WASHINGTON - Don't panic, but there is another virus out there that could cause a pandemic.
This one is an influenza strain circulating in pigs and their caretakers in China.
It is not currently causing widespread illness, and it may never do so. But it has "all the essential hallmarks of a candidate pandemic virus," according to the authors of a new study in the Proceedings of the National Academy of Sciences.
"This strain requires very close watching," said Andrew Pavia, chief of the pediatric infectious diseases division at the University of Utah, who was not involved with the research. "But at present, we just don't know what the pandemic potential is."
The new strain contains genes from the flu virus that caused a pandemic in 2009. It infects human cells and spreads easily in lab tests.
The strain has become dominant in Chinese pigs and has infected people. The study's authors tested workers at 15 hog farms in Hebei and Shandong provinces for the virus, as well as people living nearby. They found that about 10% of the workers and 4% of the locals had been exposed.
'Good news, bad news'
"There's good news and bad here," Pavia said. "I think the bad news is that once again, it looks as if we're identifying strains of flu that are emerging in populations with the potential to jump to humans."
However, only a handful of serious cases have been reported.
"The severity remains low. That's good news," Pavia said, adding, "there's no guarantee that it's going to stay that way."
Other factors also must change before alarm bells really go off, experts note.
"What is really important for influenza pandemic emergence, as well as for any viral pandemic emergence, is sustained airborne transmission," said University of Pittsburgh School of Medicine microbiologist and molecular geneticist Seema Lakdawala, who was not part of the research team.
While a few people are getting infected, she said there is no sign now of sustained transmission.
Food animals are a common source of new flu viruses. Birds, pigs and humans can all exchange flu strains. Pigs are especially welcoming environments for influenza viruses to reinvent themselves. Multiple strains can infect one animal, swap genes and emerge as a novel strain.
There is no telling when the right combination of genes will fall into place and produce a virulent, transmissible virus.
A lethal strain called H5N1 first appeared in poultry in Hong Kong in 1997 and resurfaced in 2003. It kills more than half the people it infects. But for reasons scientists do not understand, it has not gone pandemic.
"It's still a concern. It has caused hundreds of deaths," said senior scholar Gigi Gronvall at the Johns Hopkins Center for Health Security, who was not involved with the research. "But for whatever reason, even though all eyes were on that, it was this other virus that took off in 2009."
That year, H1N1 emerged from pigs and sparked a pandemic. Researchers estimate that nearly 300,000 people died from it in the first year.
Since then, health officials have increased efforts to monitor livestock farms and markets for new viruses.
"There's been a big improvement, but it's far from complete," Pavia said. "The challenge is enormous. Influenza circulates among ducks, turkeys, swine - not to mention there are strains that infect everything from horses to dogs. And tracking all of these is an enormous task."
The effort is understaffed and underfunded, "like so many things in public health," he said.
And that's dangerous.
"We've seen the consequences of inadequate public health surveillance in the emergence and failure to control COVID-19," Pavia noted.
Unlike COVID-19, health experts have tools against influenza that might help if the new strain were to launch a new pandemic.
"We know how to test for influenza viruses," Lakdawala said. Flu antivirals are only partly effective, "but we do at least have antivirals that can limit the severity of disease. We have a number of them. We also have a vaccine platform that is already approved and safe."
A vaccine could be available in a matter of months.
But there is no way to know whether the newly identified strain will spark a pandemic.
"The more you study flu, the more you realize we just don't know how to predict that," Pavia said.