To the editor:
Influenza is a complex and continually changing disease that infects individuals of all ages.1 In April 2009, the 2009 pandemic influenza A (pH1N1) virus was identified in the United States in California and in Mexico, which was followed by a worldwide spread of the infection.2 The first imported case in China was reported on 10 May 2009, followed by the wide spread of pH1N1 virus in the country, leading to an unexpected earlier influenza season in the northern China, which usually takes place on the beginning of November.
In a 2-year influenza-like illness (ILI) surveillance from 1 April (week 14) 2009 to 31 March (week 13) 2011, we performed surveillance at eight hospitals in five cities of China (the Hospital 301 in Beijing, the Children’s Hospital of Chongqing Medical University in Chongqing, four sentinel hospitals of Chinese People’s Armed Police Forces(CPAPF) CDC including the General Hospital of CPAPF, the General Hospital of Henan People’s Armed Police Corps, the General Hospital of Shanxi People’s Armed Police Corps, the affiliated hospital of Logistic University of CPAFP and two hospitals of Jinan CDC including the Fourth People’s Hospital of Jinan City and the Sixth People’s Hospital of Jinan City) (Appendix S1). All the hospitals were members of the Chinese national influenza sentinel hospitals, and the inclusion of ILI cases was performed according to the standard criteria defined by China CDC.3 Altogether 6143 ILI cases (person-time) with monthly average of 255 cases were included. An estimated 26.78% (1645/6143) cases were detected positive for influenza, in which seasonal H3N2 (sH3N2), pH1N1, and influenza B accounted for (41.76%, 687/1645), (31.98%, 526/1645), and (21.15%, 348/1645) of the total ILI cases, followed by untyped influenza (3.22%, 53/1645) and sH1N1 (1.88%, 31/1645), respectively. Temporally, the influenza were predominantly detected from August to March next year during the 2009–2010 influenza season, while from November to January next year in the 2010–2011 season (Figure 1). Geographically, the detection rates of influenza virus as a whole and the predominant influenza subtypes were similar among observed areas. Influenza attacks the population of all ages; however, the highest age-specific attack rates of pH1N1 observed in 10–14 years (16.04%,47/293), sH3N2 in 30–39 years (14.93%, 63/422), sH1N1 in more than 70 years (2.63%, 2/76), and B in 5–9 years (12.30%, 61/496). Different epidemic patterns were observed for the 2009–2010 and the 2010–2011 influenza season. In the 2009–2010 influenza season, the peaking monthly detection rate was 52.84% (251/475), significantly higher than that of the 2010–2011 season (24.12%, 123/510). According to influenza subtyping analysis, the predominant (sub)types were sequenced as sH1N1, sH3N2, pH1N1, and influenza B in 2009–2010 influenza season, which was switched to sH3N2, pH1N1, and influenza B in the 2010–2011 season. Another interesting finding is that the epidemic of sH3N2 was kept at stable level during the 2-year surveillance, while the epidemic of pH1N1 and sH1N1 decreased remarkably in the 2010–2011 season, compared with that in the 2009–2010 season. Although published data showed that exposure to pH1N1 had no impact on typical influenza seasonal peaks according to surveillance,4 the data in this paper implied that typical epidemiology of influenza may be altered because of the introduction of pH1N1 in China.3 In summary, the predominant (sub)types of influenza were seasonal H3N2, pH1N1, and influenza B during April 2009 to March 2011. As a novel introduced virus, the pH1N1 influenza in 2009 may affect the epidemic of seasonal influenza, in which the intensity and the duration of influenza epidemic, as well as each influenza (sub)types could all be influenced.