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Objectives We aimed to determine the frequency of pH1N1 transmission between humans and swine on backyard farms in Tumbes, Peru. Design Two-year serial cross-sectional study comprising four sampling periods: March 2009 (pre-pandemic), October 2009 (peak of the pandemic in Peru), April 2010 (1st post-pandemic period), and October 2011 (2nd post-pandemic period). Sample Backyard swine serum, tracheal swabs, and lung sample were collected during each sampling period. Main outcome measures We assessed current and past pH1N1 infection in swine through serological testing, virus culture, and RTPCR and compared the results with human incidence data from a population-based active surveillance cohort study in Peru. Results Among 1303 swine sampled, the antibody prevalence to pH1N1 was 0% pre-pandemic, 8% at the peak of the human pandemic (October 2009), and 24% in April 2010 and 1% in October 2011 (post-pandemic sampling periods). Trends in swine seropositivity paralleled those seen in humans in Tumbes. The pH1N1 virus was isolated from three pigs during the peak of the pandemic. Phylogenetic analysis revealed that these viruses likely represent two separate human-to-swine transmission events in backyard farm settings. Conclusions Our findings suggest that human-to-swine pH1N1 transmission occurred during the pandemic among backyard farms in Peru, emphasizing the importance of interspecies transmission in backyard pig populations. Continued surveillance for influenza viruses in backyard farms is warranted. [ABSTRACT FROM AUTHOR]