Moderna's Covid-19 vaccine is likely to offer protection of up to a couple of years from the virus, its chief executive said on Thursday, news agency Reuters has reported. He, however, added that more data is still needed to make a definitive assessment. "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," Moderna CEO Stephane Bancel has said. He added that his company was about to prove that its vaccine would also be effective against variants of the coronavirus seen in Britain and South Africa. Given vaccines development and pharmacovigilance usually requires years, the protection duration of COVID-19 shots is a lingering question for scientists and regulators.