Content area
Full Text
(ProQuest: ... denotes non-US-ASCII text omitted.)
Original Papers
Respiratory Viruses
INTRODUCTION
The potential for highly pathogenic infections to transmit from animals to humans is a major public health concern [1, 2]. Between 1 January 2006 and 1 December 2013, a total of 500 confirmed influenza A(H5N1) cases were reported to the World Health Organization. Of these, 467 cases came from five countries: Cambodia, China, Egypt, Indonesia and Vietnam.
The age distribution of cases varied substantially between countries [3-5]. In most countries, there were a disproportionally high number of cases of influenza A(H5N1) in the <5 years age group, and another peak in the 25-35 years age group (Fig. 1). If reported spillover events occurred at an equal rate across all ages, we would expect the size of a particular age group to be predictive of the number of reported infections in that group. Hence the points in Figure 2 a would form horizontal lines. However, it is possible that cross-immunity also contributed to the age distribution of reported cases. Influenza A(H1N1) and A(H5N1) viruses share the same neuraminidase subtype, N1, and there is evidence that H1N1 neuraminidase antibodies cross-react with H5N1 viruses [6-8].
Fig. 1.
Demographic patterns of influenza A(H5N1) infection. Points show reported cases per million people, stratified by age group.
Fig. 2.
Comparison of reported cases in each country and model fits. (a-e) Results from model with cross-immunity only; (f-j) model with cross-immunity and age-dependent exposure risk. Dots show confirmed H5N1 cases in each 5-year age band; solid blue line shows model estimate; dashed lines give 95% credible intervals.
To establish whether demographic profiles and heterosubtypic cross-immunity could explain observed patterns of infection, we used a mechanistic model to measure how spillover risk combines with cross-immunity from prior infection in the host population. We also analysed the relationship between exposure and infection, and examined the extent to which age-specific variation in risk of H5N1 transmission - with or without additional cross-immunity from H1N1 infection - could have shaped the observed distribution of H5N1 infection.
METHODS
We compiled a line list of confirmed cases of influenza A(H5N1) reported to the World Health Organization between 1 January 2006 and 1 December 2013. For each case we gathered...