The new KP.3 variant has climbed to 1 in 4 new COVID-19 cases nationwide, the Centers for Disease Control and Prevention estimated Friday, making it now the dominant strain of the virus nationwide. 

KP.3's ascent comes as the CDC has tracked key metrics of spread from the virus now starting to trend up. Previous years have seen surges of the virus peak around August.

Data from CDC's wastewater surveillance has tracked levels of the virus starting to accelerate in the West. Emergency room visits for COVID-19 have inched up in recent weeks for all ages. COVID-19 infections are likely growing in 30 states and territories, the CDC now estimates.

"Very, very similar" to JN.1

KP.3 is now estimated to be outpacing the KP.2 variant, a so-called "FLiRT" strain that this week inched up to 22.5% of cases. KP.2 had risen to dominance in previous weeks, but its growth has now slowed.

Both KP.3 and KP.2 are "very, very similar" to the JN.1 variant that had dominated this past winter's wave of infections.

"When you look at KP.2 and KP.3, they're nearly identical to each other with really one difference between the two of them," Natalie Thornburg, the chief lab official at the CDC's Coronavirus and Other Respiratory Viruses Division, said Wednesday.

Thornburg was speaking at a Food and Drug Administration meeting debating what strains should be targeted by this fall's vaccines.

This difference is smaller than previous jumps in the virus, like when JN.1's parent – the highly mutated BA.2.86 variant – first emerged last year.

However, KP.2 and KP.3 are also not identical. Early data suggests KP.3's mutations might be better at evading immunity.

"JN.1 and KP.2-like viruses, they're really, really on top of each other. And KP.3 is very close, but not absolutely on top of it," Thornburg said.

Picking out new COVID-19 vaccines

KP.3's rise comes as the FDA said Friday that it had decided to call for shots this fall to be updated for the JN.1 variant that was dominant earlier this year, effectively turning down a newer formula aimed at the KP.2 variant.

"Yes, we always say we shouldn't be chasing strains, but we're paying an incredibly high premium for mRNA vaccines to be able to have the freshest vaccines," the FDA's Peter Marks had told the meeting.

Moderna had presented data from animal studies suggesting its KP.2-targeted shot offered similar protection against the latest variants, compared to a shot designed for JN.1. Pfizer's shot for KP.2 triggered better antibody responses for JN.1 variants, including KP.3.

"If this evolves further in the fall, will we regret not having been a little bit closer," Marks said.

But the FDA ultimately decided to pass on the KP.2 shots, after the agency's advisers worried it might not do a better job at broadening immunity for future strains compared to JN.1.

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  • Health
  • Centers for Disease Control and Prevention
  • Coronavirus Disease 2019
  • COVID-19
  • Coronavirus
Alexander Tin

Alexander Tin is a digital reporter for CBS News based in the Washington, D.C. bureau. He covers the Biden administration's public health agencies, including the federal response to infectious disease outbreaks like COVID-19.

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