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The research, which has not been peer-reviewed, found people who had tested positive for Covid could pick up the virus again.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 did not say how the variant will behave when spreading in a highly vaccinated population 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 new study estimated that the risk of reinfection for the period from November 1 to 27, 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 November 27.
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.”