There are no signs so far that the new LB.1 variant is causing more severe disease in COVID-19 patients, the Centers for Disease Control and Prevention says, as infections have begun to accelerate in this summer’s wave around the country.

The LB.1 variant currently makes up 17.5% of new COVID cases, the CDC projected Friday, and could be on track to overtake its sibling, the KP.3 variant, which has also been growing in recent weeks.

“There is currently no evidence that KP.3 or LB.1 cause more severe disease. CDC will continue to track SARS-CoV-2 variants and is working to better understand the potential impact on public health,” CDC spokesperson David Daigle said in a statement.

The reason behind any potential shifts to the symptoms or severity of disease caused by new variants is complex, affected by people’s underlying immunity from a mix of past infections and vaccinations as well as changes to the virus itself.

The CDC has said in the past that it closely tracks the toll inflicted by the latest strains largely based on data and studies from hospitals, comparing trends from places where new variants have emerged first.

Only a fraction of facilities are still reporting figures on hospitalizations and ICU admissions to the CDC, after a pandemic-era requirement lapsed earlier this year. A proposal by the Centers for Medicare and Medicaid Services to incorporate the data reporting into routine requirements is not scheduled to take effect until October.

Instead, the agency has turned to other sources like a network of hospitals that still report more detailed data about patients to the agency as well as emergency room visits to track the virus.

Where is LB.1 rising fastest?

In California, one of the states that saw trends of the virus rise earliest this summer in wastewater to “high” levels, the CDC’s COVID-NET data shows hospitalizations were near levels not seen there since February. 

More recent data from emergency rooms in California also shows rates of COVID-19 patients have risen to levels not seen since February.

But the KP.3 variant — not LB.1 — made up the largest proportion of cases during that early surge, estimates from the CDC as well as California’s health department suggest. 

Just 7.8% of cases in HHS Region 9, which spans California and some other western states, were projected to be from the LB.1 variant through June 8.

Instead, LB.1’s prevalence looks to be largest in HHS Region 2, which includes New York and New Jersey. Through June 8, 30.9% of cases are estimated to have been caused by LB.1.

What’s different about LB.1?

Compared to highly mutated SARS-CoV-2 variants that showed up earlier during the pandemic, experts say LB.1’s changes are relatively small compared to its parent variant JN.1, which was dominant during this past winter’s wave. 

LB.1 is also closely related to KP.3, which is also a descendant of the JN.1 variant. Unlike KP.3, LB.1 has a key mutation that scientists call S:S31del that looks to be helping it spread faster.

Research by scientists in Japan this month, which was released as a preprint that has not yet been peer reviewed, found that this mutation seemed to be more infectious and better at evading the immune system.