The World Health Organization is keeping an eye on a new COVID variant—one that, in some countries, is sending wastewater levels of the virus skyrocketing to the highest point ever seen.

The global health watchdog on Tuesday promoted JN.1—an Omicron spawn experts flagged this summer for its unusually large number of mutations—to a “variant of interest.” It’s second only in alert level to “variant of concern,” a designation Omicron, Delta, and Alpha had until the organization removed their status.

How dangerous is JN.1, how likely are you to get it, and could it derail your holiday plans? Here’s what you need to know, according to the experts Fortune spoke with.

What in the heck does JN.1 even mean?

JN.1 is yet another Omicron offshoot, technically. It evolved from BA.2.86, a COVID variant that caught the attention of experts this summer because of its large number of mutations compared with the original Omicron: a whopping 30 or more, depending on how you count them. BA.2.86 was—and still is—the most genetically distinct COVID variant to have evolved since Omicron, which made a global splash in 2022. It was so distinct, many experts said, that it warranted a new Greek letter from the World Health Organization.

Along with BA.2.86’s laundry list of mutations came the ability for the virus to evade immunity and infect cells with increasing ease. Variant trackers dubbed BA.2.86 “Pirola”—after an asteroid—as a nod to their belief that the new variant, or one of its offspring, may eventually warrant a new Greek letter: presumably Pi or Rho, next in the alphabet.

BA.2.86 failed to take off in the way it seemed poised to, though it managed to show strong growth in some regions of the world. As early as August, however, experts warned that while BA.2.86 may not be the black swan “Omicron event” some thought it was, one of its descendants could be.

Enter JN.1. While the letters assigned to the strain make it sound like it’s coming from left field, it’s not. It’s BA.2.86 plus one additional major mutation that makes a lot of difference: L455S, which boosts its ability to evade antibodies from prior infection and vaccination. L455S is the reason JN.1 has “sprouted wings,” while BA.2.86 did not.

Technically, JN.1 is BA.2.86.1.1. But the naming system for COVID variants calls for the string of letters to truncate after three additional sets of numbers and periods. So BA.2.86.1.1 “rolled over” to JN.1.

Now you know.

What are the symptoms of JN.1?

It’s too early to tell if the symptoms of JN.1 differ from typical Omicron symptoms. So far, there’s not much evidence that this is the case, though there are reports of increased diarrhea that may or may not be associated with the variant.

Is JN.1 more dangerous than other Omicron variants?

It’s also too early to say. Hospitalizations are rising in some areas like New York City, considered a “bellwether state” that may forecast what’s to come for much of the rest of the country. But rising hospitalizations could be the result of waning population immunity, at least in part. And other variants undoubtedly contribute to hospitalizations and deaths.

In the U.S., COVID hospitalizations were slowly rising as of Dec. 9, while deaths were holding steady, according to the latest data made available by the U.S. Centers for Disease Control and Prevention.

JN.1 looks to be around 40-50 times more immune evasive than BA.2.86, Jay Weiland, a leading COVID modeler, told Fortune. Just how its greatly increased immune evasion will play out in various populations has yet to be seen.

Do COVID vaccines work against JN.1?

Yes. They’re thought to provide good protection against severe illness, hospitalization, and death.

“For those interested in gaining immunity with less risk than COVID-19 illness, the monovalent XBB vaccine released a few months ago provides substantial immune response to the JN.1 variant,” Dr. Stuart Ray, vice chair of medicine for data integrity and analytics at Johns Hopkins’ Department of Medicine, told Fortune.

“Like prior variants that have gained against others, it’s not clear whether JN.1 will result in significantly higher rates of severe COVID-19 or long COVID, but I’m seeing a lot of respiratory illness due to COVID-19, RSV, and influenza in my hospital patients,” he added. “So it makes sense to take reasonable precautions in this respiratory virus season.”

Should I be masking again, with JN.1 on the loose?

It would be wise to, many experts say. Ray recommends “wearing an effective respirator in public spaces, especially indoors, when it doesn’t interfere with essential tasks.”

“Not a bad way to keep your face warm, too,” he added.

Does Paxlovid work against JN.1?

Yes, though experts caution that patients who are prescribed Paxlovid and stop taking the pill midway through, owing to its off-putting metallic taste, could be contributing to the virus’s potential evolution around the antiviral, eventually rendering it ineffective.

Do at-home COVID tests still work with JN.1?

Yes, though they aren’t, and never were, completely accurate. Many people test too early in their infection or too late. At both times, viral loads may not be significant enough to turn the test positive. If you have symptoms but test negative, wait another couple of days and test again, experts recommend.

The WHO just declared JN.1 a ‘variant of interest.’ What does that mean?

When the WHO declared JN.1 a variant of interest Dec. 19, it was calling for vigilance and increased sequencing by health authorities across the globe.

But in a practical sense, and for the average person, “it doesn’t change anything,” Ryan Gregory, a biology professor at the University of Guelph in Ontario, Canada, told Fortune. For over a year, Gregory and a team of “variant trackers” have devised “street names” for COVID variants, in a bid to better communicate the evolving Omicron threat to the public.

