OBJECTIVE  To confirm the spatiotemporal distribution of the epidemic onset of influenza at city scale in China, and to elucidate the motivational relationship between meteorological factors and the timing of onset by comparing the onset time of influenza virus and its subtypes in different latitude areas and age groups.

METHODS  Nation-wide etiological surveillance data of influenza-like illness (ILI) during 2009-2018 in China were analyzed. The onset time of influenza epidemic was estimated at city scale per epidemic year by the moving epidemic method (MEM), and their association with meteorological factors were tested by additive Cox proportional hazard model.

RESULTS  A total of 3, 666, 453 ILI cases from 669 sentinel hospitals in 342 major cities of China were tested and sub-typed for influenza virus during the study period. Subtype H3N2 had the highest positive detection rate (4.67%), followed by H1N1pdm09 (4.44%), Yamagata (1.73%), Victoria (0.83%), and H1N1 (0.07%). In low- and low-mid-latitude regions, the onset of the highest epidemic waves (HEWs) mostly occurred around December and the first epidemic waves (FEWs) mostly around October. In high-mid- and high-latitude regions, HEW and FEW both typically occurred in December. FEWs of influenza started generally earlier in the south than those in northern of China; while comparing with other influenza virus subtypes, FEWs of H3N2 occurred early. The onset time of HEW was affected by temperature and peak of previous HEW in a non-linear manner but significantly postponed by relative humidity and sunshine duration in an approximately linear manner.

CONCLUSIONS  The onset timing of influenza epidemics vary due to the differences of latitude regions and virus subtypes. Assessing the temporal characteristics of influenza epidemics may facilitate early warning and prevention of influenza outbreaks.