At least 55 employees of the HCMC Hospital for Tropical Diseases were recently infected with the coronavirus despite having received two shots of the AstraZeneca Covid-19 vaccine.

The hospital, HCMC's frontline facility for treating Covid-19 patients, was locked down starting Saturday after three employees of its IT and admin departments were found infected with the coronavirus. That cluster later ballooned to a total of 55 cases as of Monday morning.

However, that doesn't necessarily mean the vaccine has been ineffective, experts say. All the vaccinated cases are largely asymptomatic, showing little chance of developing into severe cases or becoming new infection sources to others in the community.

Tran Tinh Hien, a director of the Oxford University Clinical Research Unit and former director of the HCMC Hospital for Tropical Diseases, said "there is no vaccine with 100 percent protection rate from infections."

A survey of vaccinated U.K. citizens from December last year to February revealed that the AstraZeneca vaccine has a 60 percent efficacy 28 days after the second shot. Another report in medical journal Lancet also said the AstraZeneca vaccine helped reduce instances of symptomatic Covid-19 cases, as well as severe ones and fatalities, Hien noted.

"At the HCMC Hospital for Tropical Diseases or in any other community which has been vaccinated, a 5-10 percent rate of coronavirus-positive cases doesn't mean the vaccine has failed," he said.

Le Quoc Hung, head of the Infectious Disease Department of HCMC's Cho Ray Hospital, echoed Hien's opinion, adding that the virus would find it much harder to replicate inside a vaccinated person, which lowers the chance of that person passing on the virus to someone else.

The AstraZeneca vaccine also provides the best protection against the coronavirus four to six weeks after the second shot, so people could still get infected in the time frame before full protection, he said.

"Four weeks following the second shot, the chance of being infected is reduced, but that doesn't mean there's no chance," Hung said, adding that the occurrence of infection doesn't mean the vaccine's not effective.

Hung said the vaccine helps generate antibodies, but if not enough antibodies have formed, either because a person has received only one shot or not enough time has passed for the body to generate enough antibodies, one can still get infected.

And depending on a person’s physiology, it can happen that one is fully vaccinated and still not generated enough antibodies to completely block an infection, he added.

The viral load in the 55 infected employees at the hospital was "extremely low," Nguyen Tan Binh, director of the HCMC Department of Health, said Monday. He did not specify how low the load was.

"It is a good sign that shows the vaccine's effectiveness," Binh said.

Vietnam has launched a mass Covid-19 vaccination program using the AstraZeneca vaccine since early March. So far, around 1.5 million people have been vaccinated.