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, and subsequent circulation in swine has resulted in the evolution of pdm09 variants that then jumped from swine to humans. Alexey Markin of the U.S. Department of Agriculture-Agricultural Research Service and colleagues present these findings in the open-access journal PLOS Pathogens.
Influenza A can cause the flu in humans, birds, swine, and some other mammals. In 2009 and 2010, a pandemic caused by pdm09 resulted in thousands of human deaths around the world. Since then, as demonstrated in prior studies, pdm09 has repeatedly passed from humans to swine, and circulation of the virus among swine leads to evolutionary changes in pdm09 that could make it more likely to cross back and infect humans.
To better understand this risk, Markin and colleagues analyzed pdm09 transmission data between 2009 and 2021. They also investigated how these interspecies events may have affected the genetic diversity of the virus in swine and the risk of subsequent human infection.
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.