Estimating pre-symptomatic transmission potential of influenza A and B viruses in household transmission studies

The effectiveness of case isolation and contact quarantine depends critically on the extent of pre-symptomatic transmission. However, the capacity of influenza viruses for transmission before symptom onset remains uncertain. We developed an individual-based household transmission model that explicitly accounts for transmission occurring before symptom onset by marginalizing unobserved infection times. We analyzed influenza household transmission studies of influenza A and B viruses in Hong Kong between 2008 and 2017 (493 and 255 households respectively), estimating pathogen-specific pre-symptomatic transmission proportions and identifying factors affecting individual susceptibility and infectiousness. We estimated that 9.6% (95% credible interval (CrI): 5.9%, 14.7%) of influenza A transmission occurred > 0.5 days earlier than infectors’ symptom onset, while influenza B showed no evidence of pre-symptomatic transmission. Younger age was associated with an increased susceptibility and infectiousness for both influenza A and B viruses. Influenza A infectiousness was positively associated with viral load and fever. Using an additional SARS-CoV-2 Omicron dataset in Hong Kong, our analysis confirmed substantial pre-symptomatic transmission (65.5%, 95% CrI: 52.7%, 78.9%), validating our methodology. Our modeling framework provides a robust approach for quantifying pre-symptomatic transmission across respiratory pathogens in households, offering a methodological template for resolving similar empirical uncertainties in infectious disease transmission. Our findings suggest symptom-based control measures may be feasible for influenza, though quarantine of asymptomatic contacts would provide limited additional benefit.