Researchers in South Africa reported Thursday they have found some evidence that people who had been infected once with coronavirus were more likely to be reinfected with the Omicron variant than with the Beta or Delta variants.

They said it's too soon to know for sure, but a recent spike in second infections indicates to them that Omicron is more likely to reinfect people.

"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," Juliet Pulliam, director of the South African Centre for Epidemiological Modelling and Analysis at Stellenbosch University, who helped lead the study team, said in a statement.

Omicron was only identified in November but has worried World Health Organization and other global health officials, who have designated it a variant of concern because of its many mutations affecting areas associated with transmissibility and ability to evade the immune system.

Pulliam and her colleagues looked at reports of infections covering 2.7 million people in South Africa since the beginning of the pandemic, including more than 35,000 people diagnosed more than once with Covid-19.

"We identified 35,670 individuals with at least two suspected infections (through 27 November 2021), 332 individuals with suspected third infections, and 1 individual with four suspected infections," they wrote in their report, posted online in a preprint, meaning it has not been peer-reviewed or published in a medical journal.

"Among the individuals who have had more than one reinfection, 47 (14.2%) experienced their third infection in November 2021, which suggests that many third infections are associated with transmission of the Omicron variant."

They are assuming the recent uptick in cases in South Africa reflects the spread of Omicron and not some other factor such as waning immunity. The people whose cases they describe have not had the virus sequenced, so it's not certain they were, in fact, infected with the Omicron variant.

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

"The timing of these changes strongly suggests that they are driven by the emergence of the Omicron variant," the researchers wrote.

Before Omicron, the Delta variant had been the most prevalent variant in South Africa, and it remains the dominant variant globally. More data should make it clear if Omicron is, in fact, driving reinfections in South Africa. Public health officials should know by mid-December, they said.

"Population-level evidence suggests that the Omicron variant is associated with substantial ability to evade immunity from prior infection. In contrast, there is no population-wide epidemiological evidence of immune escape associated with the Beta or Delta variants," they wrote.

This finding has important implications for public health, especially in countries like South Africa, where there are high rates of immunity from prior infection. This study did not take vaccination into account, nor could it account for possible waning of immunity over time.

"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," Harry Moultrie, senior medical epidemiologist in the Center for Tuberculosis at the National Institute for Communicable Diseases in South Africa and a co-author of the paper, said in a statement.

Michael Head, senior research fellow in global health at University of Southampton, said the situation may be different when it comes to immunity from vaccines.

"The immune response from vaccination is much stronger when compared with infection-acquired immunity. Whilst there is likely to be some impact, it is likely vaccines will still provide some level of protection," Head said in a statement to the Science Media Center in the UK. "The booster dose may be key here in maintaining a high level of protection.

"Whilst we await more data to emerge over the coming days and weeks, the message to the general public has to be -- go and get all the doses you are eligible for. Keep that protection as high as possible."

The findings may mean natural infection will not help build herd immunity, some experts said.

"Omicron has blown a big hole in the controversial argument that we should simply allow the infection to spread in an attempt to create immunity," microbiologist Simon Clarke of the UK's University of Reading said in a statement.

"We await a further indication as to whether Omicron has any ability to evade vaccine induced immunity."