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.

Recent efforts at CDC to evaluate the case detection rate suggest that it is low, of the order of 0.5% of all H3N2v-Mattributable cases (Biggerstaff et al., personal communication). For this likely scenario of low detection rates (#1%), we estimate the reproduction number of variant viruses other than H3N2v-M to be 0.2 (95% CI 0.1-0.4) ( Table 1) . For the H3N2v-M virus, the point estimate and the lower bound of the 95% CI of R are 0.5 and 0.2, respectively. The upper bound of the 95% CI of R lies between 0.8 and .1, depending on assumptions about the case detection rate and overdispersion parameter (Table 1 ). If case-tocase variation in infectiousness is very large (k = 0. 16, comparable to that seen in the SARS epidemic [12] ), we can rule out the hypothesis R$1 so long as the case detection rate is #0.7%; for a scenario with medium levels of variation (k = 0.5), the case detection rate must be #1.7% to rule out R$1 ( Figure 3 )-a detection rate of 1.7% is unlikely to be achieved by US sentinel virologic surveillance system. The point estimate of R for H3N2v-M virus is more than double that of other variant viruses, but we cannot reject the hypothesis of equality (p = 0.15), maybe due to a lack of statistical power.