Differential transmission dynamics of COVID-19 and non-COVID respiratory pathogens in households: findings from a prospective population cohort study

Introduction: With mask mandate removed in Hong Kong in March 2023, non-COVID respiratory pathogens had re-emerged and caused year-round outbreaks. Anticipating the continuing co-circulation of SARS-CoV-2 and other respiratory viruses, this study characterises the transmission dynamics of COVID-19 and non-COVID pathogens in households. Methods: Between March 2023 and January 2024, an individual from each household enrolled in an ongoing population cohort study was invited to complete monthly surveys covering their COVID-19 status, SARS-CoV-2 testing practice, presentation of respiratory symptoms, and ensuing pattern of illness among co-living members. By Student's t-test and Chi-squared test, the symptom duration and disease severity were compared between symptomatic COVID-19 and non-COVID influenza-like illness (ILI), with the latter defined as the report of fever plus sore throat and/or cough and a negative SARS-CoV-2 test. The respective proportion of episodes leading to subsequent illness in households and secondary attack rate (SAR) were assessed in overall and by household size using ANOVA. The attack pattern among co-living members of different ages was also explored. Results: Based on 36,594 monthly responses collected from 4,554 individuals (median age 48 years; IQR 36-59), totally 1,032 COVID-19 and 518 non-COVID episodes were captured during the period. With a peak of 10.6% respondents diagnosed with symptomatic COVID-19 in May 2023, a concurrent growth in non-COVID ILI was noted in 1.9% followed by another upswing in January 2024 (2.1%). A longer symptom duration (6.33 vs 5.52 days, p<0.001) and higher hospitalisation rate (2.9% vs 0.7%, p=0.002) were identified for non-COVID ILI. With a similar proportion of COVID-19 (36.5%) and non-COVID episodes (31.5%) contributing to ensuing infections in households, a higher SAR was observed for the former (25.7% vs 19.6%, p=0.044). Stratified by household size, variations in SAR were detected (p=0.006) for COVID-19 episodes, with the highest in households with 2 (32.9%), followed by 3-4 (20.5%) and 5+ persons (19.5%). A similar trend of SAR (26.9%, 15.7% and 10.4%) was also gauged for non-COVID ILI (p=0.028). Secondary transmission of COVID-19 to co-living members aged 0-14 (27.2%, 25/92 households), 45-64 (27.9%, 92/330) and 65+ years (23.5%, 46/196) had occurred more frequently compared to those aged 15-24 (15.1%, 13/86) and 25-44 years (21.2%, 51/241). For non-COVID ILI, the likelihood of secondary transmission was, however, inversely associated with infectees’ age, with the highest realised in co-living members aged 0-14 (34.1%, 14/41) relative to those aged 65+ years (18.2%, 16/88). Discussion: Alongside the resurgence of non-COVID respiratory infections with an increased severity, COVID-19 demonstrated a higher transmissibility in households. Differential risk of secondary transmission to household members of different ages was also noted for both COVID-19 and non-COVID infection. Conclusions: To protect the vulnerable populations, vaccination and other prevention strategies should continue to be advised amid seasonal respiratory outbreaks.