[Factors associated with household transmission of influenza (H1N1) 2009].

Background: A good part of the transmission of influenza occurs in the household context. The main objective of this study was to evaluate the factors associated with the index cases generating secondary cases in household.

Methods: We designed an observational, retrospective, multicenter through the implementation of a telephone survey in three regions of Spain. The study population were hospitalized and outpatient cases of confirmed H1N1 (2009) reported to the surveillance units from week 44 of 2009. We calculated the secondary attack rate within the household (TAI) and confidence interval of 95%. Factors associated with household transmission were analyzed by logistic regression, calculating odds ratios and confidence intervals at 95%.

Results: the secondary attack rate was 11.3% (95% CI 9.9 to 12.7) within the household. The number of household contacts was the main factor associated with the transmission (OR: 5,02 CI95% 1,78-14,13).

Conclusion: The factor most associated with the generation of secondary cases is the family size, being greater the larger the group of cohabiting, which is consistent with the data provided by WHO.