Household transmission of 2009 pandemic influenza A (H1N1): A systematic review and meta-analysis

Background: During the 2009 influenza A (H1N1) pandemic, household transmission studies were implemented to better understand the characteristics of the transmission of the novel virus in a confined setting. Methods: We conducted a systematic review and meta-analysis to assess and summarize the findings of these studies. We identified 27 articles, around half of which reported studies conducted in May and June 2009. Results: In 13 of the 27 studies (48%) that collected respiratory specimens from household contacts, point estimates of the risk of secondary infection ranged from 3% to 38%, with substantial heterogeneity. Meta-regression analyses revealed that a part of the heterogeneity reflected varying case ascertainment and study designs. The estimates of symptomatic secondary infection risk, based on 20 studies identifying febrile acute respiratory illness among household contacts, also showed substantial variability, with point estimates ranging from 4% to 37%. Conclusions: Transmission of the 2009 pandemic virus in households appeared to vary among countries and settings, with differences in estimates of the secondary infection risk also partly due to differences in study designs.