A new study published in the open-access journal PLOS Pathogens by Alexey Markin and colleagues from the U.S. Department of Agriculture-Agricultural Research Service reveals that the strain of influenza A responsible for the 2009 H1N1 pandemic, known as pdm09, has undergone approximately 370 instances of transmission from humans to swine since 2009. This circulation in swine has led to the development of pdm09 variants, which have then jumped back to infect humans.
Influenza A is capable of causing flu in humans, birds, swine, and other mammals. The pdm09 pandemic in 2009 and 2010 resulted in numerous human fatalities worldwide. Previous studies have demonstrated the repeated transmission of pdm09 from humans to swine, and this circulation among swine has introduced evolutionary changes in pdm09, potentially increasing the likelihood of re-infection in humans.
To gain a better understanding of this risk, Markin and colleagues conducted an analysis of pdm09 transmission data from 2009 to 2021. They also investigated how these inter-species transmissions may have influenced the genetic diversity of the virus in swine and the subsequent risk of human infection.
The analysis revealed that pdm09 has crossed from humans to swine approximately 370 times since 2009, with the majority of these transmissions occurring when pdm09 was most prevalent among humans. Despite a decrease in pdm09 circulation among humans in 2020 and 2021 due to the COVID-19 pandemic, pdm09 continued to circulate in swine as a result of approximately 150 transmissions from humans to swine between 2018 and 2020.
The researchers observed that most of the human-to-swine transmissions were isolated incidents, but a few led to sustained circulation of different pdm09 genetic lineages among swine in the U.S. These variants circulating among swine appeared to have genetic characteristics that did not match well with human seasonal vaccines, suggesting that these vaccines would have provided limited protection against them. The analysis also revealed that persistent pdm09 circulation in swine was associated with at least five instances of transmission back to humans.
These findings further support the growing evidence that managing influenza A infections in individuals working with swine can help prevent transmission to pigs and subsequently reduce the risk of spread back to humans.
The authors emphasize the importance of controlling influenza A virus infections in humans to limit the spillover of viruses into pigs and reduce the diversity of circulating viruses in swine populations. By minimizing virus diversity in pigs, the emergence of novel viruses and the potential for swine-to-human transmission of influenza A can be minimized.