A new coronavirus subvariant, XBB. 1.16, has been designated as a "variant under monitoring" by the World Health Organization. The latest omicron offshoot is particularly prevalent in India, where it has sparked a rise in infections, and a return to mask mandates in parts of the country.

Here's what you need to know about the subvariant called Arcturus, which has been documented in 29 countries so far.

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What is Arcturus, the new coronavirus subvariant?

Arcturus was first detected in a sample from January and has now been documented in 29 countries, according to the World Health Organization. It is a subvariant of the omicron variant, which emerged in late 2021 and replaced delta as the dominant variant around the world.

By late February, the Arcturus strain accounted for 0.21 percent of cases around the world. A month later, this had risen to 3.96 percent, according to WHO figures. In the United States, it is estimated to account for 7.2 percent of coronavirus infections for the week ending April 15, according to the Centers for Disease Control and Prevention.

The WHO designated XBB. 1.16 a "variant under monitoring" on March 22. This means the variant has "genetic changes" that could affect its characteristics as a virus, including a possible "growth advantage" over other variants, but the epidemiological impact is not clear. The WHO is monitoring seven variants, including the BA.2 version of omicron seen in many parts of the United States.

A "variant under monitoring" is considered to be of lesser concern than a "variant of interest," which is predicted or known to be more transmissible or virulent, or able to evade antibodies, according to the WHO. The XBB. 1.5 strain, currently the most prevalent subvariant globally, is described as a variant of interest.

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Does the Arcturus mutation make it more dangerous?

According to the WHO, Arcturus is similar to the prevalent XBB. 1.5 variant, but has "one additional mutational mutation in the spike protein, which in lab studies shows increased infectivity, as well as potential increased pathogenicity."

While this may mean that it could spread more quickly, there is not any indication yet that it will lead to more severe cases.

"We've seen this in the past," Paul Hunter, a professor of medicine at the University of East Anglia in Britain, said in an interview. "You look at the virus and it's got mutations that should make it more virulent, but then in reality you don't see that."

He explains that immunity in the body's T cells represents "one of the biggest protections," and yet "we're not seeing much evolution in the parts of the virus that T cells actually attack," meaning that the impact of the mutations may be limited. "There's no evidence that this is any more severe - and probably it's somewhat less severe than previous strains - but it's too early to be certain. And that's almost certainly because of immunity."

"It will probably become the dominant variant for a while in the U.S. and Europe and most countries around the world, but I don't see it driving up severe infections more than we've seen in recent waves," he said.

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What's the situation in India? And where else has Arcturus been reported?

In India, the number of active coronavirus cases has been steadily rising in recent weeks, with almost 50,000 recorded on Friday, according to the country's Health Ministry - compared to 13,509 on March 30.

The country was devastated by a wave of infections involving the delta variant in 2021, which killed tens of thousands of people. Cases have been rising again recently, leading some states to reintroduce mask mandates. Last week, the federal health minister also asked states to increase genome testing and conduct mock drills in hospitals, according to Reuters news agency.

Arcturus has already replaced other variants in India, according to the WHO's March report. However, Maria Van Kerkhove, the WHO's technical lead on COVID-19, said in late March that "we haven't seen a change in severity in individuals or in populations," although the group would "remain vigilant."

According to the CoV-Spectrum website, which uses data from the GISAID Initiative to track coronavirus variants, XBB. 1.16 has been detected in sequenced samples from countries including the United States, Britain, Canada and Australia. The CDC variant tracker shows it circulating at very low levels in more than a dozen states, including California, Colorado, Delaware, Florida, Indiana, Illinois, Iowa, Maryland, Missouri, New York, North Carolina and Ohio.

"It does seem to be spreading more rapidly than any other variant right now, but this is always what happens: A new variant comes along, it spreads quite rapidly for a while, and then it peters out over a period of a few weeks, ultimately to be replaced by the next one," Hunter said.

He added that previous studies showed that many people are benefiting from the hybrid immunity that comes from a combination of previous infections and vaccination, which should offer them better protection against severe disease "for quite a bit longer, probably for a few years" - meaning that, even if they become infected with Arcturus or another variant, they are less likely to require hospital treatment.