“Many of them were not wearing masks when exercising,” the Centre for Health Protection’s Dr Chuang Shuk-kwan said of the patrons. “They attended some group exercises which may involve as many as 10 students. Most of the staff have already been infected.”

Chuang warned that some infected Ursus Fitness users had passed the virus onto others who had not visited the gym, and those people could then infect others.

“I do not wish to see a fifth wave,” she said. “I hope it can be controlled, but it has been spreading very fast. One infection has already led to many others.”

Authorities knew of at least 12 other fitness centres recently visited by Covid-19 carriers, including

H-KORE venues in Central and Taikoo Place, as well as Pure Fitness’ main facility in Central.

Among the roughly 20 people who tested preliminary-positive was a 34-year-old fitness instructor who worked at the One Personal Training gym in Central and had taught dozens of people, Chuang said.

The outbreak at Ursus Fitness has prompted officials to step up safety measures at fitness centres across the city. In addition to the mandatory testing order, for six days beginning on Friday, gym members must wear masks while working out, a rule that previously only applied to those in exercise groups.

But at the same time, Chuang pointed to the risk of gyms on Hong Kong Island becoming avenues for spreading the disease, as they were popular with professionals in banking, finance and law.

While authorities are still trying to track how exactly the virus is spreading around the island, several of the most prominent financial institutions and law firms were hit with cases. HSBC and law firm Allen & Overy are among the businesses that have reported infections among staff at their Central offices. Some Citibank employees went to affected gyms and were instructed to work from home, according to a source.

As concern mounted in downtown Hong Kong, about 100 people lined up outside a mobile testing van outside the City Hall in the afternoon, with bankers among them.

Health authorities also said 14 of the latest cases involved residents in Mid-Levels. Under the government’s existing policy, anyone who lives in the same building where one person is confirmed as infected must undergo testing.

International schools have also been affected, including the British Kellet School that was forced to put about 20 eight-and-nine year old children into quarantine after discovering a “Covid-positive teacher”. One parent or a guardian will be allowed to accompany the children in isolation.

As Covid-19 cluster spreads, are Hong Kong gyms safe to remain open?

“Despite our strong protests that these children were socially distanced at all times and both the (Covid-19 positive) teacher and students were wearing masks, the children will be moved to a government facility,” Principal Mark Steed said in a letter to parents.

At least 13 other schools have also been forced to suspend classes this week, after either discovering cases or as a precautionary measure.

Professor David Hui Shu-cheong, a respiratory medicine expert at Chinese University, said the gym outbreak was similar to the cluster tied to dance clubs across the city that ballooned late last year to 732 people – the largest in Hong Kong since the start of the pandemic.

“I worry the big outbreak at the fitness centre will be the prelude to the fifth wave, but the Centre for Health Protection very quickly quarantined close contacts,” Hui told a radio programme. “Hopefully that will stop the virus from spreading everywhere in the community. We will need to closely monitor the situation.”

But Hui acknowledged Ursus gym was popular and patrons might also visit other fitness centres, potentially carrying the virus with them.

“This can be called a ‘super-spreader’ cluster … we need to find out if there was enough ventilation inside the gym, if they followed the four-person per group rule, and whether the equipment and facilities were disinfected after use,” he said. “There is definitely a huge problem if a large outbreak suddenly emerges.

“So far, there is only an outbreak at [Ursus Fitness], so it is difficult to immediately shut down all gyms for that reason,” he said. “We need to see if outbreaks occur at other fitness centres.”

Five of the latest local transmissions were untraceable and six cases were imported. The city’s tally of infections stood at 11,210, with 203 related deaths.

Separately, the Department of Health announced that since the vaccine drive started last month, it had received 71 reports of people experiencing side effects after inoculation. Some 69 received the Sinovac jabs, while two took the German-made BioNTech vaccine.

Why has Hong Kong’s vaccine drive stumbled, and should side effects concern?

The side effect rate was not particularly higher than in other places around the world, the department said.

Four people have died after receiving the Chinese-made Sinovac vaccine, although experts have concluded that in the first two cases, both of which featured pre-existing conditions, there was no direct link to the jabs. Health authorities are looking into the other cases.

The latest fatality involved a 70-year-old woman who had a history of hypertension and osteoarthritis who died on Thursday afternoon. She received the shot at Kowloon Bay Sports Centre Community Vaccination Centre 10 days ago. As of Friday, about 162,100 people had been vaccinated, according to the latest official figures.

Additional reporting by Danny Lee and Chan Ho-him