Science Note - A self-controlled case series study to measure the risk of SARS-CoV-2 infection associated with attendance at an Events Research Programme event
Published 26 November 2021
Applies to England
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Background
1. Social interaction increases transmission of all respiratory viruses, including COVID-19. As COVID-19 related social restrictions eased there was limited evidence for whether attendance at cultural events such as the theatre, festivals or sporting events is associated with any increased risk of COVID-19 transmission over and above that which those attending are exposed to within their communities, or how effective environmental controls at the venue itself may be in mitigating any additional risk.
2. In 2021 the United Kingdom government established an Events Research Programme (ERP) to examine the risk of transmission of COVID-19 from attendance at events and explore ways to enable people to attend a range of events whilst minimising risk of transmission. We aimed to measure any impact on risk of COVID-19 transmission of attendance at events held at or close to full capacity, without social distancing, in the third phase of the ERP (The protocol associated with this study was agreed by the Science Board).
3. Data were obtained for a sample of people who both attended a Phase III ERP event, and had any SARS-CoV-2 test result recorded in NHS Test and Trace in the 16 days following attendance at the event. The proportion of attendees for whom attendance data were available varied from ~3% at some events to 90% at others. Using a self-controlled case series design, a within-person incidence rate ratio was calculated comparing the rate of positive testing for SARS-CoV-2 during high risk and baseline periods of time. The high risk period was days 3 to 9 following event attendance and the baseline period was days 1,2 and 10-16 following the event when infection detection was assumed to be unaffected by attendance. Rate ratios were adjusted for estimates of underlying regional COVID-19 prevalence to account for population level fluctuations in infection risk. A rate ratio of 1 suggests COVID-19 risk was unaffected by attendance at the event, whilst a rate ratio and 95% confidence interval all greater than 1 suggests transmission risk may have been increased by attendance. The figure below shows the study timeline for all people included in the analysis.
4. Events were grouped according to type (indoor seated, mainly outdoor seated, mainly outdoor semi-seated and mainly outdoor unseated). The rate ratio for negative testing was also calculated to determine any bias in testing trends over the observation period.
Findings
5. Approximately 1.7% of attendees for whom data were available tested positive for COVID-19 during their 16 day study period.
6. Owing to heterogeneity in the rate ratio for infection between event types, a single summary measure across all event types is not reported.
7. After accounting for attendee trends in testing behaviour over the study period, there was little evidence for increased transmission by attendance at the indoor theatre events studied, mainly outdoor seated or mainly outdoor semi seated events for which data were available.
8.Attendance at the mainly outdoor unseated events studied (Goodwood, Latitude and Tramlines) was associated with a 1.7 fold increased risk of COVID-19 transmission amongst attendees (95% confidence interval between 1.52 and 1.89). For context, the risk of infection in the baseline period was ~0.9% for Latitude attendees in the study; a 70% increase would take this risk to 1.53%.
9. Reasons for this difference in transmission risk are likely to be multifactorial and could include behaviour whilst at the event, overall event size and duration (some events run over multiple days) and mode of travel to/from the event.
10. Findings may not generalise to other settings and will depend on a mixture of factors including but not limited to; individual/crowd behaviour at events, ventilation of event space, time spent at an event, likelihood of attendees being infected, mode of travel to and from the event, age distribution of attendees and vaccination coverage.
11. Caution is needed when interpreting these findings. For example, some theatre events were run at or below 50% of normal full capacity and involved low numbers of attendees. This meant the rate ratio for infection had wide 95% confidence intervals (0.53-2.57) and therefore we were unable to rule out a potentially important increased risk of transmission. These results may not generalise to other contexts where venue characteristics and individual/crowd behaviour may be different.
12. As with any observational study, there remains the possibility that results were affected by biases that could not be accounted for, such as selection bias and misclassification of the outcome. For example, some events requested attendees to test after the event, meaning outcomes may be more likely to be detected in the high risk period. We have accounted for this where possible by factoring in the trend for negative testing over the same period.
13. It should also be noted that these results are set against the background of a particular epidemiological situation, and the possibility remains that new variants arise that are more transmissible and possibly less responsive to vaccines than those encountered in our studies, which would change transmission risk.
14. Across all events, where attendee COVID-19 vaccination status was self-reported, 87% of people with a positive COVID-19 test result during the study period were unvaccinated, 8% reported having a single dose and 5% reported 2 doses.
Table 1: Association between event attendance and testing positive for COVID-19
| Event Type | |||
|---|---|---|---|
| People testing positive N | Positive test during baseline | Positive test during high risk period | Rate Ratio* (95%CI) |
| INDOOR SEATED (Piccadilly Theatre, Leeds Grand, The Grange) | |||
| 30 | 15 | 15 | 1.16 (0.53-2.57) |
| MAINLY OUTDOOR UNSEATED (Tramlines, Latitude, Goodwood) | |||
| 2,012 | 469 | 1,543 | 1.70 (1.52-1.89) |
| MAINLY OUTDOOR SEATED (Cricket, RFL Challenge Cup, Grosvenor Park, Silverstone, Wimbledon) | |||
| 783 | 380 | 403 | 1.12 (0.95-1.30) |
| MAINLY OUTDOOR PARTIALLY SEATED (Open Golf) | |||
| 532 | 285 | 247 | 0.65 (0.51-0.83) |
*adjusted for regional prevalence and accounting for trends in testing over the observation period by dividing the rate ratio for positive tests by the rate ratio for negative tests over the same period. Further results will be made available in a full peer reviewed scientific publication.
Next Steps
15. Consideration should be given to applying the SCCS design or similar methodology when investigating the impact of outbreak control measures at large cultural events in future.
16. Further work is needed to determine why transmission was increased amongst people attending mainly outdoor unseated events.