Moderna’s Covid-19 mRNA vaccine is likely to offer protection of up to a couple of years, its chief executive said on Thursday, even though more data is still needed to make a definitive assessment.

The US biotech company, which stunned the world last year by coming up with a vaccine against the disease caused by the new coronavirus in just a few weeks, received approval for its shot from the European Commission on Wednesday.

Given vaccine development and pharmacovigilance usually requires years, the protection duration of Covid-19 shots is a lingering question for scientists and regulators.

“The nightmare scenario that was described in the media in the spring with a vaccine only working a month or two is, I think, out of the window,” Moderna Chief Executive Officer Stephane Bancel said at an event organised by financial services group Oddo BHF.

“The antibody decay generated by the vaccine in humans goes down very slowly [...] We believe there will be protection potentially for a couple of years.”

Bancel added his company was about to prove that its vaccine would also be effective against variants of the coronavirus seen in Britain and South Africa.

Scientists have said newly developed vaccines should be equally effective against both variants.