A panel of expert advisers to the U.S. Food and Drug Administration will vote later on Tuesday on whether to recommend authorization of Merck & Co's  antiviral pill to treat COVID-19.

The U.S. drugmaker reported on Friday that the oral medicine's efficacy in reducing hospitalizations and deaths had dropped to 30% with data available from over 1,400 patients in its late-stage study. It had reported about a 50% reduction a month ago using data from 775 patients.

Analysts said they still expect the drug to be authorized, as medicines like Merck's molnupiravir, developed with Ridgeback Biotherapeutics, and Pfizer's  Paxlovid are likely to become important therapeutic tools that can be taken at home as soon as COVID-19 symptoms arise.

The rival oral therapies target parts of the virus that are not changed by mutations in the new Omicron variant. They could become even more vital if vaccine-induced and natural immunity are threatened by the variant.

The Merck pills are meant to be taken twice a day for five days upon onset of COVID-19 symptoms.

The FDA's staff, in briefing documents posted ahead of the meeting of outside experts, did not make a recommendation on whether the drug should be authorized, and said they would provide their assessment of the new data at the meeting.

They flagged concerns over whether the drug could cause the virus to mutate, and asked the panel to discuss if a more targeted population of patients should be offered the medicine to mitigate those concerns.

Drugs in the same class as molnupiravir have been linked to birth defects in animal studies. Merck has said similar studies of molnupiravir - for longer and at higher doses than used in humans - indicate the drug does not affect mammalian DNA.

"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," said Mizuho analyst Vamil Divan.

More countries may end up signing deals for the Pfizer drug, which taken with another antiviral, ritonavir, showed an 89% reduction in hospitalizations and deaths in a trial, Divan said.

Pfizer's trial, like Merck's was stopped early due to its high success rate.


(Reporting by Manas Mishra in Bengaluru; Editing by Caroline Humer and Bill Berkrot)