"There are so many different factors that go into it," Barbara Marston, the Covid-19 International Task Force Lead at the U.S. Centers for Disease Control and Prevention (CDC), said at a telephonic conference on Wednesday.

"It could be partly ecological, but they are certainly related to the quality of responses, and I think Vietnam has had a very strong response."

She was responding to a reporter asking about the low numbers of infection cases in Vietnam.

As of Thursday morning Vietnam has had 268 cases, 97 of them active and no deaths.

In comparison, some of its Southeast Asian neighbors have reported thousands of cases. The Philippines has had over 5,400 cases and 349 deaths, Malaysia, over 5,000 cases and 83 deaths, and Indonesia, over 5,100 cases and 469 deaths. Southeast Asia as a whole has had over 22,000 cases so far.

Since the novel coronavirus was first spotted in the country in January, Vietnam has taken a number of measures to detect new infections and contain them.

They include letting people work and study from home, closing its borders, stopping visa issuance, closing down non-essential businesses, and, recently, a nationwide social distancing campaign that required people to stay at home and not more than two people congregating in public.

The campaign, which initially lasted from April 1 to April 15, has now been extended to at least April 22 for 12 localities with high risk of Covid-19 infection, including Hanoi and Saigon.

CDC Thailand country director John MacArthur said: "The public health system in Vietnam is very strong. The government at the highest level took this seriously, and has really taken a whole-of-government approach to the response, and I think that is why we’re seeing some successes in Vietnam."

Vietnam was also open to inputs from outside technical advisors such as WHO and academic partners, and took on board advice from experts, he said.

"They developed evidence-based guidelines, and so they looked at what was being recommended by CDC, they looked at what was being recommended by WHO, they looked at what was happening within their own country, and adopted sort of those more global guidelines to be more specific to Vietnam based on the data."

He also said Vietnam's numbers regarding the Covid-19 situation are true, he said.

"We're seeing some successes (in fighting Covid-19) in Vietnam."

The global death toll has jumped to over 134,600 as the pandemic spread to 210 countries and territories.