No-one who attended a trial concert in Barcelona caught the coronavirus, a study has revealed.
After more than a year of restrictions, social distancing is potentially set to end in England on 21 June, freeing people up to attend nightclubs, festivals and concerts.
With 3,180 cases reported in the UK on 26 May – and up to a third of coronavirus carriers being asymptomatic – many are concerned that mingling with "unknown attendees" may turn into a super-spreading event.
To better understand the risk, 465 people – aged 18 to 59 – were invited to attend a five-hour indoor concert in Barcelona on 12 December 2020.
All the attendees swabbed negative for the coronavirus at the door via a lateral flow test, a fast but not always accurate assessment.
While listening to DJs and live bands, the concert-goers wore N95 masks – "designed to achieve a very close facial fit and very efficient filtration of airborne particles", as well as adhering to "crowd control in the well-ventilated venue".
Singing and dancing was permitted, however, with no social distancing rules within the building.
Eight days later, none of the attendees tested positive for the coronavirus. This is compared to two of the 495 (under 1%) people who showed up for the concert, swabbed negative and were told to go home – acting as the control group.
In both groups, 3% of the lateral flow swabs came back positive when later assessed via the "gold-standard" polymerase chain reaction (PCR) test in a laboratory.
These individuals were not thought to carry the "live" coronavirus, but rather its residual genetic material. They would therefore not be expected to spread the infection.
In Catalonia – a Spanish region, of which Barcelona is the capital – alone, music festivals brought in $2.5bn (£1.7bn) in 2019.
"The cancellation and deferment of these events in 2020 resulted in substantial economic losses, and restrictions on their celebration or capacity remain in force in 2021," the study scientists wrote in the journal The Lancet Infectious Diseases.
Lateral flow tests provide a positive or negative result in 15 to 20 minutes, but are not as accurate as the more drawn-out PCR swabs.
Of the 465 concert attendees, 13 (3%) tested positive via PCR when the lateral-flow swab was sent off for further assessment, despite the at-the-door test coming back negative. Similarly, 15 (3%) in the control group later tested positive via PCR.
Eight days after the concert, none of the attendees swabbed positive either via a lateral flow or PCR test, compared to two participants in the control group.
"In our study, lateral flow tests were around 99.9% accurate at detecting negative results, as confirmed by PCR testing, and their short-turnaround time may make them more appropriate for screening at mass-gathering events," said study author Cristina Casañ, from the Catalan Health Institute.
"Other tests that work by detecting the virus' genetic material [like PCR swabs] can also detect residual virus even when not transmissible.
"These tests could result in positive results that would prevent people who aren't infectious from attending events."
Co-author Boris Revollo – from the Germans Trias i Pujol University Hospital in Barcelona – agreed, adding: "Mass gatherings are associated with a high risk of spreading the [corona]virus and the cancellation of large events has played an important role in bringing the COVID-19 [the disease caused by the coronavirus] pandemic under control.
"As societies look toward the possibility of safely resuming cultural activities, lateral flow tests have been proposed as a means of screening people on entry to enable large events to take place".
Similar experiments have been carried out in the UK – including at the BRIT Awards, FA Cup Final and Circus nightclub in Liverpool.
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At the time of the concert, infection rates in Barcelona were "low to intermediate". Few attendees were therefore expected to catch the coronavirus.
Although COVID vaccines were not available at the time, local travel restrictions and limits on indoor gatherings were enforced.
The concert took place at the Sala Apolo nightclub, which can normally accommodate about 900 people.
As well as face masks being required, all the attendees had their temperature measured at the door. Hand-sanitiser points were situated throughout the venue, with its cloakroom closed to avoid queuing.
Alcohol was served in a separate room to the music, with social distancing only in place in the outdoor smoking area.
"Our study provides early evidence that indoor music events can take place without raising the risk of [coronavirus] transmission when comprehensive safety measures are in place," said lead author Dr Josep Llibre, from the Germans Trias i Pujol University Hospital.
Nevertheless, "it is important our findings are considered in light of the situation in Spain at the time, when cases were not high and many restrictions were in place", he added.
"As a result, our study does not necessarily mean all mass events are safe. The conditions of the pandemic are constantly shifting.
"Widespread vaccination campaigns, changes in local incidence and the emergence of variants with higher transmissibility could all impact the interventions we tested".
In a post-concert questionnaire, the attendees scored the event 8.6 out of 10 in terms of enjoyment.
When asked how willing they would be to attend a similar gathering, the average score was more than nine, with 10 being the most willing.
The scientists have acknowledged mass testing is expensive, as well as "posing logistical challenges". The participants may have also "modified their behaviour", knowing they were taking part in a study.
Writing in The Lancet, professors Rosanna Peeling and David Heymann – from the London School of Hygiene & Tropical Medicine (LSHTM) – asked: "Does screening create bottlenecks with an increased risk of transmission among those not yet tested?"
They also questioned whether the concert's measures would be similarly successful in the upcoming Olympics and Paralympics in Tokyo, "when attendees are not all from the same community".
The N95 masks worn in the study are "generally less common in the community as they are more difficult to wear for long periods", according to Dr Julian Tang, from the University of Leicester.
The LSHTM scientists wondered: "Would triple-layered masks have been sufficient?".
Ahead of further research, Dr Tang has stressed: "It is the COVID-19 national vaccination programme that will really let us get back to some degree of normality.
"The findings and generalisation of all of these studies should be regarded with some caution, until we can vaccinate everyone with at least two vaccine doses".
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