By Fenit Nirappil and Lauren Weber
Three years after the novel coronavirus emerged, a new variant, XBB.1.5, is quickly becoming the dominant strain in parts of the US because of a potent mix of mutations that makes it easier to spread broadly, including among those who have been previously infected or vaccinated.
XBB.1.5, pegged by the World Health Organization as “the most transmissible” descendant yet of the omicron variant, rose from barely 2% of US cases at the start of December to more than 27% in the first week of January, according to new estimates by the Centers for Disease Control and Prevention.
More than 70% of cases in the north-east are believed to be XBB.1.5.
While there is no evidence that XBB.1.5 is more virulent than its predecessors, a recent swirl of misinformation linking the rise of new variants to vaccination has cast a spotlight on the latest strain and raised concern among some health experts that it could further limit booster uptake.
“XBB did not evolve because people were vaccinated,” said Vaughn Cooper, a professor of evolutionary biology at the University of Pittsburgh. “The way it evolved, let’s be straight, is because people were infected by multiple viruses at the same time.”
Since the omicron variant ignited an explosion of cases last winter, it spawned a host of descendants that are even more adept at slipping past antibodies and caused most infections in the US. The XBB line emerged as a result of two other omicron sub-variants swopping parts.
Virologists who studied XBB.1.5 say it does not appear better at escaping antibodies than other immune-evading predecessors, but it is better at binding to the cell and replicating. That means it can more easily become the dominant strain in a community than its relatives, but it doesn’t necessarily worsen an individual’s Covid symptoms.
XBB prompted some concerns because of lab studies showing antibodies generated by vaccines struggle to block the variant. But that doesn’t mean the vaccines don’t work: For much of the last year, waves of infections from immune-evading variants have not caused surges of severe illness at hospitals, and experts believe that widespread immunity from shots and prior infections are making people more resilient to the virus.
While antibodies do not provide foolproof shields against reinfections, they reduce the odds of contracting the virus and greatly reduce the likelihood of severe illness and hospitalisation.
“Things like boosters are always beneficial,” said Kristian Andersen, a professor in the department of immunology and microbiology who tracks coronavirus variants at the Scripps Research Institute. “Even if you get infected, you are expected to have less viral load, and you are expected to be able to transmit the virus less.”
But the focus on boosters is also drawing criticism. An opinion piece published in the Wall Street Journal by a member of its editorial board suggested that repeated vaccination made Americans more susceptible to new variants.
Paul Offit, the director of the vaccine education centre at the Children's Hospital of Philadelphia, said the risk of mild breakthrough infection had been consistent throughout the pandemic despite the virus’s evolution, so there was no reason to blame shots for the variants.
But he said it was difficult to justify regular booster shots for the general population when the protection against mild illness was brief and immunity from earlier shots or infections provided robust protection against severe outcomes.
“At some level, we are going to have to get used to mild illness unless you want to take three doses a year and stay in your house most of the time,” said Offit, who also advises the Food and Drug Administration on vaccines.
Several studies have tried to examine whether Sars-CoV-2 variants spread faster in locations with higher vaccination rates and found no evidence to support it, said Jesse Bloom, an evolutionary biologist at the Fred Hutchinson Cancer Center in Seattle.
“XBB.1.5 is just sort of the latest variant in what is going to be a continual process of the virus evolving,” Bloom said. “There’s a lot of space for the virus to get mutations.”
The World Health Organization is monitoring XBB.1.5 because of how quickly it has supplanted other sub-variants – and each wave of mass infection brings opportunities for the virus to morph into something more dangerous.
“The more this virus circulates, the more opportunities it will have to change,” said Maria Van Kerkhove, WHO’s technical lead on Covid-19 at a Wednesday news conference.
“We do expect further waves of infection around the world but that doesn’t have to translate into further waves of death because our countermeasures continue to work.”
XBB.1.5 surged first in the north-east, where it now makes up nearly three quarters of cases. But hospitals there have not raised alarms about people coming in sicker because of the new sub-variant.
The Washington Post’s Carolyn Johnson contributed to the report.