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.

China on Monday dismissed the possibility that the virus that caused COVID-19 leaked from a lab, after the CIA said it now favors the so-called lab leak theory over natural transmission. “It is extremely unlikely the pandemic was caused by a lab leak,