The UK Health Security Agency has designated a sub-lineage of the dominant and highly transmissible Omicron coronavirus variant as a variant under investigation, saying it could have a growth advantage.

BA.2, which does not have the specific mutation seen with Omicron that can help to easily distinguish it from Delta, is being investigated but has not been designated a 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."

The United Kingdom has 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, the UK, Sweden and Singapore.

In Denmark, BA.2 has grown rapidly - it accounted for 20 per cent of all COVID-19 cases in the last week of 2021, rising to 45 per cent in the second week of 2022.

Anders Fomsgaard, researcher at Statens Serum Institut (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 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."

Initial analysis made by Denmark's SSI showed no difference in hospitalisations for BA.2 compared to BA.1.