Meta-Analysis of COVID-19 Cluster Events Suggestive of Long-Distance Airborne Transmission/Inhalation

SARS-CoV-2 spreads through both contact and airborne routes, the latter encompassing airborne transmission/inhalation as well as the direct deposition of infectious respiratory particles. During the early phase of the COVID-19 pandemic, numerous cluster events were suspected to involve long-distance airborne transmission/inhalation. We conducted a comparative analysis of these cluster events to characterize outbreak settings and associated clinical parameters. Thirteen cluster events from 2020 attributed to the original SARS-CoV-2 strain were examined, including choral activities, indoor sports, and bus tours. Incubation periods and infection-hospitalization rates (IHRs) were compared across settings and against estimates from large-scale cohort studies that predominantly reflect transmission via direct deposition. Statistical analyses were performed using the Mann-Whitney U test, Student's t-test, and Fisher's exact test (p < 0.05). The mean incubation period in suspected long-distance airborne transmission/inhalation cases was 6.1 ± 3.9 days (median: 5 days; N = 176), with indoor sports and choral events showing significantly shorter incubation periods (p = 0.034). The average IHR was 6.7 ± 12.5%, with significantly higher rates in choral clusters (p = 0.013). Age-adjusted IHRs were lower in long-distance airborne transmission/inhalation-related clusters than those reported from contact-tracing datasets. This analysis provides an integrated evaluation of long-distance airborne transmission/inhalation settings and their associated clinical characteristics. Activities involving vigorous respiration may contribute to shorter incubation periods and higher disease severity, potentially reflecting increased viral inoculum at exposure.