Using Routine Surveillance Data to Estimate the Epidemic Potential of Emerging Zoonoses: Application to the Emergence of US Swine Origin Influenza A H3N2v Virus

Cauchemez, Simon; Epperson, Scott; Biggerstaff, Matthew; Swerdlow, David; Finelli, Lyn; Ferguson, Neil M.

The 2009 A(H1N1)pdm09 influenza pandemic [1] , the SARS epidemic in 2003 [2] , and the recent emergence of a novel coronavirus [3] are recent reminders of the global health threat posed by zoonotic viruses. Prior to widespread emergence in human populations, such pathogens can cause occasional infections in subpopulations that have been exposed to reservoir species (common reservoir species include for example bats, birds, swine, non-human primates). Whilst viruses causing such ''spill-over'' infections are usually poorly adapted for sustained human-to-human transmission, they are under strong selection pressure to increase transmissibility once in humans [4] . If the reproduction number R (i.e., the average number of persons infected by a case) evolves to exceed 1, a large scale epidemic in humans may result. Over the last decade, particular concerns were raised regarding highly pathogenic H5N1 avian influenza, due to the high mortality rate seen in humans and the virus's rapid spread in avian populations. However, as the A(H1N1)pdm09 influenza pandemic demonstrated, H5N1 is not the only influenza virus that may pose a pandemic risk. Recently, a swine-origin triple reassortant influenza A(H3N2) variant virus has emerged in the United States, carrying the matrix gene (M) from the H1N1pdm09 virus (H3N2v-M) [2] [3] [4] . Studies in animal models have suggested that the presence of the H1N1pdm09 M gene may increase transmissibility of the virus [5, 6] . From January 2012 to September 2012, 307 laboratory-confirmed H3N2v-M human infections were reported to Centers for Disease Control and Prevention (CDC) [5] as opposed to 12 throughout 2011 [6] . The majority of cases have been associated with agricultural fairs but there are documented events of human-to-human transmission [7] . The surge in cases observed in summer 2012 raised public health concerns [8] . Threats from zoonoses are not limited to influenza: more than half of all recent emerging infectious disease events were zoonotic [9] .