Risk factors of influenza transmission in households
Introduction
The epidemiology of influenza transmission in households has been the subject of great investigations in the past. The occurrence and spread of infection in households has been described in particular in relation to age, family composition, crowding, circulating viral strains, exposure in the community, and prior immunity [1], [2]. This topic has recently regained interest in light of the recent debates about large-scale vaccination and prophylaxis with neuraminidase inhibitors [3], [4], [5], [6]. However, no data are available on the quantitative evaluation of the predictors of influenza transmission in households. Here, we investigate the risk factors of transmission associated with the individual characteristics of the index case and the household contact.
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Material
We analyze a prospective study of influenza transmission in households conducted in France during the 1999–2000 influenza season [7].
A household was enrolled when a member visited his/her general practitioner and met the following inclusion criteria: the patient had fever (≥38 °C) less than 48 h ago with respiratory signs, there was at least another member in the household, the consulting patient was the first case in the household, the patient was not hospitalized for his/her illness, and the
Statistical analysis
We performed a main-line analysis, excluding households in which one or more contacts had developed clinical influenza on the day of the initial visit of the index case. This reduced ambiguity about the true introducer of infection in the household. We subsequently performed two sensitivity analyses with (1) an extended dataset that included those households where co-primary cases were reported on the day of the initial visit, and (2) a more specific definition of clinical influenza based on
Study population
Overall, 279 households were included in the main-line analysis, comprising 543 contact members. The average number of children per household under fifteen years of age was 0.71, well in line with 0.68, the national figure for French households of two or more members [8].
One hundred thirty-one (24.1%) of the 543 contacts developed symptoms of influenza within 5 days of the onset of disease of the index case. Influenza transmission was observed in 97 households (35%). Of the 97 households, 67
Discussion
The present study identifies the age of index cases and the age of contacts as the main predictors of influenza transmission in families. These factors appear to be more important than other individual variables, whether they are related to the contact person or the index case.
A limitation of this study is the absence of virological tests for secondary cases. However, several arguments suggest that the proportion of real influenza among our secondary cases is high. (1) There was very little
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