Updated Covid-19 boosters offer increased protection against new variants in people who have previously received up to four doses of the older vaccine, a real-world study in the US showed.
The study of over 360,000 people, published in Morbidity and Mortality Weekly Report, offers the first evidence that the new vaccines by Pfizer-BioNTech and Moderna provide better protection compared to the original shots.
Since their introduction in September, the vaccine boosters, which contain both original and Omicron BA.4/5 coronavirus strain, provided greater benefit to younger adults aged 18-49 years that those in the older age group.
When given eight months or more apart, relative vaccine effectiveness of the new boosters compared with the original vaccine shots was 56% among people aged 18–49 years, 48% among those 50–64 years, and 43% among persons aged 65 years and older, the study showed.
The variation in effectiveness was lower, in the range of 28-31%, when the boosters were given 2-3 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 across the US, representing around 10% of the total population, as per government data.
The authors warned that the study may not be generalizable to future variants, as the dominant variants keep changing.
In just the last two months, the BQ.1 and BQ.1.1 subvariants have become the dominant strains of coronavirus in the US, taking over from the BA.5 subvariant of Omicron, based on which the vaccines were updated.