Occupational Distribution and Prevalence of Influenza, China, 2008–2012

To the Editor—From 19 February to 9 May 2013, emerging H7N9 influenza in China has caused 131 cases including 26 deaths, and reignited global concerns regarding pandemic spread of novel influenza viruses [1, 2]. Nearly 60% of H7N9 infected persons have worked with or had contact with poultry [3, 4], demonstrating occupational exposure. However, the relationship between occupation and influenza prevalence is not well understood [5, 6], hampering effective prevention and control [7]. We analyzed the relationship between occupation and prevalence of seasonal and A(H1N1)pdm09 influenza from 2008 to 2012 using data from the China Information System for Disease Control and Prevention. From 2008 to 2012, 659 067 influenza cases including 494 414 seasonal influenza (SI, consisting of H3 and H1 subtypes) and 164 653 A(H1N1)pdm09 cases were reported in China. Four occupations accounted for 76.8%–86.3% cases of SI (Figure 1A). Students (17.7%–61.0%) or peasants (9.1%–45.5%) were the dominant groups followed by scattered children (7.9%–20.0%, a group of children who are not currently in school or kindergarten, but dwelling in scattered areas) and kindergarten children (5.2%–7.0%). Yearly variations were common; in 2008, students (34.6%) and peasants (32.1%) dominated, while in 2009 students (61.0%) far exceeded the next group (peasants, 9.1%). In 2010, 2011, and 2012, peasants were most affected (29.5%, 45.5%, and 38.1%, respectively) followed by students or scattered children. Occupations of persons infected by A(H1N1)pdm09 influenza presented a different distribution, with students overwhelmingly affected (72.9%; Figure 1B). Occupation distribution (A and B) and prevalence (C and D) of seasonal influenza (A and C; consisting of H3 and H1 subtypes) and A(H1N1)pdm09 (B and D) in China from 2008 to 2012. Since 2008, SI has presented with varying prevalence, ranging from 3.2 cases per 100 000 persons in 2008 to 14.9 cases per 100 000 in 2009 (Figure 1C and 1D). With the exception of 2009, scattered children had the largest prevalence for SI. This is probably due to their relatively lower vaccine coverage compared with other groups. However, in 2009, students (52.9/100 000) exceeded scattered children (38.1/100 000) with the highest prevalence of SI; prevalence of novel A(H1N1)pdm09 influenza was also greatest in students (46.4/100 000); scattered children (11.7/100 000) were the sixth group. Emergence of A(H1N1)pdm09 influenza impacted occupational groups differently, afflicting students more often than scattered children. The higher density of schoolchildren than that of scattered children had an impact on prevalence [8]. We also observed variations seasonality of outbreaks; SI occurred between February and March and A(H1N1)pdm09 at the end of August/early September. This difference probably resulted from the start of the school year in September; during this time students move from their homes to a new and relatively closed community containing clusters of people and different living environments, potentially exposing them to new pathogen such as A(H1N1)pdm09 [9, 10]. In China, influenza vaccination is not incorporated into the national immunization program. Our results suggest that occupational distribution of influenza in China is unique compared to other countries [5–7]; scattered children and students are at the greatest risk for SI and A(H1N1)pdm09 infection, and should be the priority groups for efficient influenza vaccination. This information should direct public health policy with respect to vaccination and effectively implementing nonpharmaceutical measures including social distancing, particularly among high-density at-risk populations. For example, A(H1N1)pdm09 might be effectively controlled among students through timely school closures or social distancing within schools. Financial support. This study was supported by the Mega-projects of Science and Technology Research (2013ZX10004218, 2012ZX10004215, 2013ZX10004203, 2011ZX10004001, and 2012ZX10004801), the National Natural Science Foundation of China (21105122 and 81001267), the Beijing Science and Technology Nova Program (2010B067), Beijing Natural Science Foundation (7132164), and Innovation Foundation of AMMS (2012ZXJJ018). Potential conflicts of interest. All authors: no conflicts. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.