A total of 20 of 21 people who were exposed to either of the instructors in the 24-period before they showed signs of illness wound up becoming infected after attending a class or personal training session. Another eight patrons were exposed in the 24-hour period before that, and one of them contracted the coronavirus as well.

Out of those 21 infections traced to the instructors, 20 patients (including the second instructor) went on to develop Covid-19, and two of them were hospitalised. At the time of the outbreak, mask use was not required in fitness facilities, the authors wrote.

Some of the classes seemed designed to spread the virus. For instance, there was a high-intensity cycling class where the infected instructor faced a group of 10 maskless participants and shouted instructions at them. Three large floor fans blew air toward the patrons for cooling; doors and windows remained shut.

“They essentially created an ideal environment for transmitting coronavirus,” Chang said. “I mean, you had enclosed spaces, inadequate ventilation, masks were often not used or required, and people were vigorously exercising. That’s essentially a recipe for a super-spreader event.”

Hong Kong’s fitness industry forced outdoors as trainers push Covid-19 restrictions

The outbreak in Chicago took place around two months later. There, mask use, temperature checks and symptom screenings were required upon entry. Patrons brought their own mats and weights, and were positioned at stations at least six feet apart. However, they were allowed to remove their masks during exercise.

“I think in Chicago they did some things which we sometimes call public health theatre,” Chang said. “Asking if people have symptoms doesn’t catch people who are asymptomatic transmitters or who don’t tell the truth. Temperature checks are insufficient because not every contagious person will be running a fever. None of that really probably has a huge effect on decreasing your risk of transmitting coronavirus in your facility,” he said.

Indeed, 55 out of 81 people who attended high-intensity indoor classes at the Chicago gym sometime between August 24 and September 1 became infected and developed Covid-19. And 43 attendees with Covid-19 took part in multiple classes while they were potentially infectious, while 22 attendees with Covid-19 attended those classes on – or after – the day they started feeling symptoms.

Class participants were interviewed about their behaviour in the gym. Out of 38 attendees who had Covid-19, 32 (84 per cent) reported infrequent mask use. (So did 12 out of 20 – 60 per cent – people who did not become sick.) Three of those with Covid-19 attended the gym on the same day – or after – they received a positive coronavirus test result.

Ultimately, two of the 55 people visited an emergency department and one was hospitalised for eight days.

Both research teams suggested ways of reducing the risk of working out in a gym, such as significantly improving ventilation, enforcing consistent and correct mask use and physical distancing, reminding all patrons and staff to stay home if they’re ill, and increasing hand-hygiene opportunities.

“Conducting exercise activities entirely outdoors or virtually could further reduce SARS-CoV-2 transmission risk,” both research teams added.

Asia gets the fitness bug, inspired by sporting role models

Chang said the safest place to exercise would probably be at home or outdoors. But not everyone is able to do so, which is where gyms come in.

“Gyms are a social and public health good,” he said. “It’s important for people to exercise and some people aren’t able to exercise at home or outdoors. So I do think many gyms can stay open – but they really need to have some vigorous policies and adjustments in place.”