Scientists in the country analyzed surveillance data and say the numbers indicate the risk of reinfection tripled between the beginning of October and the end of November.
“We find evidence of a substantial and ongoing increase in the risk of reinfection that is temporally consistent with the timing of the emergence of the Omicron variant in South Africa, suggesting that its selection advantage is at least partially driven by an increased ability to infect previously infected individuals,” professor Juliet Pulliam, director of the South African Center for Epidemiological Modeling and Analysis, and other scientists wrote in the pre-print paper.
Modelers drew from records of nearly 2.8 million people who tested positive for COVID-19 at least 90 days prior to Nov. 27. Of those people, 35,670 were suspected of being reinfected. A case was counted as a reinfection if a person tested positive at least 90 days after his or her first result. A subset of the reinfections happened in October and November.
Genomic sampling has shown the Omicron variant was present in South Africa by Nov. 8, over a week after it arrived in Nigeria.
The data show the 7-day moving average of suspected reinfections, 250.7, is still lower than the peak of 349.1 seen on Sept. 19. Still, the average is increasing after a drop following the peak.
The surveillance data did not list the vaccination state of those who tested positive, leaving scientists unsure whether the decrease in protection will also be seen in the vaccinated.
“We do not have information about the vaccination status of individuals in our data set and therefore cannot make any assessment of whether Omicron also evades vaccine-derived immunity,” Pulliam said in a statement.
Limitations of the study were listed as including vast differences in detection rates of COVID-19 cases across the country, and reinfections not being confirmed by sequencing. Some outside scientists cast doubt on the findings.
It can’t be said that the reinfections were Omicron due to the lack of sequencing, Dr. Muge Cevik, an infectious diseases expert based at the University of St. Andrews, wrote on Twitter.
“Increased risk of reinfection is plausible, but it seems too early to estimate the relative risk,” she said.
COVID-19 cases in South Africa have been increasing rapidly in recent weeks, stoking concern worldwide over the Omicron variant. Officials there reported 11,535 new COVID-19 cases on Dec. 2, an increase of 25 percent from the day prior and 98 percent from Nov. 2.
While it takes time to obtain and sequence samples, “it does look like there is a predominance of Omicron,” Anne von Gottberg, a microbiologist with the country’s National Institute of Communicable Diseases, told a remote press briefing organized by the World Health Organization.
Researchers also said that natural immunity likely still provides protection against severe disease.
“We believe that they are still protected from severe disease and symptomatic disease requiring admission,” von Gottberg said. “Be need to wait for one or two weeks longer to see what’s going on to allow for the clinical course of these cases.”