The rumors began almost as soon as the disease itself. Claims that a foreign adversary had unleashed a bioweapon emerged at the fringes of Chinese social media the same day China first reported the outbreak of a mysterious virus.

“Watch out for Americans!” a Weibo user wrote on Dec. 31, 2019. Today, a year after the World Health Organization warned of an epidemic of COVID-19 misinformation, that conspiracy theory lives on, pushed by Chinese officials eager to cast doubt on the origins of a pandemic that has claimed more than...