Dr. Giovana Ciacci Zanella swabbing a pig snout to gather samples to test for influenza A virus. Credit: M.Marti and A.Grimes, USDA (CC-BY 2.0, https://creativecommons.org/licenses/by/2.0/
Key Points:
- The strain behind 2009’s “swine flu” has passed from humans to swine about 370 times since then.
- The circulation is causing the strain to evolve, possibly making transmission back to humans more likely.
- The researchers say reducing influenza A infection in humans can minimize spillover to pigs, and subsequent human recirculation.
A new study of the strain of influenza A responsible for the 2009 H1N1 pandemic—pdm09—shows that the virus has passed from humans to swine about 370 times since 2009. Since then, circulation of the virus among swine has led to evolutionary changes that could make it more likely to cross back and infect humans.
In the study published in PLOS Pathogens, the researchers from Iowa State University analyzed pdm09 transmission data between 2009 and 2021. The analysis showed that, since 2009, pdm09 has crossed from humans to swine about 370 separate times, with most of these events occurring when pdm09 burden was highest among humans.
In 2020 and 2021, during the COVID-19 pandemic, pdm09 circulation among humans dropped, but pdm09 circulation persisted in swine as a result of about 150 human-to-swine transmissions between 2018 and 2020.
The researchers found that most human-to-swine transmission events were isolated, but a few led to sustained circulation of different pdm09 genetic lineages among swine in the U.S. These swine-circulating variants appeared to be genetically poor matches for human seasonal vaccines, suggesting that the vaccines would have provided scant protection against them. The analysis also showed that persistent pdm09 circulation among swine was associated with at least five instances of swine-to-human transmission.
These findings add to mounting evidence that managing influenza A infection in people who work with swine can help prevent transmission to pigs, and subsequently reduce the risk of spread back to humans.
“Controlling influenza A virus infection in humans can minimize spillover of viruses into pigs and reduce the diversity of viruses circulating in swine populations. Limiting virus diversity in pigs can minimize the emergence of novel viruses and the potential for swine-to-human transmission of influenza A virus,” conclude the authors in their paper.