At the start of the COVID-19 pandemic, mass gathering events (MGEs) were banned globally due to suspected high SARS-CoV-2 transmission risk. This study aimed to quantify this risk by examining transmission via droplets and aerosols during a controlled indoor MGE experiment. The event industry in Germany, valued at €129 billion in 2019, employs 1.5 million people. A prolonged ban would create significant economic and societal consequences beyond financial losses, including the potential loss of creative skills and talent. SARS-CoV-2 transmission occurs via droplets, aerosols, and contaminated surfaces; however, the relative importance of each route remains debated. Proximity and hygiene are key factors, with droplet transmission assessed through measured contacts and aerosol transmission influenced by activity levels, exhalation/inhalation patterns, and airflow dynamics. This study combined experimental data on contacts during a simulated pop concert with computational fluid dynamics (CFD) modelling of aerosol distribution to create a comprehensive risk assessment.

Existing literature highlights the potential for mass gatherings to contribute to COVID-19 spread. Studies have shown the significant role of airborne transmission of SARS-CoV-2, particularly in indoor settings with poor ventilation. The debate on the relative importance of different transmission routes continues. There's also a need for data on the effectiveness of different hygiene practices in mitigating transmission risk in real-world settings.

This study used a three-part methodology: 1. **Experimental Concert:** A pop concert with 1212 participants was conducted in the Leipzig Arena under three hygiene scenarios: 1) No restrictions (pre-pandemic setting); 2) Moderate restrictions (checkerboard seating, doubled entrances); 3) Strong restrictions (pairwise seating with 1.5m spacing, quadrupled entrances). Contact tracing devices (CTDs) measured contacts within a 1.5m radius. 2. **Aerosol Distribution Simulation:** A CFD model of the arena simulated aerosol distribution under two ventilation versions (VV): VV1 (existing system, ACH 1.46 h⁻¹); VV2 (modified system, ACH 0.85 h⁻¹). 24 virtual infectious individuals were placed in the arena, and aerosol exposure of 4000 virtual participants was assessed. 3. **Epidemiological Simulation:** An individual-based model integrated contact tracing and aerosol data to estimate the excess burden of infection from MGEs under varying conditions (incidence, hygiene practices, mask use, ventilation, event size). The model incorporated contact data from the POLYMOD study, age-specific hospitalization and mortality rates from Germany, and the German SARS-CoV-2 testing strategy.

The experimental concert revealed a high overall number of contacts (>10s), substantially reduced by moderate and strong hygiene measures (Scenarios 2 and 3). Critical contacts (>15 min) were few in all scenarios, mostly during performance periods. The CFD simulations showed a tenfold increase in aerosol exposure in VV2 (poor ventilation) compared to VV1 (existing system). The epidemiological model demonstrated that the excess burden of infections from MGEs depended greatly on ventilation and hygiene practices. Under Scenario 3 (strong restrictions) and VV1 (good ventilation), the additional infections were minimal. In contrast, with no restrictions (Scenario 1) and poor ventilation (VV2), the excess infections were significantly higher, especially at higher incidences. At a high incidence (100 per 100,000 per week), the increase in positive cases attributed to MGEs ranged from 4.8% (Scenario 3, VV1, masks) to 23.6% (Scenario 1, VV2, no masks). Mask wearing significantly reduced the risk, and participant acceptance of mask use was high (89% found N95 masks acceptable).

The findings confirm that the risk of COVID-19 transmission in indoor MGEs is significantly impacted by hygiene practices and ventilation quality. Good ventilation coupled with effective hygiene measures greatly reduces the transmission risk, even with a large number of attendees. The study's simulation model successfully integrated experimental data to estimate real-world effects. The results highlight the importance of adequate ventilation systems in preventing aerosol transmission. Moreover, the study reinforces the effectiveness of mask use and strong adherence to hygiene protocols in mitigating infection risk.

This study demonstrates that with proper hygiene practices and good ventilation, indoor mass gatherings contribute minimally to COVID-19 spread. Poor ventilation dramatically increases transmission risk. Mask wearing and strong adherence to hygiene protocols are crucial for risk mitigation. Future research should focus on ventilation optimization in large indoor spaces and on the impacts of different MGE types (e.g., unseated concerts).

The study's participant number (1212) was lower than planned (4000), potentially underestimating the contact density. The aerosol exposure model simplified aspects such as minimal infectious dose and viral load. The study did not consider additional contacts outside the arena (e.g., travel, post-event gatherings). The model assumed perfect testing and compliance with contact tracing and quarantine guidelines, which may not always be the case in real-world scenarios.