A cluster of coronavirus cases has emerged from indoor fitness classes in South Korea, sparking warnings about the potential of the virus to spread in indoor facilities.
In a new research paper, it was reported 112 people were infected with the virus in 24 days after taking part in fitness dance classes at 12 sports facilities.
"Intense physical exercise in densely populated sports facilities could increase risk for infection," the authors found.
"Vigorous exercise in confined spaces should be minimised during outbreaks."
Researchers identified the outbreak started from a fitness dance instructor workshop in Cheonan on February 15.
Latin dance classes have become popular in South Korea because of their high aerobic intensity.
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Instructors trained intensely for four hours at the workshop, and among the 27 who participated, eight tested positive for the virus.
They were all asymptomatic on the day of the workshop.
By March 9, researchers had identified 112 cases after instructors went on to teach classes.
On average, students developed symptoms 3.5 days after participating in a fitness dance class.
More than half of the cases were the result of transmission from instructors to fitness class participants; 38 cases were in-family transmission from instructors and students; and 17 cases were from transmission during meetings with co-workers or acquaintances.
"Characteristics that might have led to transmission from the instructors in Cheonan include large class sizes, small spaces, and intensity of the workouts," the researchers said.
"The moist, warm atmosphere in a sports facility coupled with turbulent airflow generated by intense physical exercise can cause more dense transmission of isolated droplets."
Classes that had secondary cases had five to 22 people in them in a room about 60sq m in size for 50 minutes of intense exercise.
They found no cases in classes with less than five participants and believe lower intensity classes like pilates and yoga did not cause the same transmission effects.
"Because of the increased possibility of infection through droplets, vigorous exercise in closely confined spaces should be avoided during the current outbreak, as should public gatherings, even in small groups," they concluded.