There has been "no sign" of coronavirus infection among 5,000 unvaccinated people who took part in an indoor trial concert last month in Barcelona seeking pandemic-safe ways to celebrate mass events, organisers said Tuesday.

The participants underwent PCR tests two weeks after the March 27 event and "there is no sign that suggests transmission took place during the event," Josep Maria Llibre, a specialist in infectious diseases from the Germans Trias i Pujol hospital told a news conference.

Organised by the hospital and a group of Spanish music promoters, the concert in the Palau de Sant Jordi was billed as Europe's biggest indoor rock concert since the start of the pandemic.

It featured one of Spain's most popular bands, indie rockers Love of Lesbian.

Participants wore FPP2 surgical masks at the concert but there was no assigned seating or mandatory social distancing.

Ahead of the gig everyone underwent mass screening and antigen tests. They were tested again 15 days later and only six people were found to have Covid-19.

"We are certain that in four of these six cases, transmission did not take place during the concert," Llibre said.

"With optimised ventilation, antigen tests and the wearing of face masks, we can guarantee a safe space."

Boris Revollo, the virologist involved in the design of the health protocols at the concert, said he could not "categorically" rule out that the other two people were infected during the concert but there was a "very high probability" that they had not.

"The measures which we implemented were very safe," he added.

Another similar test concert was held in Amsterdam in March involving 1,300 people.