BA.2.75, nicknamed Centaur, appeared to have mutated in a way that could indicate “major immune escape,” said WHO chief scientist Soumya Swaminathan, adding that it showed a “clear growth advantage.” versus other variants in India. World health authorities issued a similar warning when the highly mutated Omicron variant emerged late last year. The strain has been detected in many countries, including Germany, Japan, the United Kingdom and the United States. Dutch health authorities on Wednesday were the latest to announce they had identified the strain. “We now have 169 sequences from 12 countries — the increase seen is despite the decrease [samples] in [genomic sequencing platform] Gisaid,” said Swaminathan. He stressed that it was still not known whether the variant was more severe, more contagious and immune evasive or just the latter. “Any future variant will have these advantages if it has to compete with previous strains,” he said. A growth advantage could be due to intrinsic properties of the virus or changes in population immunity. The variant was first identified in India and the nickname Centaur was later circulated on Twitter. World health authorities were not involved in providing a name. The WHO, which oversees the process, named Omicron and will not provide another for sub-variation. The strain’s progress comes as health authorities deal with a resurgence of the pandemic, fueled in large part by the BA.5 Omicron subvariant. The WHO said it had seen an “increasing trend” in case reports last week, which was “worrying” given the decline in global coronavirus testing. European health authorities this week recommended wider eligibility for second boosters, saying those over 60 and those who are immunocompromised should get one. US authorities had previously issued a similar summons. “Prudent governments are really looking at their plans,” said Mike Ryan, the WHO’s executive director for health emergencies, referring to authorities’ preparations for an expected surge in infections later this year.

Governments have also taken the unusual step of opening second boosters to wider sections of their population before the Omicron variant vaccines become available later this year. The development of shots of the Omicron variant has been supported by EU and US regulators as well as the WHO, as it seems likely that the coronavirus will continue to evolve. Peter English, a retired public health consultant, said the spread of the variant warranted caution. “The virus is still spreading, very widely. In this way it mutates. Some variants are more fit than previous variants—inherently more transmissible and/or better able to evade immune responses,” he said. “Immunity lasts less than we expected, so people can be infected repeatedly, with cumulative damage.”

Today’s vaccines are modeled after the strain of the coronavirus that first emerged in Wuhan, China, more than two years ago. They continue to maintain high levels of protection against serious disease, but their effectiveness declines as more time passes since the first rounds of vaccinations and the virus continues to evolve. WHO’s Ryan compared the protection afforded by Covid vaccines to a wall protecting a castle. “It weakens, it creates cracks, there are holes in the wall,” he said, adding that immune memory “fades over time.”