A medical staff uses blood samples for testing new coronavirus at a quick test station in Hanoi, March 31, 2020. Photo by VnExpress/Giang Huy.

Tran Dac Phu, senior advisor at the Public Health Emergency Operations Center under the Health Ministry, said there is still no clue as to who the source was that first introduced Covid-19, the respiratory illness caused by the novel coronavirus, to Bach Mai Hospital in Hanoi.

The hospital is now the nation’s largest infection hotspot. By Thursday morning, the number of cases associated with the facility had increased to at least 42; 26 of them employees of Truong Sinh Company, the food and logistic services provider to Bach Mai.

Authorities had identified all sources at previous hotspots across the nation, enabling them to track down and keep all those that had come in close contact with them under close monitor.

Most recently, "Patient 17", the first infection confirmed in Hanoi and "Patient 34" in south central Binh Thuan Province, who had both returned from abroad and then tested positive, are typical examples of such sources.

"For hotspots that we could not identify the source of transmission, the risk of spreading infection in the community is very high and therefore, everybody should stay home."

At this stage, what the health sector and local authorities could do is try to detect all suspected cases as early as they could, then locate and isolate affected areas.

The national social distancing rule the government has requested citizens to follow is for the purpose of preventing healthy people from directly contacting those that have contracted the virus, Phu told a meeting Thursday.

The new coronavirus is easily transmitted via close and direct contact and thus, the further people stay away from each other, the better.

"Once social distancing is practiced well, a hotspot will not have a chance to grow bigger. It’s just like a fire: if the fires are just small ones, we totally have the ability to kill each of them off," he confirmed.

On the contrary, if the epidemic spreads widely, authorities could not locate the affected areas, allowing the number of infections to grow out of control, placing the medical system under great pressure.

What comes next would be a high fatality rate because doctors could not focus their efforts and time on treating each patient as has been happening so far in Vietnam.

"What everybody needs to do right now is stay inside and only go out when it is truly necessary," he insisted.

Truong Huu Khanh, head of the infection and internal neurology department at the Children's Hospital No.1 in HCMC, said losing track of the source of transmission is what must be prepared for.

However, he said, if practiced well, social distancing can cut the chain of transmission and even when the sources of infection are still out there in the community, there is no way they could transmit the virus widely.

For a source of infection from abroad, Phu praised Vietnam’s efforts of sending all entries to quarantine camps, not only for two week isolation but also testing.

"This method has allowed us to resist until today and keep the number of infections from surging while many other countries have seen their cases jump from 100 to 1,000 in just one week," he said.

Phu confirmed the epidemic in Vietnam, until today, remains under control.

Aside from two nurses confirmed positive on March 20, the Bach Mai Hospital has yet to record cases among its staff.

From April 1, Prime Minister Nguyen Xuan Phuc has imposed a 15-day nationwide social distancing campaign, banning gatherings of over two people and asking people not to leave their homes.

The government also suspended all inbound international flights and halted road transport services until April 15 to limit travel in the country of 94 million.

It also shut down all "non-essential" services like bars, karaoke parlors and spas until April 15 and closed its borders with neighboring countries Cambodia, China and Laos.

It declared Covid-19 a national epidemic on Wednesday afternoon when the infection tally reached 212.

As of Thursday, the number of infections in the country has gone up to 227, including 75 who have recovered and been discharged from hospital.

Many of the active cases are people, both Vietnamese and foreigners, coming from Europe and the U.S., and others who came into contact with them.