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.

However, our methods do require that the source of infection (i.e., human or natural reservoir) can be identified for either the first detected cases of a cluster or a random subset of detected cases, depending on the surveillance scenario. In the US H3N2v-M context, where most individuals have no contact with swine, it is usually relatively easy to rule out the natural reservoir as a source of infection. Determining the source of infection might however be harder in situations where a large part of the population has regular contacts with the natural reservoir (e.g., backyard poultry in rural areas). In this case, an in-depth epidemiological investigation of the potential sources of infection is required. Clearly our approach cannot be used if the zoonotic source has not been identified yet. That being said, the examples we have presented show that proxy measures for the source of transmission (e.g., contacts with the natural reservoir) are often available. Although imperfect, these proxy measures are often already used by the scientific and public health community. For example, the World Health Organization fact sheet on Nipah virus states that ''in Bangladesh, half of reported cases between 2001 and 2008 were due to human-to-human transmission'' [23] . Our methods improve this existing practice in at least three ways: (1) it adds an essential layer of interpretation by providing estimates of the reproduction number, R, which allows assessment of how far transmission is from being sustained (R$1); (2) it clarifies sources of potential biases; and (3) it provides simple guidelines to reduce these biases (see summary in Box 1).