This combination of nirmatrelvir and ritonavir is used to treat coronavirus disease (COVID-19) in people who have mild to moderate symptoms and have a high risk for COVID-19 complications. It helps people who have COVID-19 to stay out of the hospital. It is not for use by people who are hospitalized due to COVID-19.Nirmatrelvir is a SARS-CoV-2 main protease inhibitor. It works by preventing the growth of the virus that causes COVID-19. Ritonavir increases (“boosts”) the levels of nirmatrelvir. This helps nirmatrelvir work better.This medication works by stopping the coronavirus from growing. It cannot be used to prevent COVID-19.