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More than a third of Covid-19 cases in the US are now estimated to be new, fast-growing members of a group of “FliRT” variants with small but distinct changes relative to JN.1. Stress. The JN.1 was a difference from the previous one. Winter wave of infection.

The largest of them, called KP.2 by scientists, has multiplied rapidly in recent weeks and is now the dominant new strain of COVID-19.

Every week, according to the Centers for Disease Control and Prevention Alternative assumptionsThose with the same FLiRT mutation, called KP.2 and KP.1.1, account for 35.3% of infections this week. This is up from 7.1% a month ago.

“This means that while KP.2 is the main difference in proportion, the prevalence of SARS-CoV-2 is low, so it does not lead to an increase in infections,” a CDC spokesperson told CBS News.

The strain does not have significant changes of concern, unlike the previously highly variable variants that have raised alarm over the years.

However, the fast-moving change has also affected the Food and Drug Administration. this week Delaying a key step in the process to select a strain to target for this fall’s Covid-19 vaccines, citing the need for more “current” data.

When there are federal requirements for hospitals to report Covid-19 information to authorities passed This month, the CDC said it still has reliable figures from sources like wastewater testing and emergency departments to go on. Tracking activity From the virus.

Here’s the latest we know about the differences in Covid-19 in the US.

What is the current new variant of covid-19?

According to the latest projections published by the CDC, 28.2 percent of COVID-19 cases nationwide are now caused by a virus sublineage called the KP.2 variant.

The next major variant on the rise is another JN.1 strain called JN.1.16. That strain hasn’t grown rapidly, accounting for only about 10% of cases this week.

This prediction is based on the genetic sequence of the virus reported It has fallen sharply in recent weeks, mostly in public health labs, along with a slowdown in overall cases. Other CDC information from Waste water And traveller Testing still does not differentiate KP.2 from the JN.1 parent.

KP.2 is a closely related species JN.1 variant This past summer, that changed Don’t be too hard Although it has a larger number of mutations than its predecessors.

“So that’s what we’re looking at. We’re monitoring it. And again, we reiterate the need for ongoing surveillance of SARS-CoV-2 in people around the world so that we can track this evolution.” Maria van Kerkhove He told reporters. Wednesday.

Why are these variants of Covid-19 called FLiRT?

The nickname FLiRT comes from two unique mutations that appear in JN.1 variant seeds that have been created around the world after being wiped out during the winter. Currently, the largest strains with FLiRT mutations in the US are KP.2 and KP.1.1.

“Specifically, in the spec protein F456L + R346T, or phenylalanine (F) to leucine (L) at position 456 and arginine, is just to name a word. [R] to threonine [T] It ranks 346th, Canadian biologist Ryan Gregory, a professor at the University of Guelph, told CBS News in an email.

Gregory created this nickname In MarchDuring the outbreak, it was widely accepted among the mutants who saw unique changes in the virus and gave it the nickname. Although not official, these have become nicknames Commonly used names For several variants.

FLiRT wins with another nickname – “tiLT” variants – Created by Australian consultant Mike Haney. FLiRT refers to a rapidly developing fleet of JN.1 snipers, with the trackers tracking the KP.2 among them.

“Basically, now everything is of the BA. (JN.1) variety and things are improving rapidly, so for the time being it makes more sense to focus on changes in demand rather than individual differences,” he wrote. Gregory.

Do variations in FLiRT lead to different symptoms of Covid-19?

Unlike some of the earlier heavily modified variants that caused concern about possible changes Signs In recent years, the JN.1 strain most Americans are likely to catch during the winter is closely related to the now-increasing KP.2 strain.

“Based on current data, there are no indications that KP.2 causes more severe disease than other strains,” a CDC spokesperson told CBS News.

The two unique FLiRT mutations of KP.2 were also previously seen in XBB.1.5 variants circulating in 2023, the spokesperson said.

Draft Research From the Japanese scientists, released as a preprint yet to be peer-reviewed, the variant appeared to remove more antibodies than the JN.1 variant. This “increased immunity” probably explains the increase, the scientists said.

In general, they have health officials and experts Low claims Variants were bringing different signs. Changes in a person’s immunity to vaccines and pre-infections often play a role in different symptoms rather than specific mutations.

“Mutations occur frequently, but sometimes they change the characteristics of the virus,” the CDC said He says..

Do vaccines work against FLiRT variants?

The CDC has made no changes to the current vaccination recommendations, which they were. Last updated In April. But the emergence of these new JN.1 variants, such as KP.2, could affect the FDA’s vaccine selection for the upcoming fall and winter.

Most Americans remain eligible to receive at least one dose of the updated Covid-19 vaccine this season, the CDC said. Data so far It suggests that JN.1 was up to 51% effective in reducing emergency room or urgent care visits during the growing season.

“CDC will continue to monitor community spread of the virus and how vaccines are preventing it,” the agency said of KP.2.

Last month, experts from the World Health Organization Recommended Vaccine manufacturers will produce vaccines targeting the JN.1 variant for the next season. The FDA’s own panel of vaccine experts was scheduled to weigh that approach to the U.S. vaccine market next week.

However, the agency announced recently He decided to delay the meeting until June, hoping to buy more time to select and confirm a vaccine target that is “most appropriate for use against the strain(s) expected to spread” in the fall.

“The FDA, along with its public health partners, is carefully monitoring the spread of SARS-CoV-2. As has occurred since the outbreak of COVID-19, we have recently observed changes in the main transmission patterns of SARS-CoV. -2,” an FDA spokesperson said in a statement to CBS News.

Pfizer has generated data from its vaccine research on KP.2, but a company spokesperson said they could not share the results at this time. A spokesperson for Moderna did not respond to a request for comment.

A Novavax spokesperson said they have data showing a “good response” to KP.2, their vaccine candidate targeted at JN.1. While the Novavax vaccine will take longer to develop than mRNA vaccines from Pfizer and Moderna, a spokeswoman said the FDA’s delay to the meeting “will not affect” their ability to deliver the shot this fall.

“We have developed JN.1 in line with the recommendations and are on track to deliver an improved vaccine this fall,” said a Novax spokesperson.