US regulators yesterday authorised a second booster dose of the two most commonly used Covid-19 vaccines for people 50 and older, given data showing waning immunity and the risks posed by Omicron variants of the virus.

The US Food and Drug Administration (FDA) said the new boosters - a fourth round of shots for most vaccine recipients - of the Pfizer Inc/BioNTech SE and Moderna Inc vaccines are to be administered at least four months after the previous dose.

They are intended to...