One of those principles is that if you’re going to actively expose people to something - to create a possible harm where there previously wasn’t one - you have to be honestly uncertain of the potential harm (eg, it’s ethical to give you a placebo vs experimental drug because I honestly don’t know if the drug will work).
It’s unethical to -certainly- cause harm.
In part that’s because, social dynamics and power structures being what they are, those “consenting” will skew towards the uneducated and powerless.
In part, consent isn’t even relevant - in this example, if they ended up stoking the flames of contagion, they wouldn’t just affect the concert goers but, by definition, all those exposed to the concert goers.
Amusingly, the above refers more to interventional studies - those with more than one arm. This doesn’t really have a control arm, or randomized assignment of people to exposure. It’s effectively an observational study - the sort of thing you don’t need to intentionally construct, but for the fact that people seem to not be going to concerts.
So... did they have to construct concerts artificially because not enough were occurring naturally? Did they have to create a harm that didn’t exist to begin with? Will it even be insightful? They filtered for the young, which specifically ensures there’s less of a chain of transmission available.
Considering Germany inspired so much of modern medical ethics work, it’s ironic to see them conduct such a study. I honestly wouldn’t have expected it.
I’m sure the relevant experts and authorities beat these questions to death, but even so, I’d be awfully curious to see those deliberations and what justifications were ultimately considered adequate.