Assessment of airborne infection risk and secondary cases in Addis Ababa public transport

This study assesses the risks of airborne infections and estimates secondary cases associated with SARS-CoV-2 transmission in Addis Ababa's public transportation system, encompassing shared rides, minibuses, midi buses, and the Addis Ababa Light Rail Transit (AALRT). Real-time measurements of indoor environmental quality parameters were used to calculate air exchange rates (AERs), which were then incorporated into the Airborne Infection Risk Calculator (AIRC) under different passenger activity scenarios. The findings indicate that infection risk varies substantially across modes, influenced by ventilation rate, trip duration, and occupancy levels. Shared rides exhibited the highest individual infection risks (up to 11.5 %) due to prolonged exposure times, while midi buses showed moderate per-person risks but elevated secondary cases due to larger passenger loads (up to 1.7). The AALRT, despite low individual infection probabilities, generated the highest number of secondary cases (1.9) due to high occupancy under limited ventilation (15.2 h^-1). Across all modes, speaking increased the risk of infection relative to breathing alone. These results underscore the importance of implementing integrated mitigation strategies that combine behavioral measures, ventilation improvements, and occupancy management to mitigate transmission risks in high-density public transportation environments.