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The World Health Organization (WHO) is monitoring XBB.1.16, an Omicron subvariant that has been detected in over 20 countries and contributing to a recent surge of COVID cases in India.

Known as "Arcturus," XBB.1.16 has been listed as a since March 22, with 800 sequences of the Omicron subvariant currently analyzed across 22 countries.

"Most of the sequences are from India and XBB.1.16 has replaced the other variants that are in circulation, so this is one to watch," said Maria Van Kerkhove, PhD, technical lead for COVID-19 response at the WHO, during a last week.

XBB.1.16 has high infectivity and pathogenicity, Van Kerkhove noted.

In the U.S., XBB.1.16 has been reported in several states, including California, Washington, New Jersey, New York, Virginia, and Texas, according to run by Rajendram Rajnarayanan, MSc, PhD, of the New York Institute of Technology. On , he estimated that the lineage comprises 2.9% of current U.S. cases.

Still, despite contributing to a spike in cases in South-East Asia in recent weeks, the WHO reported that the death count in the region has gone down by 6% in the last 4 weeks.

"So far reports do not indicate a rise in hospitalizations, ICU admissions, or deaths due to XBB.1.16," the WHO report stated. "Further, there are currently no reported laboratory studies on markers of disease severity for XBB.1.16."

However, according to Van Kerkhove, "we have to remain vigilant."

While she pointed out that there hasn't been a change in severity, "we have to have systems that are in place that have strong surveillance, so that we can track variants -- the known variants that are in circulation, and to detect new ones so that we can have agile systems to scale up or scale down the need for clinical care, making sure that we have good antivirals that are in use and given to patients who need them when they need them to prevent severe disease."

XBB.1.16 is a recombinant variant from BA.2.10.1 and BA.2.75. It has three additional mutations in the SARS-CoV-2 spike protein (E180V, F486P, and K478R) compared with its parent lineage, XBB. It is very similar in profile to XBB.1.5, which currently comprises 85% of U.S. cases and 45% of global cases.

While XBB.1.16 and XBB.1.5 both share a common parent and common mutation in F486P, it is the K478R mutation in XBB.1.16 that appears to be responsible for the spike in cases in India, the WHO said.

"Mutations at position 478 of the SARS-CoV-2 spike protein have been associated with decreased antibody neutralization, increased transmissibility, and pathogenicity," the WHO explained in the update.

According to the , "recombinant viral variants can occur when a single person is infected with multiple distinct variants at the same time, allowing the two different variants to interact during replication. When their genetic materials mix they create a new hybrid, or a recombinant variant. There's nothing inherently 'worse' about recombinant variants -- they could be more or less fit than their parents, or have the same fitness."

Although XBB.1.16 is currently making headlines worldwide, other variants, including XBB.1.9.1, are also currently on both the WHO's and CDC's radar.

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