COVID-19 risk assessment at the opening ceremony of the Tokyo 2020 Olympic Games

Abstract

The 2020 Olympic/Paralympic Games have been postponed to 2021, due to the COVID-19 pandemic. We developed a model that integrated source-environment-receptor pathways to evaluate how preventive efforts can reduce the infection risk among spectators at the opening ceremony of Tokyo Olympic Games. We simulated viral loads of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emitted from infectors through talking/coughing/sneezing and modeled temporal environmental behaviors, including virus inactivation and transfer. We performed Monte Carlo simulations to estimate the expected number of newly infected individuals with and without preventive measures, yielding the crude probability of a spectator being an infector among the 60,000 people expected to attend the opening ceremony. Two indicators, i.e., the expected number of newly infected individuals and the newly infected individuals per infector entry, were proposed to demonstrate the extent of achievable infection risk reduction levels by implementing possible preventive measures. A no-prevention scenario produced 1.5-1.7 newly infected individuals per infector entry, whereas a combination of cooperative preventive measures by organizers and the spectators achieved a 99% risk reduction, corresponding to 0.009-0.012 newly infected individuals per infector entry. The expected number of newly infected individuals was calculated as 0.005 for the combination of cooperative preventive scenarios with the crude probability of a spectator being an infector of 1 × 10^-5. Based on our estimates, a combination of cooperative preventions between organizers and spectators is required to prevent a viral spread at the Tokyo Olympic/Paralympic Games. Further, under the assumption that society accepts < 10 newly infected persons traced to events held during the entire Olympic/Paralympic Games, we propose a crude probability of infectors of < 5 × 10^-5 as a benchmark for the suppression of the infection. This is the first study to develop a model that can assess the infection risk among spectators due to exposure pathways at a mass gathering event.

Keywords: COVID-19; Infection risk; Mass gathering event; Solution-focused risk assessment; Tokyo 2020 Olympic/Paralympic Games.

Graphical abstract

Fig. 1

The exposure pathway model at the opening ceremony of the Tokyo Olympic Games. Arrows (→) indicate the directions of virus transmission between humans and environmental media. Dashed boxes show exposure pathways: (1) droplet spray, (2) inhalation of inspirable particles, (3) hand contact, and (4) inhalation of respirable particles. Text in italics shows seven preventive measures: (a) physical distancing among the spectators at entrances and exits, (b) decontamination of surfaces in concessions, (c) enhanced stadium air ventilation, (d) partitioning of spectators in the stands, (e) mandatory face masks at concourses, restrooms, and concessions, (f) hand washing with soap in restrooms, and (g) wearing hats or other headwear in the stands.

Fig. 2

Base scenario for infection risks among spectators (no-prevention). Comparison of overall infection risks under five simulation conditions with different crude probabilities of a spectator being an infector (P₀) (A). Comparison of infection risks among five categories of spectators obtained from simulations with P₀ of 10⁻⁴ (B). A ratio of average infection risk to P₀ corresponds to newly infected individuals per one infector entry into the stadium at the opening ceremony. Box-and-whisker plots represent the following percentiles: 2.5, 25, 50, 75, and 97.5. Closed circles represent average values (arithmetic mean). Monte Carlo simulations were performed with 1,000 iterations for each condition.

Fig. 3

Comparison of infection risks among spectators in different prevention scenarios at a crude probability of a spectator being an infector (P₀) of 10⁻⁴. Comparison of overall infection risks among no-prevention, seven individual preventions, organizer-oriented preventions, spectator-oriented preventions, and all preventions combined. ^a Physical distancing of spectators at entrances and exits. ^bDecontamination of surfaces in concessions. ^c Enhanced stadium air ventilation. ^dPartitioning of spectators in the stands. ^e Mandatory face masks at concourses, restrooms, and concessions. ^fHand washing with soap in restrooms. ^g Wearing hats or other headwear in the stands. Four preventions (a–d) are organizer-oriented; three (e–g) are spectator-oriented. Risk reduction values were calculated from the average of the no-prevention scenario and respective prevention(s) scenarios. Box-and-whisker plots represent 2.5, 25, 50, 75, and 97.5 percentiles. Closed circles represent average infection risks. Monte Carlo simulations were performed with 1,000 iterations for each condition.

Fig. 4

Infection risks among spectators in the scenario of all preventive measures combined. The expected number of newly infected individuals was obtained by multiplying the average overall infection risk by the number of non-infectors. Box-and-whisker plots represent 2.5, 25, 50, 75, and 97.5 percentiles. Closed circles represent average infection risks. Monte Carlo simulations were performed with 1,000 iterations for each condition. P₀ is the crude probability of a spectator being an infector.