Association between two mass-gathering outdoor events and incidence of SARS-CoV-2 infections during the fifth wave of COVID-19 in north-east Spain: A population-based control-matched analysis

Abstract

Background: Many countries have resumed mass-gathering events like music festivals, despite the risk of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spreading. In this study, we aimed to assess the effect of two mass-gathering outdoor events, held during a peak of SARS-CoV-2 transmission, on COVID-19 incidence.

Methods: This was a retrospective, population-based control-matched analysis. The study population included attendees to two outdoor music festivals held in Catalonia (North-East Spain). The primary objective was to compare the incidence of COVID-19 within the 3-to-10 days following the event between attendees and a population-based control group.

Findings: The analysis included 18,275 and 27,347 attendees to the first and second festivals, respectively, and their corresponding controls. The post-festival 7-day cumulative COVID-19 incidence among attendees and controls was 4.14% (95% CI 3.86-4.44) vs. 1.69% (1.51-1.88) for the first festival (RR 2.46; 2.16-2.80), and 2.42% (2.35-2.61) and 1.10% (0.99-1.2) for the second festival (RR 2.19; 1.92-2.51). COVID-19 incidence among immunized individuals was also two-fold higher in attendees than in controls. Previous COVID-19 infection, vaccination, and adequate mask-wearing were significantly associated with a lower risk of COVID-19 infection after the events.

Interpretation: Despite the proven effectiveness of preventive measures such as Ag-RDT screening, mask-wearing and vaccination, caution should be taken when holding these events during a period of high community SARS-CoV-2 transmission.

Funding: Crowdfunding campaign YoMeCorono (https://www.yomecorono.com/) and the Generalitat de Catalunya.

Figure 1

Progression of cumulative weekly cases of COVID-19 per 100,000 inhabitants before and after the events in Catalonia. Event dates are indicated with yellow bands; the first festival was held on 03 July and the second from 08 July to 10 July, 2021. The progressive role out of vaccination campaigns to younger individuals is indicated. Selected policies regarding the use of the face mask and other COVID-19 preventive measures are indicated. Incidence data was obtained from the public repository https://dadescovid.cat. ^a The time points indicate only the starting date of vaccination campaigns for each age group. Healthcare and other essential workers of all ages were vaccinated during the first semester of 2021, before the indicated date for their corresponding age group.

Figure 2

Odds ratio for COVID-19 infection after attending the events. Risk factor multivariate analysis for COVID-19 infection after attending the events. Time frame considered: 3-to-10 days. (A) Analysis including all attendees and variables recorded in the central health registries [n(cases)/n(total): 757/18,275 for the first festival; 662/27,347 for the second festival). (B) Sub analysis of attendees who responded the 14-day survey regarding behavioural factors after attending the second festival [n(cases)/n(total): 570/13,097]. Definitions: Previous COVID-19 Infection (with or without receiving any vaccine); Full vaccination (i.e., had received the full vaccination regimen >14 days before the event), partial vaccination (i.e., had received an incomplete vaccination regimen or a complete vaccination regimen <14 days before the event); Preventive Measures Compliance and Mask-wearing (OR for answering "all or most of the time"); Age (OR for an increase in one unit); Days of Attendance (OR for each extra day of assistance). Dashed vertical lines indicate Odds Ratios of 0.5 and 2. Alternative models including the interaction between age and gender showed no contributions to the model (ORs of the interaction age*gender were 0.99 [0.96-1.01] for the first festival, 1.00 [0.98-1.02] for the second, and 1.00 [0.98-1.02] for the survey). The values for OR and 95% CI are presented in Table S4.