Wondering whether it’s safe to go back to the gym? Norwegian gymgoers may have some good news for you. A study on the risk of coronavirus transmission in Oslo found that people who went to a gym were no more likely to get infected, or sick than people who didn’t. Norway has reopened its gyms based on the tentative results, which were published as a preprint yesterday and still need to go through peer review. But some epidemiologists aren’t so sure. It’s possible that no one was infected at those Oslo gyms because there were very few COVID-19 cases in the city when the study was done—not because working up a sweat on the treadmill or lifting weights in the midst of a pandemic is safe. So far, crucial decisions to reopen public spaces after lockdowns are being made on the basis of little evidence. Every child returning to school and every Zumba fan returning to class is taking part in a large, uncontrolled experiment. A much better approach is to carefully study the impact of e