Early data indicates the Omicron Covid variant may more easily reinfect people who have already had the virus or been vaccinated than previous variants, but could also cause milder disease, the World Health Organisation (WHO) said.

“Emerging data from South Africa suggests increased risk of reinfection with Omicron,” WHO chief Tedros Adhanom Ghebreyesus told reporters, adding that “there is also some evidence that Omicron causes milder disease than Delta”.

But he stressed that more data was needed before drawing firm conclusions, and urged countries everywhere to boost their surveillance to help provide a clearer picture of how Omicron is behaving.

Even if it does turn out that Omicron causes less severe disease, Tedros warned against slacking off vigilance against the virus. “Any complacency now will cost lives,” he warned.