(NEXSTAR) – A new offshoot of the COVID-19 delta variant was responsible for approximately 6% of cases in the United Kingdom as of late September, according to officials with the U.K. Health Security Agency.

The subvariant, known as AY.4.2, is not yet a variant of concern (VOC) or even a variant of interest (VOI), though officials confirmed that cases appeared to be on an “increasing” trajectory in an Oct. 15 report.

“A Delta sublineage newly designated as AY.4.2 is noted to be expanding in England,” the UK Health Security Agency wrote in its report. “It is now a signal in monitoring and assessment has commenced.”

Researchers have determined that AY.4.2 contains two spike mutations — A222V and Y145H — that have already been observed in other variants. Neither of those mutations, however, had been observed in any variant deemed to be a VOC, or variant of concern.

Commenting on the mutations, Prof. Francois Balloux of the UCL Genetics Institute in London told the U.K.’s Science Media Centre that the presence of both A222V and Y145H does not necessarily mean AY.4.2 is more transmissible than other variants, though he said it’s possible.  

“Neither mutation is a priori an obvious candidate for increased viral transmissibility, but we have learnt that mutations can have different, sometimes unexpected, effects in different strains,” said Balloux on Tuesday.

On Twitter, Balloux further noted that AY.4.2 might actually be “intrinsically more transmissible,” citing its unchanged trajectory of rising cases, as well as the areas of the UK where cases have been observed. (Infections don’t seem “region-specific,” he said).

Outside of the U.K. and Denmark, cases of AY.4.2 currently appear to be rarer, though reports indicate cases have now been observed in the U.S., Canada, and Israel.

Still, concerns are rising in the U.S. Former FDA commissioner Scott Gottlieb tweeted on Sunday that “urgent research” is needed to determine the transmissibility of the subvariant, as well as its possible resistance to vaccines.

“This is not a cause for immediate concern but a reminder that we need robust systems to identify, characterize new variants,” Gottlieb added. “This needs to be a coordinated, global priority for COVID, same as similar international efforts have become standard practice in influenza.”

Remarking on the release of the U.K. Health Security Agency’s findings, a representative for the prime minister has said the government is also “keeping a very close eye on” the numbers.

“There’s no evidence [to suggest increased transmissibility], but as you would expect, we’re monitoring it closely and won’t hesitate to take action if necessary,” the representative told Sky News.

While it’s unclear for certain whether the AY.4.2 subvariant is more transmissible, U.K. health officials are investigating the possibility of transmissibility and immunity-evading traits.

Balloux, for one, does not yet believe AY.4.2 is an “obvious” candidate for elevation to a VOC.

“The emergence of yet another more transmissible strain would be suboptimal,” he told the Science Media Centre. “Though, this is not a situation comparable to the emergence of Alpha and Delta that were far more transmissible (50% or more) than any strain in circulation at the time. Here we are dealing with a potential small increase in transmissibility that would not have a comparable impact on the pandemic.”