The U.S. Food and Drug Administration (FDA) last week authorized Merck's antiviral COVID-19 pill for high-risk adult patients, a day after giving a broader go-ahead to a similar but more effective treatment from Pfizer.

In the last week, Japan and India have also recommended approval of Merck's experimental pill -- which has already been approved in Britain, Denmark and the Philippines, among others -- while South Korea on Monday authorized Pfizer's treatment for emergency use.

Merck has signed deals to sell about nine million courses of its treatment to governments around the world, but on Dec. 22 France became the first country to publicly say it had canceled its order for the Merck pill, following disappointing trial data. It hopes, instead, to receive Pfizer's drug before the end of January.

In the European Union, both treatments are under review by the European Medicines Agency (EMA), though the regulator has said they can be used in adults ahead of any wider recommendation.