The relative contribution of close-proximity contacts, shared classroom exposure and indoor air quality to respiratory virus transmission in schools.

Close-proximity interactions are considered a key risk factor for respiratory virus transmission, but their importance relative to shared space and air quality remains unclear. We conducted a six-week longitudinal study in a Swiss secondary school (67 students, aged 14-15). We detected 87 infections in saliva samples and recorded absences to identify plausible transmissions, excluding implausible ones through genomic analysis. Time in close proximity (within 1.5 metres) was measured using wearable sensors and air quality via CO2 monitors. Students spent 21.2 minutes per day in close proximity (interquartile range 7.8-44.2) and 5.3 hours in shared classrooms (IQR 3.8-6.2), during which air quality was suboptimal for 1.9 hours (IQR 1.2-3.0). Using pairwise survival models, we found that transmission was more likely within than between classes. Close proximity was modestly associated with higher transmission risk overall (rate ratio 1.16 per doubling daily time, 95%-CI 1.01-1.33), while time in shared classrooms (RR 8.96, 95%-CI 4.85-16.88) and suboptimal air quality (RR 5.59 95%-CI 3.25-9.83) also predicted within-class risk. Prolonged exposure in shared, poorly ventilated spaces drives respiratory virus transmission more than close contact.