LONDON- The European Union drug regulator expects to announce the results of its review of Moderna's COVID-19 booster vaccine and start a rolling review of Merck's antiviral drug molnupiravir next week, a senior official said on Thursday.

Marco Cavaleri, head of biological health threats and vaccines strategy for the European Medicines Agency (EMA), said in a briefing the results of the Moderna boost shot would be announced on Oct. 25.

He said it was not clear if Russia would submit an application for its one-dose COVID vaccine called Sputnik Light in addition to the one for its two-dose Sputnik V shot.

"We don't have really any clear understanding of whether (the Russian team) intends to submit an application also for this vaccine in the European Union, but we will continue the dialogue to get more clarity on this," he said.

Reuters reported on Thursday that EMA was unlikely to decide whether to approve Sputnik V vaccine until at least the first quarter of 2022 because some data needed for the review was still missing. 

On Oct. 4, the watchdog gave the go ahead for the use of Pfizer-BioNTech 22UAy.DE boosters and recommended people with weakened immune systems should get a third dose of a shot from Pfizer-BioNTech or Moderna. 

It left it to member states to decide if the wider population should have a booster.

Merck's experimental antiviral pill could halve the chances of dying or being hospitalised for those most at risk of contracting severe COVID-19, interim data showed this month. 

(Reporting by Josephine Mason and Pushkala Aripaka; Editing by Jason Neely and Edmund Blair) ((Josephine.Mason@thomsonreuters.com; +44 207 542 7695; Reuters Messaging: josephine.mason.reuters.com@reuters.net))