• A newly-discovered mutation of the delta variant is under investigation in the U.K.
• There are worries that it could make the virus yet more transmissible, and could possibly undermine Covid-19 vaccines further.
• Still, there are many unknowns surrounding this new descendent of the delta variant and it has not been dubbed a "variant of concern" yet.

LONDON — A newly-discovered mutation of the delta variant is being investigated in the U.K. amid worries that it could make the virus even more transmissible and undermine Covid-19 vaccines further.

Still, there are many unknowns surrounding this descendent or subtype of the delta variant — formally known as AY.4.2 — which some are dubbing the new "delta plus" variant.

U.K. government health officials have said it's too early to tell whether the mutation poses a greater risk to public health than the delta variant, which itself is significantly more infectious than the original Covid-19 strain (and its successor, the alpha variant).

But they have stated that they are monitoring the mutation very closely: it now accounts for 6% of U.K. Covid cases that have been genetically sequenced at a time when infections in the country are rising rapidly.

Here's what we know, and don't know, about the variant:

What is the new variant?

Viruses constantly mutate and the coronavirus that emerged in China in late 2019 has gone through multiple minor variations that have made it virus more infectious and effective at spreading. This was first seen with the alpha variant (first sequenced in the U.K.) that went on to spread globally before it was usurped by the even more transmissible delta variant that was first discovered in India.

Delta, which was dubbed a "variant of concern" by the World Health Organization in May, remains globally dominant.

But last Friday, the U.K.'s Health Security Agency issued a report in which it said "a delta sublineage newly designated as AY.4.2 is noted to be expanding in England." The agency said it was monitoring the subtype, which includes mutations to the spike protein (A222V and Y145H) that the coronavirus uses to enter our cells.

Why is it being monitored?

AY.4.2 is being identified in an increasing number of U.K. Covid cases, with some suggesting it could be a factor in the country's growing health crisis that has prompted some doctors to call for Covid restrictions to be reimposed.

Read more: UK doctors call for urgent return of Covid restrictions as experts monitor new mutation

"This sublineage is currently increasing in frequency," the U.K.'s Health Security Agency said last week, noting that "in the week beginning 27 September 2021 (the last week with complete sequencing data), this sublineage accounted for approximately 6% of all sequences generated, on an increasing trajectory. This estimate may be imprecise ... Further assessment is underway."

Read more: The UK has one of the highest Covid infection rates in the world right now: Here’s why

The U.K. is currently seeing a prolonged and worrying spike in Covid cases, reporting between 40,000-50,000 new infections per day in the last week, prompting experts to question why the U.K. is so vulnerable to Covid right now.

The delta subtype is reported to be 10-15% more transmissible than the standard delta variant, but it is too early to say for certain whether it has been causing a spike in cases in the U.K.

Why does it matter?

It's worth remembering that although AY.4.2 is being monitored, it has not been classified as a "variant under investigation" or a "variant of concern" by the WHO — that is, it has not been identified as having genetic changes that are expected to affect virus characteristics such as transmissibility, disease severity, immune escape, diagnostic or therapeutic escape.

It also has not been confirmed that it causes significant community transmission or multiple Covid-19 clusters.

Still, that status could change following further monitoring and if it continues to be sequenced in an increasing number of cases.

Finding a potentially more transmissible variant matters because it could cause more Covid cases among the unvaccinated.

A large part of the world remains unvaccinated (only 2.8% of people in low-income countries have received at least one dose of a Covid vaccine, according to Our World in Data) while developed countries are seeing more and more "breakthrough" cases as immunity to Covid wanes around six months after being fully vaccinated.

A more infectious variant could undermine vaccine efficacy even further, although there is no indicated that is the case yet with the AY.4.2 subtype.

What do experts say?

Health officials are remaining calm about the delta subtype, for now, noting that it's crucial to keep an eye on the mutation but not to panic.

Commenting on "delta plus" on Wednesday, U.S. CDC Director Rochelle Walensky noted that "in particular the AY.4.2 variant has drawn some attention in recent days." She added that: "we have, on occasion, identified this sub lineage here in the United States, but not with recent increase frequency or clustering, to date."

As well as the U.S., Israel said it had confirmed a case of the AY.4.2 variant in an 11 year-old boy entering the country at Ben Gurion airport. On Thursday, Russia also said it had registered some isolated cases of the AY.4.2 variant. It's unknown to what extent, if any, the subtype has been found in mainland Europe.

The U.K. prime minister's official spokesman called for calm on Tuesday, telling Sky News that "[AY.4.2] is something we're keeping a very close eye on," but stressing that there was currently no evidence to suggest that this variant is more easily spread.

"There's no evidence for that, but as you would expect, we're monitoring it closely and won't hesitate to take action if necessary," he added.

U.K. government officials are very reluctant to reimpose Covid restrictions, despite calls from health professionals to do so as British hospitals face being overwhelmed by demand as winter approaches.

Andrew Pollard, head of the Oxford Vaccine Group, which helped to developed the AstraZeneca-Oxford University vaccine, said on Wednesday that the delta subvariant won't change Covid picture.

"Discovery of new variants is of course important to monitor, but it doesn't indicate that that new variant is going to be the next one to replace delta," Pollard told BBC radio, Reuters reported.

"Indeed even if it does, delta is incredibly good at transmitting in a vaccinated population and a new one may be a bit better but it's unlikely to change the picture dramatically from where we are today."

Meanwhile, professor of immunology at Imperial College London, Danny Altmann, told CNBC Monday that the subtype "needs to be monitored and, so far as possible, carefully controlled."

"Because delta has now been the dominant mutant in several regions for some six months and not been displaced by any other variants, the hope has been that delta perhaps represented [the] peak mutation performance achievable by the virus. AY.4 may be starting to raise doubts about this assertion," he warned.