The World Health Organisation says the latest research shows that Stealth Covid, a mutation of Omicron, spreads faster and could cause more severe disease than Delta or Omicron.
And it is possible to catch Stealth even if you have had Omicron.
WHO's Technical Advisory Group on SARS-CoV-2 Virus Evolution ( TAG-VE ) met to discuss the latest evidence on the Omicron variant of concern, including its sublineages BA.1 and BA.2 - also known as Stealth Covid because it is not easily detectable with standard testing.
WHO says BA.2 should continue to be considered a variant of concern.
Cases of BA.2, also known as Nextstrain clade 21L, have been increasing across the world in recent weeks.
Studies have shown that BA.2 appears inherently more transmissible than Omicron - and there have been cases of people catching BA.2 even if they already had Omicron.
The TAG-VE also looked at preliminary laboratory data from Japan generated using animal models without any immunity to Covid which highlighted that BA.2 may cause more severe disease in hamsters compared to BA.1.
They also considered real-world data on clinical severity from South Africa, the United Kingdom, and Denmark, where immunity from vaccination or natural infection is high: in this data, there was no reported difference in severity between BA.2 and BA.1.
