Tran Dac Phu, senior advisor at the Public Health Emergency Operations Center under the Health Ministry, said the world has a tradition of certifying travel for those who have been fully vaccinated against a number of diseases such as cholera, lymphadenitis and yellow fever.

However, Covid-19 is a new infectious disease, requiring newly developed vaccines. Despite various mass inoculation campaigns, the effectiveness of Covid-19 vaccines remains open to verification, Phu said.

"Different vaccines will have different protective effects and it remains unknown how long antibodies against the novel coronavirus would exist in the body of a person already vaccinated," he said.

When the strain of the virus changes, the Covid-19 vaccine might no longer work, he said.

Phu's comments came as countries like China and Israel started issuing digital health certificates for travelers who had been vaccinated against Covid-19, also dubbed "vaccine passports."

The U.S. and Britain are among countries currently considering issuing similar permits, while the European Union is also working on its own vaccine "green pass," allowing those who have had the jab to travel more freely within the region this summer.

Phu said Vietnam is yet to introduce policies regarding vaccination certificates and that anyone entering the country still have to undergo mandatory 14-day quarantine.

Echoing Phu, Truong Huu Khanh, head of the infectious and neurological diseases department at Ho Chi Minh City's Children's Hospital No.1, said there is no vaccine that guarantees 100 percent immunity.

Evidence of their effect only appears after many inoculations, with the number in Vietnam still low, meaning there is no herd immunity in the country of 98 million people as yet, Khanh added.

As of Thursday, 955 medical staff and frontline workers in the Covid-19 fight have received the first dose of AstraZeneca's vaccine, with some having developed minor complications.

Khanh advised those holding a vaccine passport and entering the country to be screened and quarantined for 14 days.

An anonymous expert at the Health Ministry said global studies showed Covid-19 inoculation only helps reduce symptoms, and have not proved to reduce the risk of infection, as with the AstraZeneca and Pfizer vaccines.

"Covid-19 vaccines are being rapidly produced, meaning those already vaccinated with two doses still need to be closely monitored," he said.

On Wednesday, Calvin Q Trinh, a Vietnamese national in the U.S., landed at Tan Son Nhat International Airport in HCMC on a repatriation flight along with nearly 300 other Vietnamese. Prior to returning to Vietnam, he had received two Covid-19 vaccine shots provided by Moderna in the U.S., evidenced by a "Covid-19 Vaccination Record Card" in his passport.

However, he is still being quarantined for 14 days as per Health Ministry protocol.

Vietnam closed its borders in March last year and banned entry of foreign nationals except for those with diplomatic or official passports or are high-skilled workers.

Earlier, experts proposed Vietnam’s tourism industry to prepare to welcome vaccinated foreign visitors back to the country from the third quarter in a similar vein to regional peers. But officials said they would need more time as the opening of international markets could meet with difficulties and public opposition for fears of further Covid-19 outbreaks.