Avian Influenza Prevalence in Pigs, Egypt

south were clear for adults and for children (Figure, panel A); the level of ILI was 3–5× for children. The infl uenza subtypes causing the 3 peaks in the north were preceded by a peak of the same subtypes in the south. During winter 2006–07, the infl uenza subtype was seasonal H1N1 and to a lesser extent H3N2. In winter 2007–08, the virus was B/Yamagata; and in 2008– 09, it was again seasonal infl uenza A (H1N1), which was almost absent in the south during April–December 2007. Antigenic characteristics of the infl uenza virus from the north were similar to those from the south in the same epidemic episode (2). Furthermore, infl uenza A (H3N2) was in southern China throughout the year, whereas in northern China, this subtype only showed a clear peak in the fi rst 2 winters of the study period. Subtype B/Victoria and B (unsubtyped) were both in northern and southern China in irregular and low numbers. Data from the 3 northern provincial areas with year-round surveillance confi rmed that infl uenza cases during April–September were negligible (data not shown). The infl uenza subtypes of seasonal infl uenza A (H1N1) and B/ Yamagata that have caused the past 3 summer peaks in southern China were followed by an epidemic of the same subtypes in northern China during the subsequent winter. This fi nding may indicate that these peaks are regular epidemic phenomena for seasonal infl uenza in China. Another possible explanation is that other subtypes were cocirculating with the predominant subtype at the time of epidemics. The dual pattern of seasonal peaks for infl uenza is well-known for the Northern and Southern Hemispheres, but apparently it is also possible on 1 side of the equator. China is a large country with climatic differences between north and south. Although most of southern China is above the Tropic of Cancer, it is warmer and more humid than northern China (Figure, panel B), which may explain the different seasonal patterns within mainland China (3). Knowledge of the dual patterns of infl uenza in China is relevant for determining effective control measures, and knowledge of the underlying mechanisms of such patterns is relevant to understanding the epidemiology of infl uenza in general.