A slightly mutated version of the delta coronavirus variant, which some observers have nicknamed "delta plus," is gaining prominence in the United Kingdom—but several infectious disease experts suggest it's unlikely to drive case surges as severe as those associated with the original delta variant.

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This isn't the first so-called 'delta plus'

According to NPR, since the original delta variant was first identified last year in India, it has continued to evolve and mutate—and both public health experts and the media have informally referred to several subvariants as "delta plus." For example, in June, a notable "delta plus" subvariant with a spike protein mutation was identified in India.

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This latest delta plus subvariant, which is more formally known as AY.4.2, was first identified by British scientists last month, AP/Modern Healthcare reports, and has two mutations on its spike protein. It has been found in 27 countries, including the United States, Canada, and Australia, and in parts of western Europe.

However, data from the GISAID virus reporting database indicates that AY.4.2 has become prominent only in the United Kingdom. As of Oct. 18, around 14,247 out of 14,970 delta plus cases that have been sequenced worldwide were from the U.K., Newsweek reports.

According to U.K. officials, the AY.4.2 variant currently makes up 6% of all analyzed Covid-19 cases in the country and is "on an increasing trajectory." In addition, early evidence from the U.K. Health Security Agency suggests that its growth rate may be higher than that of the original delta variant.

According to Newsweek, AY.4.2 is not currently considered a variant of interest or concern by the United Kingdom, World Health Organization, or CDC, although the United Kingdom has labeled it a "variant under investigation."

Should people be worried about AY.4.2?

Several health experts have said that, while it's too early to say for sure, AY.4.2 is not likely to be significantly more transmissible than the original delta variant, NPR reports.

William Hanage, an epidemiologist at Harvard University, estimates that the AY.4.2 variant is around 10% more transmissible than the original delta variant. However, according to NPR, because the delta variant is already highly transmissible, this increased transmissibility likely won't cause much of a difference.

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"[T]his 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," said Francois Balloux, director of the Genetics Institute at University College London. "Here we are dealing with a potential small increase in transmissibility that would not have a comparable impact on the pandemic."

For that reason, some health experts said the AY.4.2 variant probably won't cause Covid-19 cases to surge in the United States like the original delta variant did during the summer, NPR reports.

"There's no obvious sign that it's going to take off the way that delta did," said Jeremy Luban, a virologist at the University of Massachusetts Medical School. "I think if you polled people who are following these variants, most of them would tell you that AY.4.2 is unlikely to be a new delta. It's unlikely to totally just take over. It's more likely to be a slow, creeping increase in cases."

And Jeremy Lessler, an epidemiologist at the University of North Carolina, who has developed models for the Covid-19 Scenario Modeling Hub, said, "Even with a variant that's way more transmissible than delta-plus, cases [in the simulated models] didn't go back to the types of peaks we saw last winter or even the types of peaks we've seen in the delta wave."

Separately, former FDA commissioner Scott Gottlieb said the newest delta plus variant is not contagious enough to affect the pandemic's current trajectory in the United States.

"I don't think this is enough to really change the trajectory of the direction we're heading in," Gottlieb said. "We're much closer to the end of this delta wave than we are to the beginning." He added that Covid-19 vaccines should be protective against the AY.4.2 variant and that it is unlikely to spread as widely as the original delta variant.

According to Newsweek, there have currently been only three reported cases of the AY.4.2 variant in the United States. However, CDC Director Rochelle Walensky said the agency is "following the genomic sequencing of this [AY.4.2 variant] very carefully," particularly as cases of the variant continue to rise in the U.K. (Browne, Newsweek, 10/19 [1]; Browne, Newsweek, 10/19 [2]; AP/Modern Healthcare, 10/22; AP/U.S. News & World Report, 10/22; Doucleff, NPR, 10/22; Vakil, The Hill, 10/24; Golgowski, Huffington Post, 10/24)