Is Omicron as infectious as it gets? Scientists lay out their best and worst scenarios for the virus' future.

• Omicron raises questions about how much more infectious or dangerous the coronavirus can get.
• In scientists' best-case scenario, the virus could become endemic by the end of the decade.

For nearly six months, Delta seemed capable of vanquishing all other coronavirus variants — so much so that scientists wondered whether the virus had reached the apex of its ability to infect humans.

In November, Omicron threw scientists for a loop: After driving up cases in South Africa, it spread to dozens of countries seemingly overnight. Early lab studies suggest that Omicron increases the risk of reinfection relative to other variants and is better than the original virus at evading antibodies from two vaccine doses.

Now, scientists are wondering: Is Omicron as infectious as it gets?

There's no easy answer, but scientists have a few guesses about the virus' future. In the best-case scenario, they say, the coronavirus will become endemic, which means cases will persist at low levels, and seasonal outbreaks of relatively mild disease may come as a result. In a middle-of-the-road scenario, the virus could get even better at resisting vaccines, and more vaccinated people could be exposed to severe illness. And in a frightening scenario, the virus could recombine with another coronavirus to form a more lethal hybrid variant.

Some scientists aren't betting on that last outcome.

The virus "seems unlikely to do much worse than what we're already dealing with," Vaughn Cooper, the director of the Center for Evolutionary Biology and Medicine at the University of Pittsburgh, told Insider.

Still, he added, "I keep getting fooled."

The best-case scenario: The virus becomes a 'seasonal annoyance'

Viruses have one major objective: survival. Scientists generally agree that the best way for the novel coronavirus, SARS-CoV-2, to survive long term is to become endemic in the population, similar to the way influenza or other human coronaviruses have evolved.

If the virus became more lethal, it could be at a disadvantage, since people need to stay alive to keep infecting others. And because the virus is already skilled at transmitting, future variants may not need to drastically alter how they behave.

"Are we playing whack-a-mole forever with SARS-CoV-2? No. It's going to become an endemic coronavirus that is going to become a seasonal annoyance," Cooper said, adding: "That's going to happen this decade, maybe before the end of this decade."

While Cooper thinks some years will have worse coronavirus outbreaks than others, he expects vaccines to continue to ward off severe disease, he said. That's because antibodies aren't the body's only form of protection: White blood cells known as T cells and B cells also remember foreign invaders, often for longer periods than antibodies.

"One thing I am confident about is that my three doses of the original vaccine has created a diversity of cellular immune responses that are going to protect me against the virus several years from now," Cooper said. "I will bet good money on that. I may get sick, but it's not going to make me really sick because my T cells and B cells have seen something like this before."

The middle-of-the-road scenario: Vaccines become less effective as the virus evolves

The coronavirus may ultimately encounter a limit to how much it can spread when everyone capable of being infected has some degree of immunity. At that point, the virus might have to get better at circumventing the body's immune defenses — whether from vaccines or natural infection — to stay alive.

"The easiest way for the virus to cause new epidemics is to evade immunity over time," Adam Kucharski, a mathematical epidemiologist at the London School of Hygiene and Tropical Medicine, told Nature this month. "That's similar to what we see with the seasonal coronaviruses."

In that case, widespread immunity could put pressure on the virus to form new variants that render vaccines less and less effective.

"The evolutionary forces that could undermine vaccination — those are coming," Andrew Read, who studies the evolution of infectious diseases at Pennsylvania State University, told Insider. The virus, he added, "is a long way from having tapped all of its own mutational potential."

The worst-case scenario: The virus combines with another coronavirus inside an animal, then spills back into humans

As the coronavirus continues to spread widely, it's possible that an animal could get infected with two coronaviruses at once: the novel virus, SARS-CoV-2, plus another coronavirus found in wildlife. In a nightmare scenario, those viruses might form a hybrid variant that spills over to the human population and is more lethal than its predecessors.

"We've got an awful lot of SARS-2 around, so the potential for something to spill into humanity and recombine with SARS-2 is pretty high," Read said.

Already this century, three coronaviruses capable of causing severe disease — SARS-CoV, MERS-CoV, and SARS-CoV-2 — have spilled over from animals to humans. Coronaviruses also have a high capacity to recombine, Read said. A May study found evidence of a recombinant coronavirus in a hospitalized person with pneumonia. The study's researchers traced the origin of the virus to coronaviruses in cats and dogs. While scientists are still investigating the origins of SARS-CoV-2, a December 2020 study suggested that the virus emerged from a recombination of bat and pangolin coronaviruses.

So the possibility of another spillover event "seems pretty worrying to me," Read said.

But Cooper said it was rare for a human or animal to be infected with two viruses at once.

"We should be worried about it, but where does it stand among our worries? It's still quite low," he said, calling recombination "more of a long-term concern."

"Most recombination events fail because the parts don't work well together," Cooper said. "But if we've learned anything about this pandemic, when you have really huge numbers, really weird things happen."

Both Cooper and Read pointed to the white-tailed deer as a species to watch closely after a coronavirus outbreak tore through the US white-tailed-deer population in winter 2020.

"There are white-tailed deer all over the place in my neighborhood outside of Pittsburgh, and I can't help but look at them as all having had, or currently having, SARS-CoV-2," Cooper said. "The virus is evolving in them, too. We know this. How is it evolving? We don't know."