The coronavirus variant that has spread catastrophically in India has seeded itself in dozens of countries, and the World Health Organization has declared it a “variant of concern,” citing preliminary evidence that it is more transmissible than some earlier strains of the virus.

It is not clear to what degree the crisis in India — which reported 4,200 deaths on Wednesday alone — has been accelerated by the emergence of this variant, known as B.1.617. It is possible the main driver of the outbreak has been mass gatherings in a densely populated nation that still has low levels of vaccination.

But the WHO, which previously categorized the variant as being “of interest,” Monday elevated it to the status “of concern.” WHO epidemiologist Maria Van Kerkhove noted that any of the strains of the virus, SARS-CoV-2, are infectious, “and everything in that sense is of concern.”

She added, “So all of us at home, no matter where we live, no matter what viruses are circulating, we need to make sure that we take all of the measures at hand to prevent ourselves from getting sick.”

Although the WHO treats B.1.617 as a single variant, it has already splintered into three versions, called sub-lineages, with slightly different suites of mutations. Britain last week declared B.1.617.2, which is spreading quickly there, a variant of concern.

In the United States, that variant accounts for about 3 percent of cases but is gaining traction, according to Centers for Disease Control and Prevention surveillance. The CDC lists all three B.1.617 sub-lineages as variants of interest.

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The spread of variants of concern across the planet has raised fears the virus will find ways to remain elusive, potentially circumventing vaccines. All viruses mutate, and SARS-CoV-2 does not mutate particularly quickly compared with other viruses. But having spread explosively, it has had abundant opportunities to shape-shift through random mutation.

Some of the mutations affect the spike protein on the surface of the virus and enable it to bind more easily to cells. This makes the virus more “fit,” as scientists put it.

According to new CDC data, the highly transmissible B.1.1.7 variant first seen in Britain now accounts for an estimated 72% of new infections in the United States. A variant first seen in Brazil, named P.1, accounts for 6% of new U.S. cases, a rapid increase in recent weeks, the CDC data shows. Other variants, including ones first seen in South Africa and California, have decreased in prevalence, CDC epidemiologist Heather Scobie said Wednesday at a meeting of the agency’s Advisory Committee on Immunization Practices.

Scientists are conducting laboratory tests and studying epidemiological data to determine whether and to what degree the variants, including the B.1.617 variants first seen in India, pose threats above and beyond those posed by earlier strains. They are asking many questions simultaneously: How transmissible is the variant? Can it cause a different level of disease? Can it be detected through testing? How does it respond to treatments? Can it be blocked by vaccines or by natural immunity acquired through infection?

A paper posted online Monday, and not yet peer-reviewed or published in a scientific journal, reported that the Moderna and Pfizer-BioNTech vaccines produced fewer neutralizing antibodies against one of the India variants, B.1.617.1. But there were still enough antibodies for the vaccines to block infection.

This is akin to trying to defend a fortress against an enemy by building very high walls. The immunization wall isn’t as high in places threatened by some variants. But it’s still high enough to do the job. (To extend the metaphor: The walls are made of ice and gradually melt, lowering the defenses.)

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Mehul Suthar, a viral immunologist at Emory University and senior author of the paper, said the laboratory tests on B.1.617.1 were generally encouraging, suggesting the vaccines should be effective against many variants.

“It’s great, and it bodes well for how well these vaccines are working in being able to create a breadth of antibodies that target these variants,” Suthar said. “These vaccines are still working very well at protecting against serious disease and death.”

So far, no single variant circulating globally seems capable of escaping fully the immune responses induced by vaccines.

“The more the virus infects, replicates, spreads, the more it gives this virus a chance to mutate and continue to evade antibody responses,” Suthar said. But he added the virus does not have an infinite number of ways to mutate and still be able to infect cells.

Scientists this week made clear they are still early in their investigations of the India variants and are working with limited data that may be subject to sampling biases.

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“The data from India are not really good enough to be able to say anything, other than that 617 and relatives are plainly common there. We don’t know how common because we don’t have a good enough sample,” William Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health, said in an email. He said the decision by British authorities to call B.1.617.2 a variant of concern is based on infection numbers that are still very small.

Jeremy Luban, a virologist at the University of Massachusetts Medical School, said the variant spreading in India appears to be highly transmissible but cautioned that it is too early to make definitive statements.

“I think if you’re vaccinated, you’re going to have a pretty decent level of protection,” Luban said. “The problem is, if there are places where people are not vaccinated or where the virus is spreading, it looks like it’s going to do it faster. It might end up putting more people in the hospital faster and overloading the infrastructure.”