Ewen Callaway A health care worker swabs a person in Shanghai, China, for SARS-CoV-2.Credit: Qilai Shen/Bloomberg via Getty After the game-changing COVID-19 antiviral Paxlovid began to be used in late 2021, researchers noticed a perplexing trend. In some people taking the drug, symptoms and detectable virus vanish, only to mysteriously return days later. After months...
