People in the US passed swine flu to pigs nearly 400 times in 12 years

People have passed swine flu to pigs at least 370 times over 12 years in the US. Better understanding how the virus behind the disease moves between humans and pigs could help to reduce infections and ensure vaccines stay effective.

Swine flu is the common name for a condition that originated in pigs. It is caused by a type of influenza A, known as H1N1, of which a variant called pdm09 led to an outbreak in people in 2009. Since then, the variant has lingered in pigs and infected a small number of people, with some recorded cases of transmission between the two.

Tavis Anderson at the US Department of Agriculture and his colleagues wanted to investigate how pdm09 has spread since 2009. They analysed genetic sequences of all the H1N1 virus samples collected in people and pigs in the US between 2009 and 2021. Of these, 12,823 human and 1112 swine samples were of the pdm09 strain.

Viruses evolve quickly in a host’s body and often develop specific traits in different species, says Anderson. To assess which of these samples were a result of transmission between the two species, the researchers looked at how similar the samples were to each other. If one version of the virus in a person looked reasonably different to versions in other people, they assumed it was passed by a pig. Conversely, if a version of the virus in a pig looked different to those in other pigs, they assumed it was passed by a person.

From this, the researchers estimated there were 370 cases of human-to-swine transmission and 17 cases of swine-to-human transmission over the study’s 12-year period, with most of these probably occurring due to farming. However, the true number may be higher as not all infections lead to ill health and can therefore go unreported, says Anderson.

The virus also appeared to evolve particularly rapidly in pigs and continued to circulate in these animals. Vaccines against swine flu can become less effective due to these evolutionary changes, says Anderson.

Between 2020 and 2021, during the height of the covid-19 pandemic, cases of pdm09 fell in people but continued to circulate in pigs as a carryover from infections in these animals from previous years. This suggests it is possible to reduce the interspecies transmission of swine flu if proper control measures are taken, says Rebecca Jane Cox Brokstad at the University of Bergen, Norway.

“We have to understand all the components associated with the transmission of the viruses between different hosts,” says Anderson. “Hopefully if we do that, we could improve animal health and human health.”