The British Health Security Agency said on Friday that it had designated a sub-lineage of the dominant and highly transmissible Omicron coronavirus strain as a variant under investigation.

BA.2, which does not have the specific mutation seen with Omicron that can be used as a proxy 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 had identified 53 sequences of the BA.2 sub-lineage as of 10 January, with updated figures due to be published later on Friday.

In Denmark, BA.2 has grown rapidly. It accounted for 20 percent of all COVID-19 cases in the last week of 2021, rising to 45 percent 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 said, 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."

(With input from agencies)