Transmissibility of the Influenza Virus during Influenza Outbreaks and Related Asymptomatic Infection in Mainland China, 2005-2013

We collected 2768 Influenza-like illness emergency public health incidents from April 1, 2005 to November 30, 2013reported in the Emergency Public Reporting System. After screening by strict inclusion and exclusion criteria, there were 613 outbreaks analyzed with susceptible–exposed–infectious/asymptomatic–removed model in order to estimate the proportion of asymptomatic individuals (p) and the effective reproduction number (R(t)). The relation between R(t) and viral subtypes, regions, outbreak sites, populations, and seasons were analyzed. The mean values of p of different subtypes ranged from 0.09 to 0.15, but could be as high as up to 0.94. Different subtypes, provinces, regions, and sites of outbreak had statistically significantly different R(t). In particular, the southern region also manifested different R(t) by affected population size and seasonality. Our results provide China and also the rest of the world a reference to understand characteristics of transmission and develop prevention and control strategies.