Preliminary findings from two South African clinical trials suggest the Omicron coronavirus variant has a much higher rate of "asymptomatic carriage" than earlier variants, which could explain why it has spread so rapidly across the globe.
The studies - one of which was carried out when Omicron infections were surging in South Africa last month and another which resampled participants around the same time - found a far greater number of people tested positive for the coronavirus but were not showing symptoms compared to previous trials.
In the Ubuntu study evaluating the efficacy of Moderna's Covid-19 vaccine in people living with HIV, 31% of 230 participants undergoing screening tested positive, with all 56 samples available for sequencing analysis verified to be Omicron.
"This is in stark contrast to the positivity rate pre-Omicron, which ranged from less than 1% to 2.4%," the researchers said in a statement.
In a subgroup of the Sisonke trial evaluating the efficacy of Johnson & Johnson's Covid-19 vaccine, the mean asymptomatic carriage rate rose to 16% during the Omicron period from 2.6% during the Beta and Delta outbreaks.