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.

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.

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).

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.

But as genomic sequencing increases nationwide, health experts say there's no doubt B.1.1.7 is fueling more hospitalizations among young people.

"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 ...

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.

'Covid-19 doesn't have to kill you to wreck your life'

As more young people get infected, doctors are worried they'll see more of a disturbing trend they've noticed for months -- long-term complications.

"I cannot tell you how many people I've taken care of in the ER who are in their 20s, 30s and 40s, who are never sick enough to end up in the ER with Covid, but who now have long-lasting respiratory difficulties," Ranney said.

And that's likely fueling the spread of B.1.1.7 among young people, Reiner said.

"There is no economic hardship, and there's no personal hardship to require a person to wear a mask when they're out in public."

Ranney said young people may misinterpret the lifting of safety mandates.

"When you hear that ...

"So I think there is that mixed message."

B.1.1.7 is also spreading among children

It's not just young adults who are getting infected with this variant.