Covid is a deadly virus, especially for older people, and Paxlovid reduces Covid’s severity. It does so by inhibiting the virus’s replication inside the human body, the same process that has made H.I.V. treatments so effective.

With Paxlovid, both randomized trials and data from electronic health records have pointed to its effectiveness. Some research finds an effect across all age groups, while other research finds one only among older patients. But that is not surprising. The Covid death rate for people under 50 is already so close to zero that reducing it in a statistically significant way is difficult.