Preliminary results from two South African clinical trial studies show that the coronavirus Omicron variant has a much higher rate of “asymptomatic transmission” than previous variants, which may explain why it is spreading so rapidly around the world.
Studies, one conducted last month when omicron infections were on the rise in South Africa and the other re-sampled from previously tested subjects, showed a much higher number of people who were coronavirus-positive but showed no symptoms compared to previous studies.
In an Ubuntu study evaluating the efficacy of the Modern vs. Covid-19 vaccine in people living with HIV, 31 percent of the 230 participants screened tested positive, and all 56 samples available for sequencing analysis were confirmed to be infected with omicron.
“This is in stark contrast to the pre-omicron positivity rate, which ranged from less than 1 percent to 2.4 percent,” the researchers said in a statement.
In the Sisonke subgroup of studies evaluating the effectiveness of Johnson & Johnson’s Covid-19 vaccine, the average asymptomatic transmission rate increased to 16 percent during the omicron period from 2.6 percent during the beta and delta variants.
“The Sisonke study included 577 respondents who had previously been vaccinated, with results indicating a high rate of transmission of the infection even to those known to have been vaccinated,” the researchers said.
They added that “a higher rate of asymptomatic transmission is probably a major factor in the rapid spread of the omicron variant, even among populations with high previous rates of coronavirus infection.”
South Africa has seen an increase in coronavirus infections since late November, around the time its scientists warned the world about omicron. But since then, new cases have been declining. The first indicators show that the new wave of infection is marked by weaker forms of the disease than the previous ones.