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Limiting SARS-CoV-2 transmission at large events is possible — Experimental mass gathering

The transmission of SARS-CoV-2 at large indoor events could be limited with effective ventilation and suitable hygiene measures, suggests a paper published in Nature Communications. The research is based on an experimental event held in Germany in August 2020.

When the pandemic began, one of the first actions taken by many governments was to ban large events, which has had significant economic impacts.

Stefan Moritz and colleagues investigated the potential transmission of SARS-CoV-2 through aerosols and droplets at an experimental, seated pop concert held in August 2020.

The concert was attended by 1,212 participants, who had monitors to record their contacts, under three hygiene scenarios: no restrictions, moderate restrictions (which included checkerboard seating and twice as many entrances as the no restrictions scenario), and strong restrictions (with seating 1.5 metres apart and four times as many entrances as were in place under no restrictions). The authors found that the mean number of contacts per participant was nine people, with the highest number of contacts occurring during entry, interval and exit.

They found that under no restrictions, contacts of more than five minutes were created throughout the event, but for moderate and strong restrictions, most contacts occurred in the entry phase.

The authors then developed a model of the indoor space and simulated infectious aerosol distribution and the resulting exposure of healthy subjects to SARS-CoV-2. Their model assumed 4,000 concert attendees, including 24 who were infectious. They modelled two ventilation scenarios with different air exchange rates and different airflows. They found that under the ventilation scenario with a faster air exchange rate, the mean number of exposed people per infectious person was 3.5 and under the scenario with a slower air exchange rate, this increased to 25.5.

Implementing hygiene practices reduced the number of people exposed under both ventilation scenarios. Under moderate hygiene restrictions the mean number of exposed visitors per infectious person was 1.9 in the ventilation scenario with faster air exchange and 11.6 in the slower air exchange scenario, with strong restrictions it was 0.7 and 5.3 respectively.

The authors then used this information to simulate the impact of a large indoor event on the regional spread of SARS-CoV-2. They suggest that with the correct hygiene practices and effective ventilation, large indoor events would have very little effect on the spread of SARS-CoV-2 in the wider community.

The authors note that their results are based on a seated concert and will have limited applicability to other large-scale events, such as standing concerts or football matches.

Study details

The risk of indoor sports and culture events for the transmission of COVID-1

Stefan Moritz, Cornelia Gottschick, Johannes Horn, Mario Popp, Susan Langer, Bianca Klee, Oliver Purschke, Michael Gekle, Angelika Ihling, Frank D. L. Zimmermann & Rafael Mikolajczyk

Published in Nature Communications 19 July 2021

Abstract
Nearly all mass gathering events worldwide were banned at the beginning of the COVID-19 pandemic, as they were suspected of presenting a considerable risk for the transmission of SARS-CoV-2. We investigated the risk of transmitting SARS-CoV-2 by droplets and aerosols during an experimental indoor mass gathering event under three different hygiene practices, and used the data in a simulation study to estimate the resulting burden of disease under conditions of controlled epidemics.

Our results show that the mean number of measured direct contacts per visitor was nine persons and this can be reduced substantially by appropriate hygiene practices. A comparison of two versions of ventilation with different air exchange rates and different airflows found that the system which performed worst allowed a 10-fold increase in the number of individuals exposed to infectious aerosols. The overall burden of infections resulting from indoor mass gatherings depends largely on the quality of the ventilation system and the hygiene practices. Presuming an effective ventilation system, indoor mass gathering events with suitable hygiene practices have a very small, if any, effect on epidemic spread.

Introduction
In the course of the COVID-19 pandemic, banning mass gathering events (MGE) was one of the first countermeasures undertaken by the governments of most countries. In Germany, early in March 2020, the government issued a general ban of MGE with more than 1000 people. With a turnover of 129 billion Euro in 2019, the event sector is the sixth largest economic sector in Germany, and up to 1.5 million people depend on this industry.

Insolvencies in this field will not only have an economic impact, but may also result in the loss of creative skills, training infrastructure and a lack of upcoming young artists and athletes. The impact of this loss is not restricted to individuals, but affects an important dimension of society as a whole.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), causing COVID-19, can be transmitted via droplets, aerosols or through contaminated surfaces. While the debate on relevance of various transmission routes for the spread of COVID-19 is still ongoing, it is clear that physical proximity and hygiene determine transmission. Reported or measured personal contacts can be used to assess droplet based transmission. There are several additional factors of importance for studying aerosols. The type of activity and the resulting ex- and inhalation of emitters and recipients, as well as the airflow in the area around the recipients, must be taken into account.

To investigate the transmission risk of SARS-CoV-2 through droplets and aerosols during experimental indoor MGE, we conducted an experimental pop concert with three different hygiene practices, and measured the contacts of each spectator during the event using contact tracing devices (CTDs). We developed a computer model of the arena indoor space and simulated infectious aerosol distribution and the resulting exposure of healthy subjects. Finally, we combined information on contacts during the event and exposure to aerosols with an individual based model to estimate the excess burden of epidemic caused by indoor MGE. We incorporated various parameters, including the effects of different hygiene practices, wearing masks, event sizes, ventilation systems, and different baseline incidences in our model. We derived recommendations from this data regarding MGE during a pandemic.

Results
Contact measurements at the experimental mass gathering event

We conducted an experimental pop concert on 22 August 2020, with a total of 1212 individuals in the Leipzig Arena. All participants and involved staff demonstrated a negative test result for SARS-CoV-2, performed 48 h before the event. All people involved wore N95 masks during the event. Three different scenarios were investigated: 1) No restrictions (the pre-pandemic setting), 2) moderate restrictions (checkerboard pattern seating, twice as many entrances as in 1), 3) strong restrictions (pairwise seating with 1.5 m interspace to the next pair, four times as many entrances as in 1).

Each scenario had the same schedule: first half, half time including simulated catering, second half, and exit. Contacts within a radius of 1.5 m were measured with a CTD.

There was a high overall number of contacts when all contacts over 10 s were counted. When considering only critical contacts with a duration of more than 15 min (based on the standard definition for contact tracing15) the number of contacts decreased below 10. In each scenario a high number of contacts was observed during entry, half time and exit. Few of these contacts lasted more than 15 min during entry and half time. No contacts over 15 min were recorded in any of the scenarios in the exit phase. Few contacts were observed during the two halves, but nearly all lasted longer than 15 min.

 

Nature Communication article – The risk of indoor sports and cult (Open access)

 

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