Risk factors for community-acquired respiratory infections in a non-pandemic context: Secondary analysis of the PRIMIT study.

Objectives: Respiratory tract infection (RTI) incidence varies between people, but little is known about why. The aim of this study is therefore to identify risk factors for acquiring RTIs.

Methods: We conducted a secondary analysis of 16,908 participants in the PRIMIT study, a pre-pandemic randomised trial showing handwashing reduced incidence of RTIs in the community. Data was analysed using multivariable logistic regression analyses of self-reported RTI acquisition.

Results: After controlling for handwashing, RTI in the previous year (1 to 2 RTIs: adjusted OR 1.96, 95% CI 1.79 to 2.13, p<0.001; 3 to 5 RTIs: aOR 3.89, 95% CI 3.49 to 4.33, p<0.001; ≥6 RTIs: OR 5.52, 95% CI 4.37 to 6.97, p<0.001); skin conditions that prevent handwashing (aOR 1.39, 95% CI 1.24 to 1.55, p<0.001); children under 16 years in the household (aOR 1.27, 95% CI 1.12, 1.43, p<0.001); chronic lung condition (aOR 1.16, 95% CI 1.02 to 1.32, p = 0.026); female sex (aOR 1.10, 95% CI 1.03 to 1.18, p = 0.005), and post-secondary education (aOR 1.09, 95% CI 1.02 to 1.17, p = 0.01) increased the likelihood of RTI. Those over the age of 65 years were less likely to develop an infection (aOR 0.89, 95% CI 0.82 to 0.97, p = 0.009). Household crowding and influenza vaccination do not influence RTI acquisition. A post-hoc exploratory analysis found no evidence these subgroups differentially benefited from handwashing.

Conclusions: Previous RTIs, chronic lung conditions, skin conditions that prevent handwashing, and the presence of household children predispose to RTI acquisition. Further research is needed to understand how host and microbial factors explain the relationship between previous and future RTIs.