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Just before the busy summer travel season in the US, a new COVID variant called NB.1.8.1-also known as "Nimbus"-is quickly spreading across the world.

This version of the virus, which is part of the Omicron family, recently caused a rise in cases in parts of Asia. Now, it's responsible for over one-third of all COVID infections in the US, according to new data.

Health officials first found the variant in the US in late March through testing international travelers at airports. Since then, it's been spotted in over a dozen states, and numbers are expected to grow.

On May 23, the World Health Organization called NB.1.8.1 a "variant under monitoring" because of how fast it's spreading and changes in its spike protein that could affect how easily it spreads. The nickname "Nimbus" was given in late May on X by Canadian evolutionary biologist T. Ryan Gregory, who also came up with names for other COVID variants like the "FLiRT" strains.

While this variant is making up more cases in the US, overall COVID numbers haven't changed much. As of June 7, the test positivity rate was 3%, which is just a 0.2% increase from the previous week. Hospital stays for COVID are actually going down, according to the CDC. Their wastewater tracking, as of June 6, also shows a low level of virus activity across the country.

Still, experts believe this could shift in the next few months. Every summer since 2020 has brought a jump in COVID cases in the US, and the arrival of this new variant has many keeping an eye out for another rise.

Nimbus, common variant in US

Between May 25 and June 7, NB.1.8.1 was responsible for about 37% of COVID cases in the US, up from 15% in the previous two weeks. The top variant at the moment, LP.8.1, is causing 38% of infections.

However, the CDC said its ability to track exact numbers is "low" because not enough virus samples are being sequenced. The agency hasn't shared regional data yet, but according to sequences posted on the GISAID database, the Nimbus variant has been found in at least 14 states:

• Arizona

• California

• Colorado

• New Jersey

• New York

• Hawaii

• Illinois

• Maryland

• Massachusetts

• Ohio

• Rhode Island

• Vermont

• Virginia

• Washington

Worldwide, the WHO said that as of May 18, NB.1.8.1 had been detected in at least 22 countries and made up about 10.7% of COVID cases.

Experts believe the number of cases caused by this variant will likely continue to rise in the US, based on what's happening in other parts of the world.

Is Nimbus easier to catch?

The virus behind COVID (SARS-CoV-2) keeps changing. These mutations sometimes help it spread faster or dodge immune protection from past infection or vaccines.

Like other omicron descendants, NB.1.8.1 spreads quickly. It came from a mix of previous variants and has multiple spike protein mutations, which might help it latch onto human cells more effectively. A May study, though not peer-reviewed yet, said these changes could help it spread faster and possibly outcompete other variants in the future.

Based on its makeup and how quickly it's spreading, it's likely that Nimbus is more transmissible than older versions of the virus-but experts say it's too soon to say exactly how it compares.

So far, there's no sign that it causes worse symptoms than other variants. According to the WHO, the overall risk from this variant is still considered "low."

Will Nimbus infections spike?

It's not yet clear if NB.1.8.1 will become the dominant variant or cause a large summer spike, but experts say it's possible.

"It's hard to predict whether this variant or another will take off and cause a wave … but its likely that it won't be as severe as what we experienced before 2023," Dr. Albert Ko, professor of public health, epidemiology and medicine at Yale School of Public Health, as cited by Today report.

In the US, most people now have some level of immunity-either from a past infection or vaccination. It's estimated that about 90% of Americans have already been exposed to the virus. But immunity fades over time.

"The longer we get from the last surge or the last vaccination campaign, the more people are susceptible," Andy Pekosz, Ph.D., professor at Johns Hopkins Bloomberg School of Public Health, previously said.

Add summer travel, big events, and a new variant like Nimbus, and conditions are right for more infections. According to the CDC, COVID tends to peak twice each year in the US-once in winter, and again during summer (June to August). But how big each wave is can vary.

Based on past years, experts expect some increase this summer. "We may see an upswing (this) summer as in previous years, and this is expected to be smaller compared to what we experience in the winter," says Ko.

Symptoms linked to Nimbus

Right now, symptoms from this variant appear similar to other recent omicron strains. These include:

• Sore throat

• Cough

• Runny or stuffy nose

• Trouble breathing

• Fever or chills

• Headache

• Body aches

• Fatigue

• Loss of smell or taste

Anyone can get sick from COVID, but older adults (especially those over 65), people with weakened immune systems, and those with underlying health issues are more likely to get very sick, according to the CDC.

Should you get the COVID shot?

COVID vaccines still help prevent severe illness, hospitalization, and death. With all the changes to vaccine guidance, you might be wondering if you need another dose.

The CDC says the current 2024-2025 COVID vaccines are still available and are recommended for most adults 18 and up.

"Those in high-risk groups should consider getting the vaccine now if they haven't been vaccinated in the last six months and consider receiving an updated vaccine for the upcoming winter season," says Ko.

WHO also said the current COVID vaccines should still work against NB.1.8.1.

The next round of vaccines for 2025-2026 will be geared toward the LP.8.1 variant and are expected in the fall. But US Health and Human Services Secretary Robert F. Kennedy Jr. recently announced changes that might limit who can get these shots.

Under the new guidelines, the upcoming vaccine will be offered only to adults 65 and older, and to those aged 12-64 with at least one medical condition. Routine COVID vaccines are no longer recommended for healthy children or pregnant women by the CDC, though many doctors disagree.

If you're unsure whether you should get the COVID shot, talk to your healthcare provider.

To stay safe, health officials recommend testing when you feel sick, staying home when you're ill, masking in crowded indoor places, and social distancing. The government no longer mails out free test kits, but some free or low-cost tests are still available.