Role-specific household contact patterns in urban and rural China: Implications for influenza household transmission.

Background: Household contact patterns significantly influence the household transmission dynamics of respiratory pathogens. Previous studies have shown ignoring the heterogeneity of household contact patterns can lead to an underestimation of the relative susceptibility and infectivity of children to influenza. However, there is a lack of studies on household contact patterns across all age groups and family structures in China. Methods: We conducted two cross-sectional studies via face-to-face, paper-based surveys in Anhua County, Hunan Province (June-July and October 2021) and Kunming City, Yunnan Province (February-April 2023). Factors associated with household contact patterns were explored using linear mixed-effects models. Role-specific contact matrices were established, stratified by household structures and crowding levels. Additionally, using influenza as a representative example, we developed a discrete-time, individual-based stochastic model to investigate household transmission dynamics initiated by different index case roles and to evaluate various role-targeted vaccination strategies. Results: The mean number of household contacts reported by participants was 2.58 (95% CI, 2.50-2.66) in Anhua County and 2.26 (95% CI, 2.19-2.33) in Kunming City. Household contact patterns were mainly associated with family roles, household structures, and the number of bedrooms. Contact duration was significantly longer between father-mother pairs, between mother-child pairs in two-generation households and mother-grandchild pairs in three-generation households, and among siblings. Mothers as index cases generated the highest number of secondary cases, whereas grandfathers caused the fewest. When grandparents or parents were the index cases, transmission most frequently occurred to children or grandchildren, whereas when children or grandchildren were the index cases, infection most commonly spread among siblings. Crowded living environments increased transmission risk, leading to a higher average number of secondary cases across all index case roles. Prioritizing vaccination of children or grandchildren yielded the greatest reduction in secondary cases, with further improvement when mothers were also vaccinated. Although vaccinating grandparents had minimal impact on reducing household transmission, it remained essential for their personal protection. Notably, vaccinating both grandparents and grandchildren together was the most efficient four-dose strategy for reducing infection risk for grandparents. Conclusions: Given the heterogeneity of household contact patterns, to reduce household secondary cases, priority should be given to vaccinating children or grandchildren. However, if the aim is to reduce the infection risk among grandparents, directly vaccinating them is more effective. Unlike traditional age-based vaccination strategies, our findings provide a new perspective to optimize vaccine allocation.