A highly mutated form of the coronavirus that threatens to be the most adept yet at slipping past the body’s immune defenses is capturing the attention of virologists and health officials.

While only about a dozen cases of the new BA.2.86 variant have been reported worldwide – including three in the United States – experts say this variant requires intense monitoring and vigilance that many of its predecessors did not. That’s because it has even greater potential to escape the antibodies that protect people from getting sick, even if you’ve recently been infected or vaccinated.

The latest variant does not appear to make people sicker than earlier iterations of the virus; antiviral treatments should still work against it and tests should still detect it, according to a risk assessment published Wednesday by the Centers for Disease Control and Prevention. It’s too soon to know whether the updated booster formula expected to come out next month that targets a different subvariant would be effective against this highly mutated one.

If this feels like déjà vu, it should.

After igniting a global explosion of cases in the winter of 2021-2022, the highly contagious omicron variant spawned a slew of immune-evading descendants. Most fade into irrelevance. Others, such as the XBB lineage accounting for most U.S. cases this year, outcompete the rest with their transmissibility and ability to infect – and reinfect. Nearly every announcement of a new variant came with reassurance from public health officials that it did not cause more severe illness.

But BA.2.86 stands out in the omicron family tree because of how much it has morphed. It has more than 30 mutations on its spike protein, the part of the virus that pierces through the cell and that vaccines train the body to fend off. Experts believe the antibodies forged through battles with earlier variants will have a difficult time recognizing this new foe.

“This is a radical change of the virus like what happened with omicron, which caught a lot of people defenseless,” said Eric Topol, director of the Scripps Research Translational Institute. “Even if they had a vaccine or prior infection, it could still get into them and infect them again or for the first time. We are facing that again.”

The biggest unknown is whether the BA.2.86 will be transmissible enough to cause a surge. A variant adept at evading immunity would not take off in a population if it does not spread efficiently and multiply.

“It is still a possibility we either see this variant spread very widely as happened with the original omicron,” said Jesse Bloom, a computational biologist who monitors coronavirus variants at the Fred Hutchinson Cancer Center in Seattle, “or it doesn’t end up spreading very widely and we continue to have these XBB variants.”

Experts have paid close attention to Bloom’s analysis of BA.2.86, which stressed that the evolutionary jump is on par with the one that gave rise to omicron.

Officials say they should have a clearer understanding of the variant’s transmissibility in the coming weeks as surveillance ramps up in response to the threat.

On Aug. 12, virologists in Israel were the first to announce that BA.2.86 had been detected in a patient. Scientists in Denmark, Britain, South Africa, Switzerland and Thailand soon followed. Some of the samples were collected in late July. The sporadic reports add up to evidence of international transmission, but the magnitude is unclear.

In one Swiss town, roughly 2 percent of coronavirus particles sequenced in a wastewater sample taken in early August were BA.2.86 variants. That was the first sign of a variant spreading within a community.

“Finding BA.2.86 in yet an additional country – now in wastewater rather than a patient – is another piece of evidence that BA.2.86 is globally widespread by now,” Tanja Stadler, a professor at the Swiss Federal Institute of Technology who announced the findings, wrote in an email. “It is still too early to tell if this variant will cause a major wave.”

The United States is experiencing an uptick of cases believed to be driven primarily by XBB and the new EG.5 variant, but several BA.2.86 cases have been announced in the past week. One of the BA.2.86 cases was discovered in Michigan in an older adult who was not hospitalized, while another was detected in a traveler arriving at Dulles International Airport in Virginia. A third case was detected in an Ohio wastewater sample.

Areas where BA.2.86 have been detected have not seen disproportionate increases in hospitalizations, the CDC said, suggesting it’s not more likely to cause more severe illness than other omicron subvariants.

Even if variants are adept at escaping antibodies to enter cells, those antibodies are just the front lines of the immune system, which has other mechanisms to fight off the virus and keep the infection less severe. It’s like a teenager who can sneak past a bouncer to get into a club but gets thrown out by security before causing trouble. When omicron struck, a huge swath of the country lacked immunity because they never had covid or had not been vaccinated – but that’s no longer the case.

“Nearly all the U.S. population has antibodies to SARS-CoV-2 from vaccination, previous infection, or both, and it is probable that these antibodies will continue to provide some protection against severe disease from this variant,” the CDC said in its risk assessment of BA.2.86 this week. “This is an area of ongoing scientific investigation.”

BA.2.86 poses a public health messaging challenge. For a year and a half, headlines announced new omicron subvariants with alphanumeric names that resemble preset WiFi passwords, accompanied by similar descriptions of immune evasion without causing severe illness. Officials worry about a covid-fatigued public tuning out news of a variant that could pose a serious threat. It’s only been a few weeks since EG.5 captivated public attention, even though it’s not all that different from its XBB cousin.

The rise of BA.2.86 has raised concerns about declining visibility into the evolution of the virus. Genetic sequencing is necessary to track variants but these efforts have been pared back globally. GISAID, a global data sharing initiative for the coronavirus, has seen specimens collected for genetic sequencing plunge from 425,000 in August 2022 to 38,000 last month.

In the United States, the CDC no longer offers variant estimates for most regions of the country because of declines in surveillance.

Variant watchers say they’re worried the world may be blind to the true reach of BA.2.86 as a result.

“The situation served as a significant wake-up call,” said Rajendram Rajnarayanan, a computational biologist who monitors genetic sequencing data as part of an international consortium of virus watchers, in an email. “This is especially true as global testing and sequencing infrastructure started disappearing (or is in the process of getting dismantled) following the end of pandemic declarations earlier this year.”

The new coronavirus booster expected to become available in late September is designed to target the XBB.1.5 variant, which is largely similar to the other variants in circulation. Testing is underway to determine whether the booster will be effective against BA.2.86. But because BA.2.86 is so significantly mutated, some virologists are concerned that it can easily bypass the protection from infection the new booster would provide.

In a Thursday media briefing about vaccines, a Food and Drug Administration official told reporters the agency feels comfortable saying the updated booster would at least protect people from severe outcomes from BA.2.86 infections. But the Biden administration would not let that official express such confidence on the record.

Results from tests designed to demonstrate the boosters’ effectiveness against BA.2.86 are not expected until after Labor Day.

Regardless of whether BA.2.86 becomes a threat, Topol, the director of the Scripps Research Translational Institute, said the potential mismatch underscores the folly of attempting to annually update vaccines for an ever-evolving virus. He said government officials should steer resources toward developing vaccines that are essentially variant-proof with the ability to target a broad range of coronaviruses.

For now, experts say the rise of BA.2.86 is more of a subject for scientific scrutiny than public panic.

“My level of concern changes every hour,” said Katelyn Jetelina, an epidemiologist who writes a newsletter about covid-19 developments. “It’s hard. It’s like seeing a storm form out in the distance. We don’t know if it’s going to fizzle away or become a Category 5 or somewhere in between.”

(c) 2023, The Washington Post · Fenit Nirappil