The most prevalent strain of influenza spreading among pigs in China has sparked concern among scientists, who say it has certain properties that give it pandemic potential. Chief among them: The virus has infected humans. So far it isn’t known to spread from person to person, but the worry is that with further mutations, it could start to do so. Populations still battling the coronavirus SARS-CoV-2, which also emerged in China and is believed to have originated in animals, could be vulnerable again.
1. What’s the problem?
Pigs are routinely catching variations of the flu and mostly it’s not a big deal, because the viruses usually don’t spread to people. But that’s not the case with the most-common strain infecting pigs in China since 2016, dubbed the G4 EA H1N1 virus. In past years it jumped the species barrier to infect possibly dozens of humans, according to so-called serosurveys that look for the presence of antibodies in a person’s blood, which indicates prior exposure to the virus. New research published June 29 in the Proceedings of the National Academy of Sciences showed the virus can replicate in cells lining the human airway, and can be efficiently transmitted among ferrets, an animal used to study flu viruses. Those features led that research team to declare that it “possesses all of the essential hallmarks of a candidate pandemic virus” and poses “a serious threat to human health.”
2. How worried should we be?
It’s hard to say. Influenza pandemics occur when a virus, against which there is little or no existing immunity, emerges in the human population and efficiently transmits from person to person. Most people lack immunity to this strain, which has a novel variant of hemagglutinin, the surface protein flu viruses use to grip onto cells targeted for invasion. There are also no vaccines available. Michael Ryan, executive director of health emergencies at the World Health Organization noted that many novel flu viruses, including ones circulating in birds, have pandemic potential, and that G4 EA H1N1 has been closely watched by scientists in China and around the world. Anthony Fauci, director of the National Institute for Allergy and Infectious Diseases in the U.S., said it’s not an immediate threat, but bears watching.
3. How prevalent is it?
Researchers say that it gradually spread across the 10 Chinese provinces with the highest swine density from 2011 to 2018. (China has almost half the planet’s pigs.) Numbers have declined since 2018, when African swine fever began ravaging hog farms. That virus, however, isn’t related to influenza and isn’t known to infect people, so it doesn’t pose a pandemic risk. Among humans, the serosurveys conducted from 2016 to 2018 in China found that, of 338 people working on hog farms who were tested, as many as 10% might have been infected. Prevalence of antibodies fell to 4.4% among 230 people who were not connected to farms. Liu Jinhua, the study’s lead author and a professor of veterinary medicine at the China Agricultural University in Beijing, told local media that the study results may have overestimated the number of people exposed.
4. Did anyone get sick?
It isn’t known if anyone in the serosurveys developed an illness or symptoms. But related strains caused at least two cases described in scientific studies: a 46-year-old man in Fujian province who developed severe pneumonia in October 2016 and died from multiple organ failure; and a 9-year-old boy in northern China, who experienced mild flu-like symptoms in December 2018 and recovered days later. It appears to cause little illness in pigs.
5. What can be done to mitigate the threat?
Biosecurity measures aimed at preventing the spread of the virus from farm to farm is key, along with surveillance. The more the virus spreads, the more chance it has to mutate. The WHO recommended in 2016 that “seed strains” be produced and stockpiled from which vaccines could be made to protect people. China hasn’t announced any plans on how to nip this virus in the bud, but it has managed to do so with other zoonotic diseases – those that can infect both animals and human. In 2017, China launched a program to vaccinate chickens against the H7N9 avian flu virus to prevent it from spilling over to humans. The poultry inoculation significantly reduced the number of human infections to just three in the year through September 2018, indicating success.
6. Where did the virus come from?
G4 EA H1N1 is a Eurasian, avian-like virus with genetic material from the 2009 H1N1 pandemic strain, dubbed “swine flu” because of genetic similarities with viruses known to circulate in pigs. That strain circulated worldwide in humans and is estimated to have caused 12,469 deaths in the US and as many as 575,400 globally. (Unusually for a flu virus, the vast majority of fatalities were under 65 years old.) People then passed the virus back to swine, which are considered to be mixing vessels for influenza viruses. Scientists believe G4 EA H1N1 is the result of the H1N1 flu virus mixing with one or more other influenza strains circulating in hogs.