Beijing has never fully co-operated with international investigators, impeding the search for a definitive answer. But scientists continue their quest to establish the causes of a global health crisis that killed millions of people, caused trillions of dollars of economic damage and increased fears over the risks of virus research.

Since COVID-19 first took off, political leaders, infectious disease specialists, and the lay public have debated its origins. Eventually, two hypotheses emerged: Either the virus crossed over from animals to humans in a wet market in Wuhan—the city in which the disease first appeared—or it leaked from a Chinese lab,