The story of swine flu, or the H1N1 virus, is a continually evolving narrative, marked by its ability to adapt and spread across species. In recent developments, 2023 has seen notable instances of swine flu transmission, not only among pigs but also crossing over to humans.

In the U.S., two human infections with swine flu viruses were reported in Michigan. Both cases involved exposure to pigs at agricultural fairs, with the infections caused by influenza A(H3)v and influenza A(H1N2)v variants. These variants typically associate with pigs. The affected individuals were treated with flu antivirals and have recovered, highlighting the importance of swift medical intervention​​.

A study from the U.S. Department of Agriculture-Agricultural Research Service revealed that since 2009, the H1N1 pandemic strain (pdm09) has passed from humans to swine about 370 times. This inter-species transmission has resulted in the evolution of pdm09 variants. Significantly, some of these variants have made the jump back to humans, posing a potential pandemic risk. This continuous cycle of transmission between humans and swine emphasizes the need for vigilant monitoring and controlling of influenza A virus infections, especially in those who work with swine​​.

Further research indicates 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 variants were found to be genetically poor matches for human seasonal vaccines, suggesting that these vaccines might provide limited protection against them. This finding underscores the importance of developing and updating vaccines to keep pace with the evolving strains of the virus​​.

The CDC’s recent reports of human infections with flu viruses from pigs in 2023 reflect the ongoing public health challenge posed by swine influenza. These cases occurred in people who had exposure to pigs at agricultural fairs in Michigan, and they were caused by two different types of flu viruses that normally spread among pigs​​​​​​.

Swine flu continues to be a significant public health concern due to its ability to mutate and cross species barriers. The constant evolution of the virus, the interspecies transmission, and the potential mismatch with existing vaccines make it a complex and persistent global health threat. Therefore, continuous surveillance, both in human and swine populations, and proactive measures, including vaccination and biosecurity practices at agricultural fairs, are crucial to manage and mitigate the risks associated with swine flu.