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Nearly 400 Cases of Swine Flu Transmission from People to Pigs in the US in 12 Years

In the United States, there have been approximately 370 documented cases of people transmitting swine flu to pigs over a span of 12 years. Understanding how this virus spreads between humans and pigs is crucial in reducing infections and ensuring the effectiveness of vaccines.

What is Swine Flu?

Swine flu is a condition that originated in pigs and is caused by a specific type of influenza A virus known as H1N1. This virus variant, called pdm09, led to a significant outbreak in humans in 2009. Since then, pdm09 has continued to infect both pigs and humans, with instances of transmission between the two species.

Research on Swine Flu Transmission

Tavis Anderson and his colleagues at the US Department of Agriculture conducted a study to investigate the spread of pdm09 since 2009. They analyzed genetic sequences of H1N1 virus samples collected from both humans and pigs in the US between 2009 and 2021. Out of the samples, 12,823 were from humans and 1,112 were from pigs, all of which were pdm09 strain.

Assessing Transmission

Viruses evolve rapidly within a host’s body and develop specific traits as they infect different species. To determine cases of transmission between humans and pigs, the researchers compared the genetic similarity of different samples. If a version of the virus in a person was distinct from other human versions, it was assumed to be transmitted by a pig. Conversely, if a pig virus differed from other pig versions, it was assumed to be transmitted by a person.


Based on their analysis, the researchers estimated that there were approximately 370 cases of human-to-swine transmission and 17 cases of swine-to-human transmission over the 12-year study period, with most likely occurring in farming settings. It is important to note that the true number of cases may be higher as not all infections lead to reported illness. Additionally, the virus showed a high rate of evolution within pigs, posing challenges to swine flu vaccine effectiveness.

Implications and Control Measures

During the COVID-19 pandemic from 2020 to 2021, cases of pdm09 in humans decreased, while it continued to circulate in pigs due to previous infections. This suggests that implementing proper control measures can reduce interspecies transmission of swine flu. Rebecca Jane Cox Brokstad from the University of Bergen in Norway emphasizes the need to understand all components associated with virus transmission between different hosts to improve both animal and human health.

In conclusion, the transmission of swine flu from people to pigs in the US has been observed in approximately 370 cases over a 12-year period. Studying and understanding the dynamics of this transmission is critical in preventing infections and maintaining the efficacy of vaccines.

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