The US has authorised a fourth dose of either the Pfizer-BioNTech or Moderna Covid-19 vaccines for people 50 and older, as authorities warn of a possible new wave driven by the BA.2 variant — a sub-variant of Omicron, reports AFP.

The Food and Drug Administration (FDA) said in a statement it had based its decision on emerging evidence that an additional booster, given four months after the last, improved protection against severe Covid-19 and wasn't associated with new safety concerns.

Additionally, people with immune compromising conditions who have already received four shots, with their latest at least four months ago, are now eligible for a fifth dose.