The newest concerning variant of covid-19 is rapidly advancing across the U.S. Data from the Centers for Disease Control and Prevention shows that NB.1.8.1 has started to supplant other circulating strains of the coronavirus.

According to the CDC’s latest Nowcast, covering the last two weeks through June 7, NB.1.8.1 is likely causing 37% of all covid-19 cases in the U.S., just barely behind the current dominant variant, LP.8.1 (38%). It’s an estimate, but the CDC’s numbers indicate a large surge in NB.1.8.1 cases since last month, when officials first detected it in the country. Though not certain, it’s possible that NB.1.8.1 could drive a renewed peak of covid-19 this summer.

The World Health Organization first recognized NB.1.8.1 as a variant worth monitoring in mid-May, though it was initially detected in late January. Since then, it’s become an increasing threat. As of early June, about a quarter of all global cases are thought to be NB.1.8.1, according to WHO data. Covid-19 activity in general has also been rising in some regions of the world, and NB.1.8.1 has been linked to recent surges of cases and hospitalizations in parts of Asia, including China.

So far in the U.S., however, the situation doesn’t seem to be dramatically worsening. The covid-19 test positivity rate as of late week is 3%, just a tick above the previous week. Hospitalizations and deaths related to covid-19 have also remained steady and low. And NB.1.8.1 doesn’t seem to cause more severe illness on average than other circulating variants. Today’s variants still belong to the Omicron lineage of the virus, meaning they haven’t substantially changed on a genetic level and updated vaccine boosters are still expected to be effective against NB.1.8.1.

At the same time, NB.1.8.1 may be just different enough from other strains that it can become the dominant variant moving forward; some early evidence suggests NB.1.8.1 possesses mutations that could increase its transmissibility. This past winter was also unusually quiet for covid-19, with low circulation in the U.S. and elsewhere, but that may set the stage for a larger peak this summer, experts have warned.

“We do not anticipate the NB.1.8.1 variant to pose a greater public health risk than other Omicron-descendant variants, nor do we expect a significant impact on vaccine effectiveness against severe disease,” said Edoardo Colzani, head of the Respiratory Viruses unit at the European Center for Disease Prevention and Control, in a statement released over the weekend. “However, following a winter with low SARS-CoV-2 circulation, population immunity against SARS-CoV-2 may have partly waned—particularly among older adults and other individuals who are at higher risk of severe disease—potentially increasing vulnerability as virus activity rises.”

Thanks to vaccination and population immunity, the danger of covid-19 has greatly lowered over time. But this summer could certainly provide an unwelcome reminder of the misery that it can cause.