Scientists in South Africa say omicron is at least three times more likely to cause reinfection than previous variants such as beta and delta, according to a preliminary study published Thursday.
Statistical analysis of some 2.8 million positive coronavirus samples in South Africa, 35,670 of which were suspected to be reinfections, led researchers to conclude that the omicron mutation has a "substantial ability to evade immunity from prior infection."
Scientists say reinfection provides a partial explanation for how the new variant has been spreading. The elevated risk of being reinfected is "temporally consistent" with the emergence of the omicron variant in South Africa, the researchers found.
The team's paper was uploaded to a preprint server and has not been peer-reviewed.
Questions about the level of protection vaccines provide against the new variant remain unanswered, as the scientists did not have access to immunization data. But Juliet Pulliam, a South Africa-based epidemiologist and one of the study's authors, said vaccines are likely to still offer the most effective protection against severe disease and death.
Knowing that omicron may lead to more reinfection is important, Pulliam wrote on Twitter. Acquired immunity from previous infection has been key in helping countries such as South Africa and Botswana, which have relatively low vaccination rates, manage the pandemic.
"Our most urgent priority now is to quantify the extent of Omicron's immune escape for both natural and vaccine-derived immunity, as well as its transmissibility relative to other variants and impact on disease severity," said Harry Moultrie, an infectious-disease expert who co-wrote the study, in a statement.
Other variants have been known to cause reinfection: Several patients reinfected with the beta variant were identified in Israel earlier this year. But the latest study suggests the relative risk of getting infected again remained stable in other variants, underscoring the significance of the findings on omicron.
"Contrary to our expectations and experience with the previous variants, we are now experiencing an increase in the risk of reinfection that exceeds our prior experience," Pulliam said in Thursday's statement.
South Africa's National Institute for Communicable Diseases said Wednesday that omicron overtook other virus variants in November, accounting for 74 percent of the genomes sequenced last month. Delta had previously been dominant. Overall case numbers have also risen rapidly over the past three days.
"Omicron is probably the fastest-spreading variant that South Africa has ever seen," said Tulio de Oliveira, a public health professor at South Africa's Stellenbosch University.
Only 6 percent of Africa's population has been fully vaccinated. In South Africa, just under 30 percent of people have been fully immunized, according to Our World in Data, but the country's public health officials warned that misinformation on social-media sites is hindering the rollout, especially among young adults.