British health officials on Friday said they were formally looking into a subvariant of the Delta strain of the coronavirus, after it was seen in a growing number of cases.

The UK Health Security Agency (UKHSA), a public health protection body, said AY.4.2, which was last week seen in six percent of cases, had been “designated as a variant under investigation” but not yet a “variant of concern”.

“The designation was made on the basis that this sub-lineage has become increasingly common in the UK in recent months, and there is some early evidence that it may have an increased growth rate in the UK compared to Delta,” the agency said.

“More evidence is needed to know whether this is due to changes in the virus’ behaviour or to epidemiological conditions,” it added.