Omicron is spreading at lightning speed. Scientists are trying to figure out why

So far, omicron's best trick — what helps explain its success more than anything else — is dodging our immunity: the antibodies and other immune defenses put on by the body after vaccination and/or prior infection.

The variant's many mutations on the spike protein allow it to infect human cells more efficiently than previous variants could, leaving many more people again vulnerable. Because of that, "immune escape" alone could be the major reason why the variant looks so contagious compared with delta, which was already highly transmissible.

In fact, omicron has been spreading at a pace that's comparable to how fast the original strain of the coronavirus spread at the very beginning of the pandemic, despite the world's newfound levels of immunity.

"The playing field for the virus right now is quite different than it was in the early days," says Dr. Joshua Schiffer, an infectious disease researcher at Fred Hutchinson Cancer Research Center. "The majority of variants we've seen to date couldn't survive in this immune environment."

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With so many mutations, it's still quite possible that omicron does have additional advantages that make it more contagious than other variants — advantages that rely on more than breaking through our prior immunity.

Maybe omicron can produce more copies of itself in a cell? Or maybe it sticks to cells more effectively? Or maybe it's better at hanging in the air and staying infectious?

"Any of those things would make the virus more contagious," says Schiffer of Fred Hutchinson.

One key difference emerging with omicron is just how quickly someone who gets infected becomes contagious.

Omicron appears to have a shorter incubation period and that can substantially speed up infections across the population. A study of the Oslo Christmas party outbreak found the incubation period could be about three days, compared with 4.3 days for delta and five days for other variants. A small study from CDC also puts the incubation period at about three days.

"That's actually a fairly significant difference," says Schiffer. This would mean there are many more cycles of infections and less time for people who are exposed to take precautions not to expose others.