The UK Health Security Agency has designated the Omicron variant sub-lineage, known as BA.2, as a variant under investigation – with current case rates very low.
Just 53 sequences of the sub-lineage had been identified in the UK by 10 January, the HSA said.
The health body said further analysis will be undertaken.
However, initial analysis from Denmark – where it makes up around half of Omicron cases – shows no difference in hospitalisations between BA.1 and BA.2.
Vaccines are also expected to be effective against BA.2 in preventing severe illness.
BA.2 is S-gene positive (SGTP) – making it harder to identify as Omicron than the original, dominant strain, BA.1, which accounts for 99% of cases.
S-gene target failure (SGTF) is a useful indicator of Omicron, because as a rule Delta cases have the S-gene and Omicron cases do not.
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The HSA said last week that 2,093 sequences of BA.2 had been recorded on an online database from 22 countries including the UK.
Denmark’s Statens Serum Institut, part of its ministry of health, said this week that hospitalisations appeared to show no difference for the two Omicron lineages but that “analyses regarding infectiousness and vaccine efficiency etc. are ongoing”.
“It is expected that vaccines also have an effect against severe illness upon BA.2 infection,” it added.