“FLiRT” variants of COVID now account for more than a third of cases in the United States. Scientists share what we know so far about them.

More than a third of COVID-19 cases in the United States are now estimated to come from a new, fast-growing member of a group of so-called “FLiRT” variants, nicknamed for their small but distinctive changes from JN .1 stump. JN.1 was the variant behind this past winter wave infections.

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

According to the Centers for Disease Control and Prevention’s biweekly variant estimates, KP.2 and another strain with the same FLiRT mutations, called KP.1.1, together account for 35.3% of projected infections this week. That’s an increase from 7.1% a month ago.

“This means that even though KP.2 is proportionally the most predominant variant, it is not causing an increase in infections because transmission of SARS-CoV-2 is low,” a CDC spokesperson told CBS News in a press release.

The strain also doesn’t show any big, worrying changes, unlike some previously heavily mutated variants that have caused alarm in years past.

However, the rapid change in circulating variants has led the Food and Drug Administration to this week delaying a key step in its process of selecting which strain to target with this fall’s COVID-19 vaccines, citing the need for more “up-to-date” data.

While federal requirements for hospitals to report COVID-19 data to authorities expired this month, the CDC says it still has reliable numbers from sources like wastewater testing and wards urgently to continue monitoring virus activity.

Here’s the latest on COVID-19 variants in the United States

What is the current new variant of COVID-19?

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

The second largest variant on the rise is another descendant of JN.1 called JN.1.16. This strain has not grown as quickly, reaching only about 10% of cases this week.

This projection is based on genetic sequences of the virus reported primarily by public health laboratories, which have declined significantly in recent weeks alongside the overall slowdown in cases. Other CDC data from wastewater and traveler testing still does not distinguish KP.2 from its parent JN.1.

KP.2 is a closely related descendant of variant JN.1 from last winter, which turned out not be significantly more serious than the variants that were dominant before it, despite its large number of mutations.

“So this is an issue that we are monitoring. This is an issue that we are monitoring. And again, let’s reiterate the need for continued surveillance of SARS-CoV-2 in people around the world, so that we can monitor this development,” the World Health Organization said. Maria Van Kerkhove told reporters on Wednesday.

Why are these COVID-19 variants called FLiRT?

The FLiRT nickname comes from two distinct mutations seen in several descendants of the JN.1 variant that cropped up around the world after it passed over the winter. Some of the largest strains currently exhibiting FLiRT mutations in the United States are KP.2 and KP.1.1.

“It’s basically putting a word on the specific amino acid changes in the F456L+R346T spike protein, or from phenylalanine (F) to leucine (L) at position 456 and arginine (R) in threonine (T) at position 346,” Canadian biologist Ryan Gregory, a professor at the University of Guelph, told CBS News in an email.

Gregory invented this nickname in March, and it has gained traction among variant trackers who have spotted and dubbed many of the virus’s distinctive changes during the pandemic. Although unofficial, these nicknames have become commonly used names for a number of variations.

FLiRT trumped another nickname — “tiLT” variants » – coined by Australian consultant Mike Honey. FLiRT refers to a set of faster-growing JN.1 offshoots that trackers monitor, including KP.2.

“Basically almost everything right now is a descendant of BA.2.86.1.1 (JN.1) and things are changing quickly, so it makes more sense to focus on mutations of interest rather than individual variants at the moment” , wrote Grégoire.

Do FLiRT variants cause different COVID-19 symptoms?

Unlike some previous heavily mutated variants that had raised concerns about potential changes in symptoms In recent years, the JN.1 variant that many Americans have likely already caught over the winter has been closely related to the KP.2 strain, which is currently on the rise.

“Based on current data, there is no indicator that KP.2 would cause more severe disease than other strains,” a CDC spokesperson told CBS News.

The two distinctive so-called FLiRT mutations of KP.2 have also been seen before, in the XBB.1.5 variants that were circulating throughout 2023, the spokesperson said.

A draft study by Japanese scientists, published as a preprint that has not yet been peer-reviewed, found that the variant appeared to dodge antibodies better than the JN.1 variant. This “increased immune resistance” probably explains its increase, according to scientists.

In general, health authorities and experts have downplayed claims that the variants caused different symptoms. Changes in a person’s immunity from vaccines and previous infections often play a role in different symptoms, rather than specific mutations.

“Mutations occur frequently, but only sometimes change the characteristics of the virus,” explains the CDC.

Will vaccines work against FLiRT variants?

The CDC has made no changes to its current vaccine recommendations, which were last updated in April. But the emergence of these new descendants of JN.1 variants like KP.2 could affect which vaccine the FDA chooses for the coming fall and winter.

Most Americans remain eligible to receive at least one dose of last season’s updated COVID-19 vaccine, which CDC data so far suggests was up to 51% effective against emergency room visits or to emergency care at a time when JN.1 was on the rise. .

“The CDC will continue to monitor community transmission of the virus and the effectiveness of vaccines against this strain,” the agency said of KP.2.

Last month, World Health Organization experts recommended vaccine makers produce shots targeted against the JN.1 variant for next season. An FDA panel of vaccine experts was expected to evaluate this approach for the U.S. vaccine market next week.

However, the agency recently announced he had decided to delay the meeting until June in hopes of buying more time to ensure he chooses a vaccine target that is “most appropriate for use for the strain(s) expected to be circulating” at autumn.

“The FDA, along with its public health partners, is carefully monitoring trends in circulating strains of SARS-CoV-2. As has occurred since the emergence of COVID-19, we have recently observed changes in the dominant circulating strains of SARS-CoV. -2,” an FDA spokesperson told CBS News in a statement.

Pfizer has generated research data on its KP.2 vaccines, but a company spokesperson said it is currently unable to share the results. A Moderna spokesperson did not respond to a request for comment.

A Novavax spokesperson said it had data showing its fall vaccine candidate, intended for JN.1, had “good cross-reactivity” for KP.2. Although Novavax’s vaccine takes longer to manufacture than Pfizer’s and Moderna’s mRNA shots, the spokesperson said the FDA’s delay in the meeting “will not affect” its ability to administer a shot this autumn.

“We have manufactured JN.1 as recommended and are on track to deliver an updated vaccine this fall,” the Novavax spokesperson said.