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.

Medical reference: CNN Health