A team of researchers in South Africa said they found evidence that people who had been infected once with Covid were more likely to be re-infected with the Omicron variant than with the Beta or Delta variants.

According to CNN, the research team said it was too early to be sure, but a recent spike in second infections tells them Omicron is more likely to re-infect people.

“Contrary to our expectations and experience with previous variants, we are now experiencing an increased risk of reinfection that is beyond our previous experience,” said Juliet Pulliam of Stellenbosch University.

Omicron was not identified until November, but worried the World Health Organization (WHO) and other global health officials, who pointed to it as a variant of concern because of its numerous mutations affecting areas associated with transmissibility and the ability to evade the immune system.

Pulliam and his colleagues have examined reports of infections covering 2.7 million people in South Africa since the start of the pandemic, including more than 35,000 people diagnosed more than once with Covid-19.

“We have identified 35,670 people with at least two suspected infections (through November 27, 2021), 332 people with a third suspected infection and one person with four suspected infections,” they wrote in their report, posted online in a pre-publication.

“Of those who had more than one re-infection, 47 (14.2%) experienced their third infection in November 2021, suggesting that many third infections are associated with transmission of the Omicron variant,” said they added.

They speculate that the recent increase in cases in South Africa reflects the spread of Omicron and not some other factor such as declining immunity.

The people whose cases they describe did not have the virus sequenced, so it is unclear whether they were in fact infected with the Omicron variant.

However, officials said the Omicron variant is now the dominant coronavirus strain in South Africa, accounting for 74% of genetically sequenced samples as of November. More sequencing is underway to determine the true prevalence of the variant.