Paxlovid works by slowing replication of the covid virus, so that your immune system kick in and can more easily attack it. This means that once Paxlovid ends, the virus starts ramping up and so some people feel sicker (rebound effect). Your immune system is working by attacking the virus and, unfortunately also making you feel worse. Paxlovid and other antivirals decrease the severity of illnesses. Paxlovid is only given in the first 5 days of covid because the viral load is less this early.