For a third season, COVID-19 cases are rising ahead of the holidays, this time with a more recent variant driving infections.

JN.1, a variant of omicron , now accounts for an estimated 21% of new infections in the United States, the Centers for Disease Control and Prevention reports.

Cases of JN. 1 infections rose from just more than 8% at the end of November to double digits by Dec. 9, according to CDC data.

International health experts say the sudden jump isn’t cause for concern – yet.

The JN.1 variant is closely related to another omicron subvariant named BA.2.86, according to the CDC.

Health officials started tracking BA.2.86 in August but noticed some cases had a single change to the spike protein, the part of a virus that penetrates host cells to cause infection, according to the CDC.

JN.1 was first detected as its own variant in the United States in September, the CDC reported, and has since been identified in 11 countries.

The variant made up just 0.1% of cases at the end of October, according to CDC data, but that number has now increased exponentially.

“The continued growth of JN.1 suggests that it is either more transmissible or better at evading our immune systems,” the CDC said at the beginning of December.

This means the change to the spike protein may make it easier for the virus to get into cells than previous variants or make it harder for human immune systems to identify the virus as COVID-19.

International health experts also noted the trend, with JN.1 officially classified as its own variant by the World Health Organization on Dec. 19.

“Based on the available evidence, the additional global public health risk posed by JN.1 is currently evaluated as low ,” the WHO said in a post on X, formerly known as Twitter. “Despite this, with the onset of winter in the Northern Hemisphere, JN.1 could increase the burden of respiratory infections in many countries.”

The CDC says the J N.1 variant is closely related enough to previous variants that the updated COVID-19 booster shot will provide protection.

“The spike protein — called a ‘spike’ because it looks like tiny spikes on the virus’ surface — plays a crucial role in helping the virus infect people. Because of this, the spike protein is also the part of the virus that vaccines target, meaning vaccines should work against JN.1 and BA.2.86 similarly,” the CDC said.

The agency continued by saying it expects “treatments and testing to remain effective” against JN.1, in line with previous variants.

The most recent COVID-19 booster was approved by the Food and Drug Administration and then recommended by the CDC in September, but a large number of Americans have opted out of the shot.

The vaccine is recommended for everyone over the age of 6 months , regardless of previous vaccine status, the CDC says.

Despite this, fewer and fewer Americans are adding the shot to their regular seasonal vaccines, like the flu shot.

By the middle of November, only 36 million adults and 3.5 million children had received the latest shot, equivalent to about 14% of the American public , ABC News reported.

That’s compared to the nearly 70% of Americans who completed the primary series of COVID-19 vaccines, McClatchy News reported.

Cases of JN.1 have seen some of the highest increases in Midwestern states, CDC data shows, correlating with a pocket of the U.S. that has been particularly hesitant to get the new shot.

Ahead of the holidays, the CDC reiterated the vaccine, masking and keeping your distance when you might be sick are still the most effective ways to prevent a COVID-19 infection and prevent its spread.

The agency said this also extends to other respiratory illnesses making the rounds this season.

“Many viruses spread more during the holiday season, so it is important to get all recommended vaccines, including flu, COVID-19 and RSV, as soon as possible,” the CDC said. “This will give you the best protection against these respiratory diseases, including while traveling and gathering with family and friends. These vaccines will also make your illness less severe if you do get sick.”