The study, published on Tuesday, has not yet been peer reviewed.

The findings are in contrast to earlier studies that showed the vaccine, CoronaVac, was 74.5% effective in preventing infections in children aged 6 to 16 years, when Delta was the dominant variant.

Countries are increasingly recommending booster doses in the face of the circulating Omicron variant, which the World Health Organization has said can bypass some protection imparted by vaccines. Most vaccines are still effective against severe infections.

Sinovac’s vaccine was 64.6% effective in preventing hospitalisations and 69% effective in preventing Covid-19 related ICU admissions, the Chilean study also showed.

A study by New York State researchers in February showed that Pfizer Inc’s vaccine quickly lost most of its ability to prevent infections in children aged 5 to 11 during an Omicron-led wave.