Phan Trong Lan, head of Ho Chi Minh City Pasteur Institute, Vietnam's leading epidemiology center, said the Delta variant, or B.1.167.2, first detected in India, has been labeled highly transmissible by the World Health Organization, and able to reduce the efficacy of available Covid-19 vaccines.

Previously in 2020, an infected person could transmit the virus to between two and four others and the Alpha, or the U.K. strain that appeared earlier this year, to up to seven. The Delta variant, meanwhile, is said to be 40-60 percent more transmissible than the Alpha.

Gene sequencing results show the Delta variant is dominant among infections in the ongoing outbreak in Vietnam, especially among cases recorded in central and southern localities, including Ho Chi Minh City.

Because of this strain, the southern region has recorded many cases to have developed Covid-19 symptoms or tested positive a day after coming into contact with an infected person, said Lan.

"It is still uncertain whether the Delta variant causes more severe Covid-19 symptoms, though the increased number of cases has caused healthcare system overload, which can lead to a higher mortality rate," he stressed.

In the fourth Covid-19 wave that broke out in Vietnam on April 27, the Delta strain was first detected in northern Yen Bai Province before spreading to epicenter Bac Giang Province in the north.

Currently, more than 15,200 cases have been recorded in 52 out of 63 cities and provinces.

Le Quoc Hung, head of the Tropical Diseases Department at Cho Ray, a leading hospital in southern Vietnam, said when a new coronavirus strain struck, there will be groups experiencing high and lower virulence.

Specifically, the group of higher virulence will see early symptoms, allowing early detection of Covid-19 patients for early isolation and treatment.

Meanwhile, those with lower virulence often sees no, or only mild symptoms, which impedes detection.

This results in community infections and affords the virus an opportunity to spread and multiply, he noted.

Hung said the more infectious Delta variant retains the basic core of the virus, with the only difference found on virus spikes.

The number of spikes in this strain is higher, leading to a stronger ability to attach to cells of the pharynx, making it easier to enter the body and cause infection.

"For example, with previous strains, the number of viral spikes may be less and simply by the action of swallowing, could be removed from the oropharynx, avoiding infection," Hung said.

Nguyen Tri Dung, director of the Ho Chi Minh City Center for Disease Control, concurred while addressing a city meeting on June 25.

He said after transmission over many generations, the new coronavirus would either mutate and increase its virulence or stay the same and see its virulence decrease.

Though virulence may decrease, transmission would persist, though with no or mild symptoms. Therefore, the virus would quietly spread throughout the community without anyone knowing it.

Nguyen Huy Nga, former head of the General Department of Preventive Medicine under the Ministry of Health, said the Delta strain has made this outbreak more complex.

About 80 percent of patients show no, or only mild symptoms, making it difficult to determine who is carrying the virus into the community, posing a high risk of infection.

Those infected might not be aware they have caught the virus and so easily transmit it to others. Even if patients were to recover thanks to a healthy immune system, the virus would already have been transmitted, Nga said.

"Therefore, it is necessary to track down all community infections, but given the ongoing situation, it seems impossible."

Accounting for the second most community infections so far in this wave, HCMC has since June 26 carried out a mass testing campaign to collect as many as five million samples in 10 days to detect any possible Covid-19 cases in the community.

The city and other infected localities have placed infected areas under lockdown and imposed social distancing measures to curb the spread of infection.