An omicron subvariant is once again demonstrating immune-dodging abilities, posing a threat to both vaccinated and previously infected individuals.
A report published Wednesday in the New England Journal of Medicine suggests that the subvariant, called BA.4.6, could drive reinfections.
As of Friday, BA.4.6 accounted for just over 12% of new Covid cases in the U.S. BA.5, meanwhile, has been detected in nearly 68% of new cases, according to the Centers for Disease Control and Prevention.
These subvariants are far from the only strains that experts worldwide are keeping their eyes on. Other omicron subvariants that have piqued the interest — and concerns — of scientists read like a viral alphabet soup: BQ.1, BQ.1.1, BF.7. (Those three, as it turns out, each account for around 5% of new U.S. cases.)
Gone are the days of identifying Covid by Greek letters, such as alpha and delta. Ever since the omicron variant emerged, it's been omicron all the way down, with omicron subvariants splitting off into their own subvariants.
"It's astonishing to see how the virus keeps mutating at such a rapid rate," said study author Dr. Dan Barouch, director of the Center for Virology and Vaccine Research at the Beth Israel Deaconess Medical Center in Boston. "This is essentially viral evolution on steroids."
Barouch's study was small, including just 35 people who'd had either the Covid vaccine or an omicron infection. Most, regardless of prior infection, had at least three doses of Covid vaccine. Blood samples showed that antibodies meant to neutralize BA.4.6 were about twofold lower than antibodies for BA.5.
"This suggests that omicron continues to evolve and continues to evolve in a way that becomes more transmissible and more effective at escaping vaccines and immune responses," he said. "The results are actually a harbinger to new variants that might be even more worrisome."
Viruses mutate at random, but mutations that give the virus an advantage over the immune system, vaccines or treatments tend to be the ones that stick around.
"The virus is very rapidly diversifying," said Bill Hanage, an associate professor of epidemiology at the Harvard T.H. Chan School of Public Health.
Compounding matters further, people have different levels of protection, depending on which vaccines they got (or didn't get) and how many infections they've had.
"There's a patchwork of immunity, which enables lots of hopeful monsters to emerge and transmit," Hanage said.
The updated Covid booster shots, which became available just last month, target both BA.4 and BA.5. But there's still no data on how the new shots work against BA.4 and BA.5, not to mention the slew of other subvariants.
The World Health Organization is tracking more than 300 omicron subvariants worldwide, Maria Van Kerkhove, head of the WHO’s emerging diseases and zoonosis unit, said during a media briefing Wednesday. All of them, she said, show increased transmissibility.
Adriana Heguy, a professor of pathology and director of the Genome Technology Center at NYU Grossman School of Medicine, said her team has detected omicron subvariants XBB and XAZ in New York City. Those are not yet listed in the CDC's variant tracker.
"Everything comes to New York first," Heguy added.
Even as the subvariants take off around the globe, there's no evidence yet that they're making people sicker.
"We don't see a change in severity yet," Van Kerkhove said.
Heguy echoed the message. Her team is not yet seeing an increase in Covid-related hospitalizations.
"This is not March 2020," she said, "but I think we should exercise caution."