HONG KONG — Thousands of Hong Kong residents were locked down on Saturday in an effort to contain a worsening outbreak of coronavirus in the city. There have been more than 4,300 cases of coronavirus in Hong Kong since November, making up nearly 40% of the city's total.

Cases in a working-class neighborhood, the Yau Tsim Mong district, made up half of infections last week.

Sewage testing in the area picked up more concentrated traces of the virus, prompting concerns that poorly built plumbing systems and a lack of ventilation in subdivided units may present a possible path for the virus to spread.

During the 2003 outbreak of SARS in Hong Kong, the virus likely spread through "inadequate plumbing", according to a World Health Organization (WHO) technical analysis that year.

Experts at WHO wrote in 2003 that "inadequate plumbing and sewage systems could continue to enhance the potential spread of SARS and certain other diseases."

Authorities said on Saturday that an area of the Yau Tsim Mong district with 16 buildings will be locked down until residents are all tested. Residents will not be allowed to leave their homes until they receive test results.

“Persons subject to compulsory testing are required to stay in their premises until all such persons identified in the area have undergone testing and the test results are mostly ascertained,” the government said in a statement.

Hong Kong has previously avoided lockdowns in the city during the pandemic, with leader Carrie Lam stating in July last year that authorities would avoid taking such “extreme measures”.

Hong Kong placed 10,000 residents in one of its most crowded districts under COVID-19 lockdown on Saturday, as the city leader said officials would investigate how to avoid a repeat of people fleeing in advance of any future confinement orders.

Officials from 4 a.m. sealed off about 200 buildings in Yau Tsim Mong district and deployed around 3,000 government workers to carry out emergency testing in a drastic bid to stem the alarming spread of COVID-19 in the city.

News of the first lockdown under legislation introduced last month was leaked ahead of its roll-out, offering residents the opportunity to escape the clutches of the city’s toughest pandemic restrictions yet.

During a visit to the lockdown scene in Jordan on Saturday afternoon, Lam admitted there was room for improvement, while noting it was the first time the government had taken this form of action under legislation approved last December.

She said the government would look into the issue of residents having advanced warning of the lockdown, while acknowledging it was difficult to ensure the information would not be leaked because 3,000 workers were involved in the operation.

She said the lockdown was necessary after 162 confirmed cases were found in 56 housing blocks in the Jordan area between Jan. 1 and Jan. 20. Residents who left the area ahead of the curbs were still legally bound to take a COVID-19 test, Lam pointed out.

The government has ordered everyone in the area to get tested by the end of Saturday, as it aimed to allow residents to leave the sealed-off section for work from about 6 a.m. on Monday, — Agencies