Covid-19 transmission in fitness centers in Norway - a randomized trial oleh Lise M. Helsingen, Magnus Løberg, Erle Refsum, Dagrun Kyte Gjøstein, Paulina Wieszczy, Ørjan Olsvik, Frederik E. Juul, Ishita Barua, Henriette C. Jodal, Magnhild Herfindal, Yuichi Mori, Solveig Jore, Fridtjof Lund-Johansen, Atle Fretheim, Michael Bretthauer, Mette Kalager, for the TRAiN study group