In September, a Chicago resident called their gym with alarming news: They’d recently come to an indoor workout class despite feeling sick and then later tested positive for the coronavirus.

The gym quickly shut its doors, but it was too late. Fifty-five of the 81 people who attended high-intensity classes at the facility between Aug. 24 and Sept. 1 would eventually test positive. A similar case tied to three gyms in Honolulu over the summer resulted in 22 total infections.

Citing both cases, the Centers for Disease Control and Prevention on Wednesday urged gym users to wear masks during intense workouts – even when socially distant – and asked gyms to improve ventilation and push for outdoor activities when possible.

The new research is a reminder that working out indoors with other people carries a significant risk of infection, public health experts said.

If you can wait until the spring and work out outside, it will be a lot safer,” Joshua Epstein, an epidemiology professor at NYU’s School of Global Public Health, told The Washington Post. “We are not out of the woods by any means. It’s not the time to relax.”

Studies have long pointed to gyms as a particularly dangerous venue for spreading the coronavirus, as many people from different households pack into indoor rooms where they breathe heavily and sometimes work with instructors who yell instructions during classes.

In Chicago, the CDC found an outbreak was tied to a gym that was operating at 25 percent capacity. Although gym members brought their own weights and mats to high-intensity classes, remained six feet apart and were screened for symptoms, including temperature checks, upon arrival, two major basic guidelines to curb the spread of the virus were not met, the report found.

The majority of attendees did not keep their masks on throughout the entire class. Added to this, 22 people attended the high-intensity workouts despite exhibiting COVID-19 symptoms and three after having tested positive for the virus.

None of those infected through the gym died, but one person was hospitalized for eight days. The gym reopened 13 days after it closed its doors, and only those who had proof of a negative coronavirus test were allowed to return, the CDC report added.

The quick spread inside the gym was not surprising, Epstein said, because many of the best practices to prevent an outbreak were ignored.

“This was a very high risk behavior with predictably unfortunate consequences,” Epstein said. “It’s high respiration in a closed space. Yes, people brought masks but evidently (a majority) said they wore them rarely, including some attendees with COVID. Some were symptomatic and some knew that they were positive. All of those are very, very high-risk circumstances.”

The nature of the high-intensity (HIT) classes also posed a risk, public health experts said.

“One of the features of this outbreak was that they were engaged in HIT and that makes a difference,” said Saad B. Omer, an infectious-disease epidemiologist and director of the Yale Institute for Global Health.

The cluster of cases tied to the Honolulu gyms began in late June, the CDC said. Researchers traced the outbreak at the three gyms to a 37-year-old fitness instructor who, before exhibiting any symptoms, taught classes at two fitness facilities on the island.

On June 27, two days before his first symptoms began, the man taught a one-hour yoga class to 27 people. Although the instructor wore a mask, none of the participants did. At that time, masks were not required at gyms. The following day, the instructor taught a high-intensity cycling class where neither he nor the 10 participants wore masks.

The instructor, who said he was more than six feet away from class participants, stood on a pedestal as he shouted instructions and words of encouragement. On June 29, four hours before he began exhibiting symptoms, he taught his last cycling class to a group of 10 people. The class took place in the same room he had taught the day before and no one wore masks.

One of the participants in that class was a 46-year-old personal trainer who taught at a separate facility. Two days after attending that class, he taught five personal training lessons and small-group kickboxing lessons. According to the report, everyone who attended the personal training sessions, including the trainer, wore masks. But no one who attended the kickboxing sessions did. He continued to teach personal training sessions and kickboxing lessons at least 12 hours before his symptoms began. The usage of masks at these sessions varied.

In the end, authorities linked the case of the first instructor to 21 people who later tested positive. One person was admitted to an intensive care unit.

The findings should be a warning signal to Americans not to let down their guard even as case numbers are dropping and vaccines are rolling out, experts said.

“It’s the time to maintain all of these measures throughout the vaccination (process),” Epstein said. “It’s the worst time to get complacent.”