A new COVID-19 subvariant that is spreading rapidly in the United Kingdom is already here in Massachusetts, health experts say.
Known as AY.4.2, the so-called "delta plus" variant is believed to be even more contagious than the highly transmissable delta variant. One expert said it could be 10% to 15% more contagious than delta.
So far, the mutation has been detected in Massachusetts and some other U.S. states, according to Outbreak.info.
"Ever since at least about May or June here in the U.S., the vast majority, so almost 100% of the virus that we are seeing here, is the delta variant," said Dr. Mark Siedner of Massachusetts General Hospital. "Now the delta variant is starting to change a bit. New offshoots, almost like grandchildren."
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Concerns about delta plus are currently very high in the U.K., which is seeing a surge in cases. The new subvariant had accounted for about 6% of all cases and is being watched carefully by the U.S. Centers for Disease Control and Prevention.
But 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.
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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.
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.
"I think it's a little early to say what the significance of this so-called delta plus variant is," said Dr. Daniel Kuritzkes of Brigham and Women's Hospital. "But I don't think there is cause for concern at this point that they are either resistant to the vaccine or more likely to be transmitted than any other of the delta variants."
Commenting on "delta plus" last week, 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."
For the moment, delta plus bears watching in large part because the delta variant has proven so virulent.
"This is an extremely contagious variant," Siedner said. "The vaccines are still working against it, and we're going to need more information before we are convinced that anything new has come along."