We've been open for over a month now.

Two weeks ago, one of our guys got sick & tested positive for Covid. He immediately informed everyone using our group chat, and everyone he was on the mat with got tested.

No one tested positive. No one got sick.
Even the guy's girlfriend, who spent the night with him after training didn't catch the bug.

The guy himself is a stud. 2 meters tall, super healthy, shredded, kitesurfer & blue belt beast. He beat it in a few days, and has doctors premission to return to the mat next week.

Some key points.
We train in a room with all windows open, and two ventilators refreshing the air from inside to outside.

It's interesting to note that eventhough he rolled with and breathed on everyone, no one caught it. This points towards the aerosol hypothesis, where only prolonged exposure to aerosols within a confined, closed space can cause infection.

TL;DR version.
Train with the windows open & make sure the air in your dojo gets refreshed with outside air (no regular air conditioning / air recycling).