“I do think that, in some ways, it’s good that they’re indicating this is still evolving and still worth monitoring,” he added.

Just how fast is JN.1 spreading?

With sequencing of COVID viral samples at a near all-time low, it’s hard to say. We do know, however, that JN.1 “appears to be outcompeting other circulating variants worldwide, including its recent ancestor BA.2.86 and siblings,” Ray said.

A good way to gauge how vast a variant is spreading is its “doubling time,” or the time it takes to double its number of sequences in a particular area. It’s hard to nail down an accurate doubling time on new variants due to low sequencing. JN.1’s immediate predecessor, BA.2.86.1, was doubling about once every two weeks. That pales in comparison to the doubling time of the original Omicron BA.1.1: every 2.5-3 days, according to Weiland. Still, the doubling time of the JN.1 family is enough to make it stand out in the current landscape, comprised of much slower-growing variants.

In what countries is JN.1 causing, or contributing to, a record level of COVID spread?

Countries that are currently seeing all-time highs of COVID in wastewater—higher than even the Omicron spike of early 2022—include Germany, the Netherlands, and Denmark, according to Weiland.

A JN.1 spike in the U.S. could eclipse all other spikes except the initial Omicron spike, experts say. Each country, however, has slightly or even vastly different COVID waves, depending on variables we know of—like weather, social distancing, population immunity, and the like—and even ones we’re not yet aware of. European countries had smaller initial Omicron waves than the U.S. saw, which may be why they’re seeing record-setting JN.1 waves now.

Where did JN.1 come from?

Scientists aren’t exactly sure where JN.1 and its predecessor, BA.2.86, came from and likely never will be. It’s thought that BA.2.86 evolved in the body of an immunocompromised patient—or one with a long-term infection—of BA.2 for over a year. BA.2, if you will remember, hit radars in early 2022 and was known as “stealth Omicron” for its ability to evade detection on PCR tests.

In most people, the immune system defeats the COVID virus, and others, in relatively short order. In those with a compromised immune system, however, infections can remains for months or even years, giving the virus a chance to repeatedly evolve around human immunity. In these cases, the virus may “hole up” somewhere we wouldn’t expect it to, like the GI system. If this happens, typical COVID tests, which involve the respiratory system, will return negative. But the person is still infectious and can spread the virus, potentially through fecal contamination.

New variants that evolve in those with long-term infections are unlikely to immediately out-compete top COVID strains, on the rare occasion that they spill back over into the population. Why? Contained within a single host, a variant has no need to develop increased transmissibility to win an evolutionary survival-of-the-fittest. Globally circulating variants, on the other hand, have had months to pick up new mutations and refine their ability to spread.

Once a highly mutated variant like BA.2.86 escapes into the broader population, it’s likely to begin its own process of refinement, Gregory told Fortune this fall. “The fact that Pirola has gained enough of a foothold to be evolving among hosts now means we need to be wary,” he said at the time.

As predicted, BA.2.86 underwent further evolution, eventually picking up the signature spike protein L455S mutation, which boosts its ability to evade antibodies from prior infection and vaccination. It’s the reason JN.1 has taken off, whereas BA.2.86 did not.

It’s a similar situation to what occurred with the original Omicron, experts say. BA.1.1.529 was the original, highly mutated Omicron, with very little similarities to its predecessor, Delta. It never took off. When it acquired a new mutation that allowed it to transmit faster, it became BA.1.1.529.1.1, shortened to BA.1.1—and that’s what eventually took over the world.

Where is JN.1 going?

In short: Nowhere.

JN.1 will continue to evolve—just as BA.2.86 did—or die, as all strains of COVID do. Variants continually compete in a veritable survival of the fittest, acquiring the same or similar mutations through a form of parallel evolution known as convergent evolution. There are a number of mutations JN.1 doesn’t currently have that it could acquire, potentially making its descendants more troubling than JN.1 itself.

Some experts say that most new major COVID variants could evolve from JN.1, in the way that all new major COVID variants currently evolve from Omicron. In other words, this could be the start of a new era in the pandemic.

On the other hand, COVID could always throw the world another curveball. Highly mutated variants from patients with long-term infections of seemingly ancient strains exist and can always re-enter the population. That’s why public health officials can’t simply focus on the current COVID landscape when planning for the future, Gregory said. Because sometimes, “weird, one-off things come back.”

Will the variant trackers assign JN.1 a new ‘street name’ like Pirola or Kraken?

The group has had “many discussions” as to whether it should give JN.1 its own nickname, Gregory said. On the one hand, only one major mutation separates it from its parent, BA.2.86 “Pirola.” On the other hand, it’s taking off in a way Pirola never did.

For now, the group has decided not to assign a new name to JN.1, and to reference it as a member of the Pirola family.

“What matters is not individual variants and whether they are going to cause a wave as big as the first Omicron. It’s the idea of evolving lineages,” he noted. “It’s consistent with what we said initially: It’s not the variant [BA.2.86], per se, that’s going to be an issue,” but likely what will evolve from it—a whole new line of COVID variants, potentially, in the way Omicron expanded into a family of diverse-yet-similar variants that became increasingly adept at evading immunity and infecting hosts.