The UK Health Security Agency on Friday said it had designated a sub-variant of the highly-transmissible Omicron coronavirus strain as a “variant under investigation".
BA.2, which does not have the specific mutation seen with Omicron that can easily distinguish it from Delta, is being investigated but has not been given the more serious designation of “variant of concern".
“It is the nature of viruses to evolve and mutate, so it's to be expected that we will continue to see new variants emerge,” Dr Meera Chand, incident director at the UKHSA, said.
“Our continued genomic surveillance allows us to detect them and assess whether they are significant.”
Britain has so far sequenced 426 cases of the BA.2 sub-lineage, and the UKHSA said that while there was uncertainty around the significance of the changes to the viral genome, early analysis suggested an increased growth rate compared to the original Omicron lineage, BA.1.
UKHSA said that 40 countries had reported BA.2 sequences, with the most samples reported in Denmark, followed by India, Britain, Sweden and Singapore.
The Statens Serum Institut (SSI), a disease research centre in Denmark, said there are now three known sub-variants of Omicron: BA.1, BA.2 and BA.3.
Initial studies showed that instances of the BA.2 sub-variant have risen rapidly since the start of 2022, and now account for almost half of all Omicron cases in the country.
SSI researchers said the difference between BA.1 and BA.2 is greater than the difference between the original variant and the Alpha variant. The two sub-variants also exhibit “many differences in their mutations in the most important areas”, they said.
Anders Fomsgaard, researcher at the SSI, said he did not yet have a good explanation for the rapid growth of the sub-lineage, adding he was puzzled, but not worried.
"It may be that it is more resistant to the immunity in the population, which allows it to infect more. We do not know yet," he told broadcaster TV 2, adding that there was a possibility that people infected with the original Omicron, called BA.1, might not be immune from then catching BA.2 soon after.
"It is a possibility," he said. "In that case, we must be prepared for it. And then, in fact, we might see two peaks of this epidemic."
Early analysis shows little differences in hospital admissions for BA.2 compared to BA.1 and that vaccines also seem to have an effect against severe illness upon BA.2 infection.
Meanwhile, the UK government said there were 95,787 new cases of Covid-19 on Friday.
The Government also said a further 288 people have died within 28 days of testing positive for Covid-19, bringing the total to 153,490.Separate figures published by the Office for National Statistics show there have been 177,000 deaths registered in the UK where Covid-19 was mentioned on the death certificate.