A group of H1N1 swine influenza viruses have essential hallmarks of being highly adapted to infect humans and are of potential pandemic concern, health officials say.

These viruses — referred to as G4 Eurasian (EA) avian-like H1N1 viruses — have been spreading in pigs in China since 2016 and are now the predominant set of genes that can be passed down from parents to offspring, according to the Centers for Disease Control and Prevention.

The CDC cited a study in the journal Proceedings of the National Academy of Science that shows the viruses have the right characteristics for causing infections in people, including the ability to grow well in human lung cells and to spread by respiratory droplets and direct contact in an animal model.

G4 EA H1N1 has not yet been shown to infect humans, but it is exhibiting "re-assortment capabilities," Dr. Anthony Fauci, the nation's top infectious disease expert, told the Senate Health, Education, Labor and Pensions Committee during a hearing last month.

Fauci said the new strain of flu carried by pigs in China has characteristics of the 2009 H1N1 virus and 1918 pandemic flu.

“When you get a brand new virus that turns out to be a pandemic virus, it’s either due to mutations and/or the reassortment or exchanges of genes,” Fauci told lawmakers. “And they’re seeing virus in swine, in pigs now, that have characteristics of the 2009 H1N1, of the original 1918, which many of our flu viruses have remnants of that in it, as well as segments from other hosts, like swine.”

The 1918 flu, which Fauci has often compared to COVID-19, is estimated to have killed between 30 million and 50 million people, according to the CDC.

The study found that about 10% of swine workers from whom blood samples were taken in China had evidence of prior infection with G4 viruses, which suggests human infection is more common than previously believed.

The CDC emphasized there are no reports of G4 viruses spreading from person to person, which is a required characteristic for declaring a pandemic. Also, G4 viruses have not been detected in pigs or people in the United States.

However, like all flu viruses with pandemic potential, the CDC is taking a number of actions to monitor and prepare against this emerging public health threat, including:

  • Coordinating with public health partners in China, including requesting a virus sample
  • Assessing the risk of the virus causing a pandemic using CDC’s Influenza Risk Assessment Tool (IRAT)
  • Evaluating whether an existing candidate vaccine virus (CVV) against a closely related flu virus (called “G5”) would protect against this virus,
  • If needed, creating a new CVV specific to G4 viruses, and
  • Studying whether existing flu antiviral drugs offer protection against this group of viruses.

Experts believe most people would lack immunity against G4 viruses, and despite seasonal flu vaccines protecting against the 2009 H1N1 virus, G4 viruses are different enough that seasonal flu vaccines would be unlikely to provide protection or prevent onward human-to-human transmission.

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