The European Union's drug regulator is set to decide whether to approve Pfizer's COVID-19 pill at the end of this month, before a final review of Merck's similar but less effective drug in February, a source with knowledge of the matter said.

Late last year, the European Medicines Agency (EMA) gave guidance to member states on how to use both antiviral pills as emergency treatments, while it carried out rolling reviews to help member states decide on quick adoption ahead of any formal EU-wide approval. 

Italy, Germany and Belgium are among a handful of EU countries that have bought the drugs. 

The EMA's review of Merck's pill, developed with Ridgeback Biotherapeutics, is taking longer than Pfizer's because Merck released trial data in November showing its drug was significantly less effective than previously thought.

"Merck had applied earlier than Pfizer, but then they had sent the final data on efficacy later, and therefore it took European regulators more time to examine that very last data," the source said.

The source declined to be identified because the EMA's reviews are confidential.

EMA Executive Director Emer Cooke told Irish broadcaster RTE the regulator may be able to conclude its review on one of the antivirals this week and was "looking at a little bit more review on the second oral antiviral, which might push it into next month's discussion."

She did not identify which drug could be approved first.

While vaccines are central to fighting the pandemic, regulators are looking at antivirals, which can be taken at home, as a new weapon against the fast-spreading Omicron variant.

U.S. and UK regulators have given the go ahead for both drugs to treat certain high-risk adult patients. 

The EMA said on Jan. 10 it could issue "within weeks" a decision on whether to approve the use of Pfizer's two-drug regimen Paxlovid. 

In a clinical trial, it was nearly 90% effective in preventing hospitalizations and deaths when given at an early stage to patients at high risk of severe illness. 

The source also said the EMA planned to issue its decision on Valneva's VLA2001 COVID-19 vaccine candidate in March, in line with the timeline given by the French company on Jan 6.

(Reporting by Reuters reporters Editing by Josephine Mason and Mark Potter) ((; +39 06 8030 7744; Reuters Messaging:))