Later on Tuesday, a panel of expert advisers to the US Food and Drug Administration will vote on whether to recommend approval of Merck's antiviral tablet to treat COVID-19.

With data from over 1,400 patients in a late-stage study, the American drugmaker announced on Friday that the oral medicine's efficacy in reducing hospitalizations and deaths had dropped to 30%. It had reported a 50% reduction a month prior, based on data from 775 patients.

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Analysts believe the drug will be approved because of medicines like Merck's molnupiravir, which was developed with Ridgeback Biotherapeutics, and Pfizer's Paxlovid, which was developed with Pfizer, are likely to become important therapeutic tools that can be taken at home as soon as COVID-19 symptoms appear.

The competing oral medicines target portions of the virus that are unaffected by the new Omicron variant's mutations. They may become even more important if the variation threatens both vaccine-induced and natural immunity.

When COVID-19 symptoms appear, the Merck pills should be given twice a day for five days.

The FDA's staff did not offer a recommendation on whether the drug should be approved, and stated they will provide their opinion of the new data during the conference, according to briefing documents provided ahead of the meeting of outside specialists.

They raised concerns about whether the treatment could cause the virus to evolve, and requested the panel to consider if the therapy could be administered to a smaller group of patients to alleviate those fears.

In animal research, drugs in the same class as molnupiravir have been related to birth abnormalities. Similar tests with molnupiravir - conducted for longer periods of time and at larger doses than those used in people - have shown that the medicine has no effect on mammalian DNA, according to Merck.

"We anticipate significant discussion on molnupiravir's mechanism of action and theoretical safety concerns, especially related to potential birth defects from the drug and whether the drug should be used in pregnant women or women of childbearing age," Mizuho analyst Vamil Divan said.

According to Divan, other countries may sign deals for the Pfizer medicine, which when combined with another antiviral, ritonavir, resulted in an 89 % reduction in hospitalizations and fatalities in a trial.

Pfizer's experiment was halted early, as was Merck's, due to its high success rate.