Nguyen Trung Cap, deputy director of the National Hospital for Tropical Diseases, said most of the patients in Hai Duong did not have noticeable symptoms, making it difficult to determine the exact time of exposure. Cap and his colleagues were sent by the Ministry of Health to Hai Duong, Vietnam's biggest Covid-19 hotspot at present, to work at Chi Linh Town's Field Hospital 1, where 165 patients are being treated.

"We felt that signs of lung damage seem to appear earlier than in patients infected by previous strains. But as the sample size is still small, no definitive conclusion could be drawn yet," he said, adding the findings are insufficient to prove whether the new variant could be deadlier or cause more severe effects.

Echoing Cap's observation, Tran Van Giang, deputy head of the Virus-Parasite Department at the National Hospital for Tropical Diseases, said he has also not seen "more severe symptoms" in patients infected with the new variant. The percentage of carriers with noticeable signs of pneumonia and respiratory failure is not much different compared with that during the previous wave, he added.

Due to concerns the new variant may progress the disease more quickly, patients have been placed under extra surveillance, Cap said.

More transmissible

The new variant has already been known to allow airborne transmission, including through droplets and aerosols, according to Cap.

Nguyen Van Vinh Chau, director of the HCMC Hospital for Tropical Diseases, said the new variant could also infect human cells more easily. Studies on the U.K. coronavirus variant have shown mutations on the viral genome that encodes its "spike" protein make it easier for the virus to infect human cells.

Le Quoc Hung, head of the Tropical Disease Department at Cho Ray Hospital in HCMC, said the U.K. variant is 30 percent more virulent and 70 percent more transmissible than previous strains. As northern Vietnam is currently experiencing cold weather, that factor may allow the virus to stay viable for longer periods of time in the environment, compared to hotter regions like in central Vietnam, he added.

The Ministry of Health on Tuesday said the new Covid-19 wave sweeping through Vietnam since late last month was caused by the U.K. coronavirus variant, after gene sequencing revealed 11 Covid-19 cases in Hai Duong and one in HCMC to be infected by the strain.

Health Minister Nguyen Thanh Long said the variant's higher infectivity and shorter incubation period explained the rising number of community cases through the past week, which has now amounted to 366 in 10 cities and provinces.