Health authorities say they have been tracking a handful of new SARS-CoV-2 variants climbing in prevalence around the country, including the XBB.1.16 sublineage. That strain has been dubbed by some scientists on social media as "Arcturus," to the frustration of some health officials.
Scientists say XBB.1.16 is relatively similar to previous strains that have recently been dominant in the United States, including thethat drove the last wave of infections this past fall and winter, and does not seem to be leading to worse or different symptoms compared to other variants.
"We have not observed a dramatic shift in spike since that Delta to Omicron shift," the Centers for Disease Control and Prevention's Dr. Natalie Thornburg said at an April 27 vaccines meeting hosted by the FDA.
Thornburg said XBB.1.16 has only a handful of mutations on SARS-CoV-2's spike protein compared with its recent predecessors, fewer than the more than two dozen changes seen when the original Omicron strains overtook other variants earlier in the pandemic.
XBB.1.16 could be on track to outcompete another variant called XBB.1.9, which makes up 12.7% of circulating virus across the U.S. But XBB.1.16 is also similar enough to its siblings that federal officials think a single vaccine recipe might be able to boost against all of them this fall.
"We have continued to see accumulating substitutions in the spike protein more incrementally over time. And that can be described as drift, which happens more slowly," said Thornburg.
Here's the latest about what we know about XBB.1.16.
How many people have infections from this COVID variant?
Weekly projections published April 28 by the CDC estimate that XBB.1.16 has inched up to 11.7% of virus circulating nationwide. There were more than 88,000 reported cases nationwide over the past week, but CDC officials have said that cases are being substantially undercounted because of at-home testing and states no longer regularly reporting data.
At the regional level, XBB.1.16 has climbed to more than 1 in 10 infections across multiple parts of the country. It makes up the largest estimated proportion of cases at 15.6% in the region spanning Arkansas, Louisiana, New Mexico, Oklahoma and Texas.
Among travelers at international airports, the CDC's testing program has detected XBB.1.16 in around 1 in 5 positive samples pooled from arriving flights through early April.
Globally, India – which has seen a wave of new infections over recent months – has reported the largest share of XBB.1.16 sequences to global virus databases. The strain has been dominant there since February.
Is this COVID strain causing "pink eye" or other different symptoms?
Health authorities around the globe, including at the World Health Organization, have downplayed claims that XBB.1.16 is causing new or worse symptoms compared with other Omicron variant strains that have driven previous waves.
In India, where XBB.1.16 had surged, the WHO reported on April 17 that hospitalizations and other measurements of disease severity were not worse compared with other circulating variants.
Some have pointed to "pink eye" – also known as conjunctivitis – as a potential new symptom caused by XBB.1.16. But at a news conference on April 18, WHO officials described it as a "known symptom that already is part of COVID."
"I'm not aware of any major shift in symptomatology for this variant, but we are seeing characteristics associated with increased transmission capacity," the WHO's Dr. Mike Ryan told reporters.
Will this COVID strain cause a new wave?
The WHO has described the variant's growth advantage as only "moderate" compared to other strains. Since it was first reported in early January, the variant has only gradually increased around the world.
"What we're really seeing is a kind of an estimated growth advantage, some evidence of immune escape characteristics, and therefore this variant may spread more globally and it may cause a rise in incidence," Ryan said.
Preliminary analyses suggest "there is little difference" in the ability of antibodies from vaccination to fend off XBB.1.16 compared to earlier XBB strains, the United Kingdom reported on April 21. Relative to the earlier XBB.1.5 variant, data from animal tests described by the WHO on April 17 found "comparable" ability to evade prior infections.
While XBB.1.16 has been found across the surge of infections in India, it remains unclear what the exact role has been of this variant's mutations in driving that increase. India saw a "strikingly similar" surge at the same time in 2021, Swiss variant trackers noted on April 27, making it difficult to figure out how much this is simply a "seasonal effect."
For now, COVID-19 metrics are continuing to trend downward around the U.S. The pace of hospital admissions with the virus is nearing some of the record lows seen in the spring of 2021 and 2022, before both years saw renewed surges over the summer.
"It has not fallen into an exact seasonal pattern yet, but over the past few years we have seen a late summer, early fall surge, or mid-summer, early fall surge, and then another surge over the holidays," Thornburg said.
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