By Joel Achenbach, Ben Guarino and Aaron Steckelberg
Washington - Coronavirus infections are down across much of the United States. Hospitalizations, too.
Deaths are finally dropping from their dismaying late-summer peak of more than 2 000 a day. Most people are vaccinated, and booster shots are gaining approval. Officials in the United States are hoping the worst of the pandemic is over.
But so much depends on the virus itself. It is not static. It mutates. Delta, the variant of SARS-CoV-2 now causing virtually all infections in the United States, is more than twice as transmissible as the virus that emerged in Wuhan, China.
The possibility of further significant mutations in the virus looms like a giant asterisk over any discussion of the trajectory of the pandemic.
In recent weeks, scientists who closely monitor the virus have said it still appears to have plenty of room to evolve.
"I see nothing that suggests this virus is quieting down," said Kristian Andersen, an immunologist at Scripps Research. "I don't think this virus is as transmissible as it can be."
Scientists are tracking dozens of "sublineages" in the delta line of viruses, each with a slightly different array of mutations. One of those sublineages has spread with unusual speed in the United Kingdom recently and is gaining attention from researchers.
So far, there is no compelling evidence that any of the delta offspring have evolved into new, more dangerous variants. The attention-grabbing sublineage in the United Kingdom has taken several months to reach 8% of new infections there, so although it may have an advantage, it is not spreading with the kind of explosive speed seen with the ancestral delta strain, noted William Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health.
"This is nothing like that, but still worth keeping an eye on," Hanage said in an email. But he stipulated, "We'd have to be idiots to think the virus is done with us, and it will continue to evolve."
Many scientists suspect that the next "variant of concern," if and when one does emerge, is most likely to descend from delta. But viral evolution is inherently unpredictable.
"You can't predict the future - biology is too complicated. No one should even try," said Joel O. Wertheim, a biologist at the University of California at San Diego who studies how viruses evolve.
'They didn't want to believe it'
There was a time, early in the pandemic, when the scientific orthodoxy held that the coronavirus didn't mutate much, certainly not as promiscuously as influenza. The virus has a proofreading mechanism that limits genetic errors as it replicates.
But the virus surprised the experts. The first significant change in the virus was identified by Bette Korber, a theoretical biologist at the Los Alamos National Laboratory in New Mexico.
She had been scrutinizing the genomes of virus samples from around the world and noticed that one mutation, known as D614G, had become common in the virus in dozens of geographic locations. This mutation altered the positioning of the virus's spike protein - its tool for binding to cells.
Korber, in collaboration with researchers at Duke University and the University of Sheffield in England, concluded that the strain with the mutation was more transmissible than the first strain that circulated in China. They posted their findings online - and slammed into a wall of scientific skepticism.
Maybe, other scientists suggested, it was just a coincidence. Others simply didn't believe that a single mutation was likely to change the virus dramatically.
"They didn't want to believe it. They wanted to believe it was holding still," Korber said, recalling a "rough" period for her in the days after she posted her findings. "People wanted to hope for the best."
The smoking gun in the case of D614G is that the virus never randomly mutated back to the original strain.
"We could show that within weeks after it entered a new local population, the new form would soon dominate; the reverse, a switching from the mutant D614G to ancestral, just wasn't happening, and random events don't all go in one direction," she said in an email.
"It is very likely that the next variant will be a daughter of delta, and it may emerge anywhere," said Vaughn S. Cooper, who studies the evolution of microbes at the University of Pittsburgh.
Related video: