WASHINGTON - Updated Covid-19 boosters offer increased protection against new variants in people who have previously received up to four doses of the older vaccines, a real-world study in the United States showed.

The study of more than 360,000 people, published in the Morbidity And Mortality Weekly Report digest, offers the first evidence that new vaccines by Pfizer-BioNTech and Moderna provide better protection compared with the original shots.

Since their introduction in September, the vaccine boosters, which target both the original and Omicron BA.4 and BA.5 coronavirus strains, provided greater benefit to younger adults aged 18 to 49 years than those in the older age group.

When given eight months or more apart, relative vaccine effectiveness of the new boosters compared with the original shots was 56 per cent among people aged 18 to 49 years, 48 per cent among those aged 50 to 64 years, and 43 per cent for those aged 65 years and older, the study showed.

The variation in effectiveness was lower, between 28 per cent and 31 per cent, when the jabs were given two to three months apart.

The authors of the study, however, warned that participants may not have recalled their vaccination status, previous infection history and underlying medical conditions, and that low acceptance of bivalent boosters could have biased the results.

So far, around 35 million updated boosters have been administered in the US, representing about 10 per cent of the total population, as per government data.

The authors warned that the study may not be generalisable to future variants, as the dominant variants keep changing. In just the past two months, the BQ.1 and BQ.1.1 subvariants have become the dominant strains of Covid-19 in the US, instead of the BA.5 subvariant of Omicron, which the updated vaccines were based on. REUTERS