When the virus that caused the COVID-19 pandemic first emerged, many scientists thought it would evolve slowly, like other coronaviruses.
But that was one of the first big surprises from the virus dubbed SARS-CoV-2. It evolved like crazy.
"SARS-CoV-2 so far has probably been even faster than influenza virus, which is really remarkable," says Jesse Bloom, who studies viral evolution at the Fred Hutch Cancer Center in Seattle. "I thought it would undergo some evolution, but the speed at which it's undergone that evolution and the ability it's shown to undergo these big evolutionary jumps is really remarkable."
In fact, SARS-CoV-2 has been evolving the ability to evade the immune system about twice as fast as the fastest-evolving flu virus, punctuated by several large evolutionary jumps, scientists say. Most notoriously, SARS-CoV-2 jumped a huge evolutionary hurdle to spawn the omicron variant, which spread around the world with shocking speed.
And the pace of the virus's evolution hasn't changed much. The virus has kept evolving, averaging more than a dozen significant changes every year as the virus begets new additions to the rogue's gallery of variants.
"It's possible that the evolution of the virus has slowed down a little bit. But the evolution of the virus has not stopped," Bloom says.
But all of the meaningful new viral offspring have been descendants of omicron. So instead of getting new Greek letter names, the omicron subvariants go by tongue-twisters like BA.2.86, XBB.1.6 and JN.1.
Each new member of this menagerie of mutants just keeps finding new ways to try to get around the human immune system, enabling the virus to infect more than 700 million people worldwide and kill more than 7 million, according to the World Health Organization.
The immune system strikes back
But something else has changed: the human immune system.
"As we got infected by the virus — but also really importantly as we saw the vaccines roll out — that just built up immunity in the population, which means that SARS-CoV-2 all of a sudden ran into this immunity wall," says Kristian Andersen, an evolutionary biologist at the Scripps Research Institute in California. "And with the immunity wall, we have SARS-CoV-2 at least a little bit cornered now."
That's because the immunity wall keeps most people from getting very sick when they catch the virus.
"It's the fact that we have immunity that is largely responsible for making it seem like this virus is less dangerous," says Michael Worobey, who studies the evolution of viruses at the University of Arizona.
That doesn't mean the virus isn't dangerous anymore. SARS-CoV-2 still makes lots of people sick, even so sick they sometimes end up in the hospital or die. Hundreds are still dying every week in the U.S. from COVID, mostly older people and people with other health problems.
But the virus has essentially become endemic, meaning: This is the new normal. The virus is here to stay. But SARS-CoV-2 is no longer upending daily life.
"The immunity we've built up — it makes it like trying to start a forest fire after a few weeks' heavy rain versus when everything's tinder dry," says Jeremy Kamil, a microbiologist and immunologist at the University of Pittsburgh.
Most experts think this dynamic will continue for the foreseeable future. The virus will continue to evolve new ways to try to get around immune defenses. But most people's immune systems are able to keep the virus in check by learning from mild infections and vaccinations.
"We may just continue on in this phase of subvariants of subvariants of subvariants. [This] might just be what we deal with for decades and decades and decades," Worobey says.
The "immunity wall" needs maintenance
Many experts say it's unlikely that a much more dangerous new variant might emerge anytime soon. But they stress that it remains crucial to maintain the immunity wall to try to keep that from happening.
"That wall of immunity is something that is built up over time but it also can erode over time. So to keep up that wall, it's important that we continue to use tools to protect ourselves," says Jennifer Nuzzo, who runs Brown University's Pandemic Center.
People can protect themselves by getting updated vaccines once or twice a year to boost waning immunity, she says.
"Every time the virus replicates it's basically buying a couple evolutionary lottery tickets," says Angela Rasmussen, a virologist at the University of Saskatchewan. "Most of those are losers. But sometimes it hits a winner. So the key that people should think about, if they want to slow that process, is: Don't give the virus more opportunities to replicate. Don't let it buy any new lottery tickets."
That's one of the dangers of cutting international aid funding for HIV drugs to AIDS patients, experts say. Major new variants like omicron are believed to have emerged from a chronic infection in a patient whose immune system was suppressed.
"We've been surprised again and again by the evolution of this virus," says Sarah Otto, a theoretical biologist at the University of British Columbia. "New variants with a lot of change can emerge like this and spread like wildfire."
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