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(CNN) — What used to be a mysterious new variant first detected in the UK is now the most dominant coronavirus strain in the US.

And unlike the original strain of the novel coronavirus, the more contagious B.1.1.7 strain is hitting young people particularly hard.

“(Covid-19) cases and emergency room visits are up,” said Dr. Rochelle Walensky, director of the US Centers for Disease Control and Prevention.

“We are seeing these increases in younger adults, most of whom have not yet been vaccinated.”

Now doctors say many young people are suffering Covid-19 complications they didn’t expect.

And it’s time to ditch the belief that only older adults or people with pre-existing conditions are at risk of severe Covid-19.

Why B.1.1.7 is more contagious

Viruses mutate all the time, and most mutations aren’t very important. But if the mutations are significant, they can lead to dangerous new variants of a virus.

“The B.1.1.7 variant has mutations that allow it to bind more” to cells, said Dr. Jonathan Reiner, a CNN medical analyst and professor of medicine and surgery at George Washington University.

“Think of this mutation as making the virus stickier.”

Coronavirus latches onto cells with its spike proteins — the spikes surrounding the surface of the virus.

“There is a little difference in the way the (B.1.1.7) spike protein holds that makes it stick to your cells a little more easily,” said emergency physician Dr. Megan Ranney, director of the Brown-Lifespan Center for Digital Health.

With the original strain of the novel coronavirus, “you need a certain inoculum — a certain amount of virus — in order for the infection to basically stick,” Reiner said.

“Is one viral particle enough to make you sick? No, probably not. On the other hand … sometimes a massive inoculum can kill an otherwise healthy person. And we’ve seen that in health care workers,” he said.

“So these new variants, particularly the UK variant, seem to be stickier. So the notion is that it’s more contagious, so to speak, because potentially you don’t need as much of an inoculum to get sick.”

What this means in real life: “You can be in a place and maybe have a briefer exposure or have a smaller exposure — more casual exposure — and then get infected,” Reiner said.

And because B.1.1.7 is stickier, “you may indeed have a higher viral load.”

“If you have a higher number of viral particles in your respiratory tract, then it’s going to be easier to spread it to other people,” Ranney said.

That’s another reason why it’s so important for young adults to get vaccinated.

More young people are being hospitalized with Covid-19

B.1.1.7 cases have now been reported in all 50 states, the CDC said.

“What we’re seeing in a bunch of places now is sick, young people — hospitalized young people. Whereas earlier on in the pandemic, it was primarily older people,” Reiner said.

“The reason for this might be as simple as the older population in this country has either been exposed to this virus, killed by the virus, or now vaccinated against the virus.”

As of Saturday, more than 78% of people age 65 and over have received at least one dose of vaccine, and 60% have been fully vaccinated, according to CDC data.

“The unvaccinated — those are the people who are getting infected — we’re seeing a large number of young people, and they’re the ones we’re seeing in hospitals now.”

In March, New Jersey saw a 31% jump in Covid-19 hospitalizations among young adults ages 20 to 29, the state health commissioner said. And the 40-49 age group saw a 48% increase in Covid-19 hospitalizations.

Ranney said she’s also noticed a stark change in who’s getting hospitalized.

“This has been kind of a gradual increase in the proportion of folks who are younger over the last couple of months,” she said, citing data from COVID-NET — which tracks cases from more than 250 hospitals in 14 states.

“Looking at the week of December 26 or January 2, age 65-plus would be, say, 3,000 (hospitalizations). And then everything else together is 3,000. More than 50% were age 65-plus.”

But by March 27, “it was about one-third (ages) 18 to 49 … about one-third ages 50 to 64, and then about one-third 65-plus,” Ranney said.

As an emergency room doctor, Ranney said she regularly sees young, previously healthy patients struggling with coronavirus.

“I see at least a few people on every ER shift that I work who are there because they are having persistent trouble breathing or other side effects as a result of Covid-19,” she said.

Ranney said she generally defines “young people” as those under 50. But “no matter which age cutoff you use, right now, we’re seeing more B.1.1.7 than the older variants.”

“We’re certainly seeing it more in 20s and 30s as well,” she said. “And people in their 20s and 30s are less likely to be vaccinated and more likely to be out and about.”

The vast majority of positive coronavirus tests don’t go through genomic sequencing to figure out whether it’s B.1.1.7 or another strain. But as genomic sequencing increases nationwide, health experts say there’s no doubt B.1.1.7 is fueling more hospitalizations among young people.

Dr. Justin Skrzynski is a Covid hospitalist — or specialist in the care of Covid-19 patients — at Beaumont Hospital, Royal Oak in Michigan. He said the facility sends a portion of its coronavirus samples to the state for DNA analysis.

“Right now, the regular Covid test we do — that’s still just showing Covid (or) no Covid,” Skrzynski said.

“But we do send a lot of those out to the state, and we are seeing something like 40% of our patients now (with) B.1.1.7.”

Reiner said he thinks both human behavior and the “stickiness” of B.1.1.7 are leading to more Covid-19 hospitalizations among young people.

“It may be simply because of just (more young people) getting infected … and perhaps the inoculum (viral load) is higher,” he said.

Sometimes, young people can be victims of their own strong immune systems.

Throughout the pandemic, doctors have noticed some young, previously healthy patients suffer from Covid-19 cytokine storms. That’s basically when someone’s immune system overreacts — potentially causing severe inflammation or other serious symptoms.

As B.1.1.7 keeps spreading, it’s possible the number of young people with cytokine storms will increase, Reiner said.

“We’ve certainly seen people come into our hospital, very young people (in their early 20s) … need to be put on ECMO, which is basically a heart-lung machine for days or even weeks because they come in with cardiomyopathy — which is a response to a cytokine storm,” he said.