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.

The CIA on Saturday offered a new assessment on the origin of the Covid outbreak, saying the coronavirus is "more likely" to have leaked from a Chinese lab than to have come from animals. But the intelligence agency cautioned it had "low confidence" in this determination.