Estimates of age-specific incidence rates of outpatient influenza in a prospective study in south China, 2008–2009.

“i” is the number of influenza-like illness (ILI) patients in the sentinel hospitals. “ii” is the number of outpatient influenza cases in the sentinel hospitals. This number is obtained by multiplying “i” by the age-specific influenza isolation rate associated with ILI. The age-specific influenza isolation rate of ILI is obtained through laboratory detection for a representative random sample of ILI cases. “iii” is the estimated number of outpatient influenza cases in the community . It is obtained by correcting “ii” for a catchment area ratio of 80%. “iiii” is the incidence of outpatient influenza cases in the community. It is obtained by dividing “iii” by mid-year population.*There were 203 and 53 ILI cases in 2008 and 2009, respectively, whose age registration information was lost.“/” means that no case counting or analysis was done on corresponding variables.