The US Food and Drug Administration said it shortened the interval between the primary series of the Moderna Covid-19 vaccine and a booster dose by a month for people aged 18 or above to at least five months.

“Vaccination is our best defense against Covid-19, including the circulating variants, and shortening the length of time between completion of primary series and a booster dose may help reduce waning immunity,” Peter Marks, director of the FDA's Centre for Biologics Evaluation and Research, said.

The move follows the agency's decision on Monday to reduce the necessary interval to get a booster dose of Pfizer and BioNTech Covid-19 vaccine to five months from six, saying such a change may provide better protection sooner against the fast-spreading Omicron variant.