The cluster’s first infections developed symptoms in mid-May, but it was not until two weeks later that the city confirmed the first three patients after they visited a local hospital.

"We have already let the virus spread through four or even five generations in the community without knowing it. And as it spreads exponentially, the situation is now extremely dangerous," said Long.

With the ongoing development, more clusters are likely to appear in the city, with the source of transmission being people who contracted the virus from those originally related to the mission cluster, he said.

Industrial parks affected

Municipal officials have confirmed that the Covid-19 cluster at the Christian congregation in Go Vap District has led to infections in several dozen companies and three industrial parks.

"HCMC has 1.6 million workers and without proper measures for controlling the pandemic, it could lead to a crisis," Long said, citing the northern province of Bac Giang as an example.

Bac Giang had to shut down four industrial parks at once last month. It remains the fourth wave’s epicenter with over 2,300 cases.

The cluster emerged last Wednesday and virus gene sequencing results among infected members of the mission found they are infected with the highly transmissible virus strain B.1.167 first found in India.

Various studies have said that it takes just two-three days for this strain to spread and infect another host.

Until now, the city has recorded over 200 cases linked to the mission cluster in 20 out of its 22 districts, including those working in a closed environment using air conditioners such as factories in industrial parks and office buildings, which facilitate the spread of the virus.

"The risk of spreading Covid-19 from the community into the industrial parks already existed. The collective working environment that allows a large concentration of people in those parks is a favorable condition for the disease to spread quickly and strongly," said Nguyen Tan Binh, the city’s Health Department director.

Thien Tu FN Service Investment Co. Ltd in Tan Binh District has recorded 34 infected employees, while IDS Vision Solution Development Co. Ltd in Tan Phu District has got nine cases.

The Quang Trung Software Park in District 12 has reported four cases while three industrial parks - Tan Binh in Tan Binh District, Tay Bac in Cu Chi District and Vinh Loc in Cu Chi District, having 500-1,000 workers each, have recorded one case each.

Health department director Binh said it was possible that the mission cluster will be responsible for around 500 cases in the city as many are waiting test results.

HCMC has isolated Go Vap District with its 676,000 people and imposed a social distancing period across the city starting Monday.