Preliminary results from two South African clinical trials suggest that the Omicron coronavirus variant has a much higher rate of “asymptomatic carriage” than previous variants, which could explain why it has spread so rapidly across the world.
The studies found that a much larger number of people tested positive for the coronavirus but had no symptoms compared to previous trials.
In an Ubuntu study evaluating the effectiveness of Moderna’s COVID-19 vaccine in people living with HIV, 31% of 230 participants screened tested positive, with all 56 samples available for sequencing analysis verified to be Omicron.
This contrasts sharply with the pre-Omicron positivity rate, which ranged from less than 1% to 2.4%, the researchers said in a statement.
In a subgroup of the Sisonke trial evaluating the effectiveness of Johnson & Johnson’s COVID-19 vaccine, the average asymptomatic carriage rate increased to 16% during the Omicron period, compared to 2.6% during the Beta and Delta.
“The Sisonke study included 577 previously vaccinated subjects (…) with results suggesting a high carriage rate even in those known to be vaccinated,” the researchers said.
They added that “the higher asymptomatic carriage rate is probably a major factor in the rapid and widespread spread of the variant, even among populations with previous high coronavirus infection rates.”
South Africa saw an increase in COVID-19 infections starting in late November, around the time its scientists alerted the world to Omicron.
But new cases have since fallen and the first indications show that the wave was marked by a less serious disease than the previous ones.