XBB.1.5— dubbed ‘Kraken’ by Canadian biology professor Dr. Ryan Gregory and his following in the Twitterverse—is the most transmissible COVID variant yet, according to the World Health Organization.

A risk assessment is currently being drawn up for the new mutant strain by WHO’s technical advisory group on virus evolution, Maria Van Kerkhove, technical lead for COVID-19 response at the authority, said on Wednesday.

XBB.1.5 began alarming scientists at the tail end of last year after the number of Kraken cases in the U.S. rose from 1% of all cases at the start of December to 41% just three weeks later.

This week, the U.S. Centers for Disease Control and Prevention projected that it comprised around 75% of infections in regions 1 and 2, which include Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, Vermont, New York, New Jersey, Puerto Rico, and the U.S. Virgin Islands.

The strain is believed to be in at least 28 other countries—including Europe—with cases of XBB.1.5 now thought to make up 4% of COVID cases in the U.K.

What are the symptoms of ‘Kraken’ COVID variant XBB.1.5?

Dr. Allison Arwady, the Chicago Department of Public Health commissioner, said in a press conference on Tuesday that Kraken “basically just a combination of two of the earlier subtypes, two variants” from the Omicron strain.

She added that although XBB.1.5 is a new mutation its symptoms have not hugely changed because it is a descendant of the variant that was discovered in mid-2020.

Arwady explained: “We’re seeing more people actually just have cold-like symptoms”—such as a runny nose, sore throat, cough and congestion—“but are less likely to have those flu-like, really feeling very sick [symptoms such as] the high fevers.”

This is especially the case in people who are fully up to date on vaccines or who have preexisting immunity built up from having a COVID infection in the past.

More widely, the CDC’s COVID symptoms to look out for are fever or chills, difficulty breathing, fatigue, body aches and headaches, loss in taste or smell, nausea, and diarrhea.

WHO researchers are currently focusing on the variant’s ability to quickly spread and overtake other strains of Omicron, but Van Kerkhove added that disease severity was also being explored.

There is not yet any evidence to suggest that Kraken prompts a more severe reaction, she said.

XBB.1.5 is causing concern as it binds tightly to the cells it infects, WHO officials added, which means the virus replicates easily in a host.

Does being vaccinated help protect against ‘Kraken’ COVID?

Dr. Raj Rajnarayanan, assistant dean of research and associate professor at the New York Institute of Technology campus in Jonesboro, Ark., previously told Fortune that the best form of protection from mutations is to get a booster vaccination.

Speaking following the Omicron spawn BA.2.75, dubbed Centaurus, Rajnarayanan confirmed that escaped mutations such as Centaurus and Kraken are “immune evasive” to some extent—but won’t be able to defy all of the human body’s resistance.

Professor Paul Hunter, of the U.K.-based National Institute for Health Research Health Protection Research Unit, added that the extent to which Kraken can bypass immunization is not yet known.

Pavitra Roychoudhury, the director of COVID-19 Next Generation Sequencing at the University of Washington, told Forbes there are no “spectacular” measures members of the public should be taking bar the bivalent vaccine, which Roychoudhury described as the “best defense against severe illness”.

What has the impact been on health services?

The seven-day average to January 3 of COVID hospitalizations has increased 16.1% compared to the prior weekly rolling average, according to data from the CDC.

From Dec. 21 to 27, 2022, 5,613 people were admitted with positive COVID tests, compared to 6,519 from Dec. 28 to Jan. 3.

However this is still a far cry, down 69.7%, from the peak seven-day average in mid-January 2022 when 21,525 were admitted with COVID.

The WHO also reported a 20% increase in global COVID deaths Thursday over the past month; however, Van Kerkhove was quick to confirm that the trend—or variant—behind the deaths is unknown.

She added it could be due to more people meeting indoors around the public holidays as opposed to threats from a new and more dangerous strain.