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A new strain of the COVID-19 virus, dubbed Delta-plus in the media, is increasingly infecting people in the United Kingdom, raising concerns among scientists that it could rekindle the pandemic here just as the virus seems to be receding.

So far, only a handful of Delta-plus cases have been identified in Massachusetts and only about 130 in the whole United States, vanishingly small numbers that suggest it’s not a major threat. However, scientists say it’s crucial to remain vigilant, saying there’s evidence that the new strain, known officially as Delta AY.4.2, may be 15 percent more transmissible than its parent, the Delta variant.

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And Delta AY.4.2 is so new that there’s a lot scientists don’t know yet. In fact, some lab equipment in this country can’t even detect it.

“It could be growing [in the UK] because it’s more transmissible, or that it just happens to be growing in populations where more transmission is happening, like twentysomethings who are partying and not vaccinated,” said Dr. Nathan Grubaugh, an associate professor of epidemiology at Yale School of Public Health.

Grubaugh’s lab has not yet encountered Delta AY.4.2, and just one such sample has been found in all of Connecticut, according to Outbreak.info, the central database many researchers are using.

Delta AY.4.2 infections have steadily risen in recent weeks in the UK, accounting for about 11 percent of all sequenced COVID-19 cases there, according to Outbreak.info. The UK Health Security Agency, which has one of the best virus surveillance systems in the world, recently declared AY.4.2 to be a Variant Under Investigation , which means it has displayed concerning properties that warrant closer scrutiny.

Agency officials said that more evidence is needed to determine whether infections with the AY.4.2 strain in the UK are growing faster than the more common Delta variant.

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Concern over the new strain of COVID began earlier this month when the UK reported its biggest one-day COVID case increase in three months just as the new subvariant started significantly climbing there.A tweet on Oct. 17 from Scott Gottlieb, former Food and Drug Administration commissioner, about the UK phenomenon generated an intense Twitter conversation among health specialists over the need for better monitoring of COVID variants in this country.

So far, Delta-plus has been detected in at least 30 countries and 33 states in the United States, according to Outbreak.info. But it is still less than 1 percent of the cases sequenced in this country. It is not listed as a variant of concern or interest by the Centers for Disease Control and Prevention, which means the agency has not seen evidence that it is more transmissible, or causes more severe illness, or could reduce the effectiveness of vaccines or other treatments.

The new strain can be difficult to detect. US researchers discovered in late summer that the solution many labs use to prepare the virus samples for sequencing was causing many to miss the AY.4.2 mutation. Many labs, such as Grubaugh’s, have updated or are in the process, he said.

Now, they’re waiting and watching for the new strain. Scientists from Yale and the Broad Institute in Cambridge are scrutinizing samples in the New England region and meet weekly with federal regulators and other public health officials.

“If there are technical glitches, we talk about them to make sure we are detecting anything that is emerging,” Grubaugh said.

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In particular, scientists are closely watching the UK to see whether the new relative of the Delta variant is a more dangerous form of COVID.

“There is nothing showing that it is a significant public health concern yet,” Grubaugh said.

Viruses mutate all the time, and already scientists have detected at least 47 mutations in Delta alone, said Dr. Wesley Long, an associate professor of pathology and genomic medicine at Houston Methodist, which has not yet seen its first case of AY.4.2.

Most viral mutations don’t matter to public health, but sometimes a new form can be more infectious or hard to stop. The Delta variant, which emerged earlier this year in the United States, proved about 50 percent more infectious than the COVID strain it replaced, and within months of its arrival became the dominant strain here.

Long, whose lab equipment has been updated to detect the new AY.4.2 strain, said disease watchers want to know if the new strain is different enough to present a greater threat. “It becomes a waiting game to see if [AY.4.2] really has a fitness advantage, or transmission advantage, over all the other Delta subvariants,” he said.

Dr Amesh Adalja, who studies emerging infectious diseases as a senior scholar at the Johns Hopkins Center for Health Security, calls AY.4.2 a puzzle.

“Although it is increasing in the UK, it is decreasing in other places, it hasn’t gotten a foothold,” Adalja said. “The overarching point is that we have to continue to be alert to new variants to understand what they may represent.”

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Despite the uncertainty about AY.4.2, scientists agree that vaccines and other tools in the anti-COVID arsenal, including masks, physical distancing, and avoiding large crowds, are the public’s best defense against COVID, including the AY.4.2 strain.

Dr. Pedro A. Piedra, professor of molecular virology and microbiology at Baylor College of Medicine in Houston, said vaccinating as much of the world as possible as quickly as possible is critical to “declawing” COVID.

“But at the end of the day,” he said, “we have to wait to see what this virus will decide to do.”



Kay Lazar can be reached at kay.lazar@globe.com Follow her on Twitter @GlobeKayLazar.