The Omicron variant of coronavirus has "substantial" ability to cause reinfection in people who have previously had Covid-19, a study from South Africa suggests.
The research, which has not been peer-reviewed, found people who had tested positive for Covid could pick up the virus again.
It comes as experts said Omicron is fuelling a steep surge in infections in South Africa but relatively few people are being hospitalised, as patients so far reported mild symptoms.
The country recorded 11,535 new cases today, mostly in the epicentre Gauteng, the province home to the biggest city Johannesburg and the capital Pretoria.
That's five times as many cases as were reported just one week ago, when South African scientists alerted the world to the new variant.
Three quarters of new cases in South Africa are now Omicron. But deaths and hospitalisations so far are rising at a much lower rate.
Meanwhile the new Omicron study did not say how the variant will behave when spreading in a highly vaccinated population such as Ireland, or whether the virus can evade the protection offered by vaccines against severe disease.
Breakthrough infections are occurring with other variants in those who have had Covid previously and those who are vaccinated.
The research paper estimated that the risk of reinfection for the period from 1 and 27 November, when Omicron was circulating, was 2.39 higher than in the first wave of the pandemic in early 2020.
In contrast, the risk of reinfection in South Africa was lower in the Beta and Delta waves than in the first wave.
The findings suggest Omicron could cause a wave of infections in people with some prior immunity.
The researchers, from the National Institute for Communicable Diseases (NICD), looked at data from almost 2.8 million people with Covid-19 who had a positive test result at least 90 days before 27 November.
Some 35,670 suspected reinfections were identified among the 2,796,982 people (1.2%).
The authors concluded: "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.
"This finding has important implications for public health planning, particularly in countries like South Africa with high rates of immunity from prior infection.
"Urgent questions remain regarding whether Omicron is also able to evade vaccine-induced immunity and the potential implications of reduced immunity to infection on protection against severe disease and death."
Michael Head, a scientist at the University of Southampton in England, praised the research as "high quality."
"This analysis does look very concerning, with immunity from previous infections being relatively easily bypassed. Might this all still be a 'false alarm'? That is looking less and less likely," he said in a statement.
Earlier, top South African scientist Anne von Gottberg, an expert at the National Institute for Communicable Diseases, forecast a surge in cases but said authorities expected vaccines would still be effective against severe outcomes.
"We believe the number of cases will increase exponentially in all provinces of the country," she said in a news conference with the World Health Organization's Africa region.
"We believe that vaccines will still however protect against severe disease," she added.