Volunteers dance during an experimental concert held in Barcelona in December. (Emilio Morenatti/AP)

In December, hundreds of people packed into a Barcelona arena to dance and sing along to five hours of live music and D.J. sets, gleefully abandoning social distancing guidelines for the night.

The show was a large-scale science experiment, designed to see if large indoor concerts could take place safely amid the coronavirus pandemic. But such scenes could soon become far more common: Researchers found that not one of the concertgoers tested positive for the coronavirus after the event, according to results published Wednesday on the website of the Lancet Infectious Diseases journal.

The safety protocols put in place for the concert could offer a preview of what live music events might look like for the foreseeable future, in some places with relatively low community transmission attempting to balance safety with the drive for a full return to normalcy.

While social distancing wasn’t required, attendees were required to wear N95 face masks throughout the event. The only exceptions were at a designated outdoor smoking area where only 20 people could gather at a time and distancing was mandatory, and an indoor bar area where masks could be briefly removed while drinking.

Researchers find ‘glimmers of hope’ after inviting thousands to indoor concert amid pandemic

The Washington Post's Rick Noack attended a concert in Germany co-organized by one of the country's university hospitals in the name of science. (Video: Alexa Juliana Ard, Rick Noack, Stefan Czimmek/The Washington Post)

To make wearing masks more comfortable, temperatures inside the venue were kept under 70 degrees. While the building lacked windows, doors were left open to allow additional ventilation.

The concert took place before coronavirus vaccines were available in Spain, but at a time when the number of infections was relatively low, with roughly 221 cases being reported for every 100,000 people in the Catalonia region. All participants were required to take rapid antigen tests ahead of the concert — a method of testing for the coronavirus that is less accurate than others but delivers results within minutes.

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More than 1,000 people passed the test and became part of the experiment, with half ushered into the concert and the other half sent home as a control group. Two members of the control group tested positive for the coronavirus eight days later, the researchers found, but none of the concert attendees did.

While similar experiments have taken place in Germany, the United Kingdom, and the Netherlands, the Barcelona researchers appear to be the first to publish their findings in a peer-reviewed scientific journal. Some of the funding for the experiment came from Primavera Sound Group, a Spanish concert company that also employs one of the study’s authors.

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“Our study provides early evidence that indoor music events can take place without raising the risk of SARS-CoV-2 transmission when comprehensive safety measures are in place,” the study’s lead author, Josep Llibre of Germans Trias i Pujol University Hospital, said in a statement. “It is important that 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. As a result, our study does not necessarily mean that all mass events are safe.”

In March, researchers experimented by hosting an even larger concert in Barcelona with similar safety measures in place. Roughly 5,000 people attended, and six people tested positive for the coronavirus within 14 days of attending the event, the BBC reported. Scientists determined that four had been infected elsewhere, and noted that the incidence rate among attendees was lower than in Barcelona as a whole. Those results have not yet been peer-reviewed.

Germany’s experimental concert study, which has also not yet been peer-reviewed, found that mandatory masks and enhanced ventilation significantly lowered infection risks.

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