Comparative epidemiology of human infections with avian influenza A H7N9 and H5N1 viruses in China: a population-based study of laboratory-confirmed cases

Abstract

Background: The novel influenza A H7N9 virus emerged recently in mainland China, whereas the influenza A H5N1 virus has infected people in China since 2003. Both infections are thought to be mainly zoonotic. We aimed to compare the epidemiological characteristics of the complete series of laboratory-confirmed cases of both viruses in mainland China so far.

Methods: An integrated database was constructed with information about demographic, epidemiological, and clinical variables of laboratory-confirmed cases of H7N9 (130 patients) and H5N1 (43 patients) that were reported to the Chinese Centre for Disease Control and Prevention until May 24, 2013. We described disease occurrence by age, sex, and geography, and estimated key epidemiological variables. We used survival analysis techniques to estimate the following distributions: infection to onset, onset to admission, onset to laboratory confirmation, admission to death, and admission to discharge.

Findings: The median age of the 130 individuals with confirmed infection with H7N9 was 62 years and of the 43 with H5N1 was 26 years. In urban areas, 74% of cases of both viruses were in men, whereas in rural areas the proportions of the viruses in men were 62% for H7N9 and 33% for H5N1. 75% of patients infected with H7N9 and 71% of those with H5N1 reported recent exposure to poultry. The mean incubation period of H7N9 was 3·1 days and of H5N1 was 3·3 days. On average, 21 contacts were traced for each case of H7N9 in urban areas and 18 in rural areas, compared with 90 and 63 for H5N1. The fatality risk on admission to hospital was 36% (95% CI 26-45) for H7N9 and 70% (56-83%) for H5N1.

Interpretation: The sex ratios in urban compared with rural cases are consistent with exposure to poultry driving the risk of infection--a higher risk in men was only recorded in urban areas but not in rural areas, and the increased risk for men was of a similar magnitude for H7N9 and H5N1. However, the difference in susceptibility to serious illness with the two different viruses remains unexplained, since most cases of H7N9 were in older adults whereas most cases of H5N1 were in younger people. A limitation of our study is that we compared laboratory-confirmed cases of H7N9 and H5N1 infection, and some infections might not have been ascertained.

Funding: Ministry of Science and Technology, China; Research Fund for the Control of Infectious Disease and University Grants Committee, Hong Kong Special Administrative Region, China; and the US National Institutes of Health.

Figure 1

Geographical distribution of 130 and 43 laboratory-confirmed cases of human infection with avian influenza A(H7N9) and A(H5N1) viruses respectively in urban and rural areas of mainland China, with dates of illness onset between November 25 2003 and May 3 2013. Provinces are shaded according to population density, and A(H7N9) and A(H5N1) cases with more recent calendar dates of illness onset are represented by symbols with darker shades.

Figure 2

Occurrence of laboratory-confirmed cases of human infection with avian influenza A(H7N9) and A(H5N1) viruses over calendar time. Panels (A) and (B) show the number of laboratory-confirmed cases of infection with avian influenza A(H5N1) virus among urban and rural residents by calendar year and calendar month of illness onset, respectively. Panel (C) shows the number of laboratory-confirmed cases of infection with avian influenza A(H7N9) virus by week of illness onset.

Figure 3

Panels (A) and (B) compare the age and sex profiles of laboratory-confirmed cases of infection with avian influenza A(H7N9) and A(H5N1) viruses. Panels (C) and (D) show the age and sex profiles for A(H7N9) and A(H5N1) cases in residents of urban areas. Panels (E) and (F) show the age and sex profiles for A(H7N9) and A(H5N1) cases in residents of rural areas.

Figure 4

Comparisons of time-delay distributions for laboratory-confirmed cases of human infection with avian influenza A(H7N9) and A(H5N1) viruses. Panel (A) shows the estimated incubation period distributions i.e. the days from infection to illness onset. Panel (B) shows the days from illness onset to admission. Panel (C) shows the days from illness onset to laboratory confirmation of A(H7N9) virus infection. Panels (D) and (E) show the days from admission to death and days from admission to discharge, respectively.