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[Epidemiological characteristics of infection for close contacts of pandemic (H1N1) 2009 in Beijing].

OBJECTIVE
To examine the epidemiological characteristics of infection for close contacts of pandemic (H1N1) 2009 and to provide scientific evidence for preparedness and response for the next pandemic.


METHODS
A total of 613 index cases with clear information of close contacts and their 7099 close contacts, determined between May 16 and September 15, 2009, were included in this study. Based on data of epidemiological investigation, sampling and test of index cases and close contacts, the characteristics of infection for close contacts were described.


RESULTS
56.8% (348/613) of the index cases were male, and 43.2% (265/613) were female, and the median age was 20 years (range: 1 - 75 years). 49.6% (3518/7099) of the close contacts were male, and 49.5% (3514/7099) were female, but the sex information of 0.9% (67/7099) could not be recorded. The median age of the close contacts was 27 years (range: 0 - 99 years). 2.4% (167/7099) of close contacts were infected. The attack rates decreased with increasing age of close contacts (χ()2 = 27.87, P < 0.001), and were significantly different between various contact patterns of close contacts (χ()2 = 109.76, P < 0.001). 14.4% of the infected close contacts were asymptomatic. For close contacts with symptomatic infection, virus could be shed 4.5 days before illness onset, and the median generation time was 2.4 days.


CONCLUSION
The attack rate of close contacts was very low; and the attack rates were different between various ages and contact patterns of close contacts. In this series 14% of cases with pandemic (H1N1) 2009 were asymptomatic. The symptomatic cases might have infectivity 1 day earlier before illness onset.