The newest subvariant of the Omicron variant, XBB.1.16, also known as Arcturus, has been detected in over 20 countries, with India being one of the hardest hit. XBB.1.16 is a descendant of the BA.2.10.1 sub-lineage of the B.1.617.2 Delta variant, which was first identified in India in December 2020 and rapidly spread to other nations.
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The WHO has been closely monitoring XBB.1.16 since March 22. According to Dr. Maria Van Kerkhove, technical lead for COVID-19 response at the WHO, XBB.1.16 is of particular concern due to its high infectivity and pathogenicity. It is a recombinant from BA.2.10.1 and BA.2.75 that has three additional mutations (E180V, F486P and K478R) in the spike protein compared to its parent lineage, XBB. One of these mutations, K478R, is of particular interest, as it is also present in the Delta variant. K478R involves a substitution of the amino acid lysine (K) for arginine (R) at position 478 in the spike protein, and it's located in the receptor-binding domain (RBD) of the spike protein, which is responsible for binding to the ACE2 receptor on human cells.

XBB.1.16 has been reported in 18 states in the U.S., including California, Delaware, Washington, New Jersey, New York, and Texas. According to a tracker run by Dr. Rajendram Rajnarayanan of the New York Institute of Technology, XBB.1.16 comprises 4.6% of current COVID-19 cases in the U.S. However, XBB.1.5, another subvariant of the Omicron variant, is currently driving the vast majority of new COVID-19 cases in the U.S.

While some people may experience mild or no symptoms from XBB.1.16, it's important to remember that there is still a risk of developing long-term health issues. Research suggests that up to 30% of people who contract COVID-19 may experience ongoing symptoms that can last for months or longer. Therefore, it's crucial that we continue to prioritize our health and take steps to protect ourselves and those around us.