Health chiefs have revealed they are officially monitoring a new version of Covid-19 – which has been nicknamed “stealth Omicron”.

       The UK Health Security Agency has marked BA.2 a “variant under investigation” – one level below a “variant of concern” – after early data suggested it may be both more transmissible and better able to evade vaccines than previous strains of the killer virus.

       It is a sub-lineage of the original Omicron – BA.1 – but appears to have certain differences that may make it both faster at spreading and harder to detect.

       According to the World Health Organisation, it is now probably outpacing the earlier strain with some 8,000 cases identified in more than 40 countries, including the US, India, Germany and Australia.

       In the UK, some 426 cases have been detected since the new lineage was first designated in December. Although that number may seem small in comparison with the tens of thousand of daily cases of Covid-19 over the last two months, it is thought the real figure of BA.2 will be many times higher because only a fraction of infections are checked for variations.

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       Its stealth nickname comes from one of its key differences with the original Omicron.

       That variant was relatively easy to track because of a specific quirk – the deletion of a spike gene – made it stand out on the widely-used PCR tests without the need for extra genome sequencing.

       But the new strain does not appear to have this feature making it more difficult to monitor.

       It means that while PCR tests will still spot if someone has this version of Covid-19, the samples would need to be sent for further lab analysis to determine if someone had ‘stealth’.

       Even so, while health chiefs will now work round the clock analysing BA.2, there appears little fear that it will cause another wave of infections.

       Dr Tom Peacock, one of the first virologists to raise the alarm over Omicron, said “Even with slightly higher transmissibility this absolutely is not a Delta to Omicron change, and instead is likely to be slower and more subtle.

       “That said, I would not be surprised if BA.2 slowly replaces [Omicron] over the coming months with a slightly more "optimised" mutations.'

       The scientist, from Imperial College London, told the Daily Mail: “Very early observations from India and Denmark suggest there is no dramatic difference in severity.”

       Dr Meera Chand, the UKHSA's Covid incident director, said an altered form of Omicron was not unexpected because, by their very nature, viruses are constantly evolving.

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       She said: “So far there is insufficient evidence to determine whether BA.2 causes more severe illness than BA.1, but data is limited and UKHSA continues to investigate.”

       More data will allow more accurate conclusions to be drawn within a week or two, she added.