Living in Ho Chi Minh City’s District 10, a woman named Thanh Hoa said her parents, both in their 70s, have been asked by ward authorities to receive their second Covid-19 vaccine jabs five weeks after the first.

Though their first shots were Moderna, their ward announced the second shots would be Pfizer on Monday.

A week ago, Hoa reached out to ask when her parents would have their second jabs as they had been vaccinated with the Moderna four weeks previously, but was told "to wait because there’s no supply for now."

Eventually, Hoa’s parents had decided to get the Pfizer.

Both Moderna and Pfizer vaccines are developed using mRNA technology, which teaches human cells how to make a protein that triggers an immune response.

An, who lives in Thu Duc City, said her family had also received the same offer.

She consulted a relative who is a doctor working in France. "When that doctor told us that in Europe people have been mixing Pfizer and Moderna and that it’s ok, we decided to follow suit."

In another case, a Covid-19 doctor in HCMC had been notified by ward authorities in District 10 to get Pfizer for his mother, 85, after she had her first Moderna shot on Aug. 2.

The city’s Department of Health has yet to make any official statement on mixing the two vaccines.

But since last week, many people have expressed worries as it had already been the due date for either them or their parents to get their second shot, yet no one had informed them of anything.

As recommended, the two doses of Moderna should be administered four weeks apart.

Duong Thi Hong, deputy director of the National Institute of Hygiene and Epidemiology, said the Ministry of Health had previously instructed localities to reserve half of the allocated Moderna vaccine for the second dose because Vietnam has no deals to purchase more of this vaccine and has to depend on aid.

For now, the ministry had already allocated all the vaccines Vietnam has received and there is now no Monderna left in stock, Hong said.

"Many localities, meanwhile, had boosted the progress to cover as many people with the first vaccine dose as possible and therefore, have now run out of vaccines to administer the second dose. They will all have to wait until there are sources of vaccine aid," she told VnExpress.

Vietnam has received five million Moderna doses donated by the U.S., and more than 40,000 doses gifted by the Czech Republic, with all distributed to localities for their mass vaccination programs.

The Ministry of Health said Tuesday it is discussing mixing other vaccines with Moderna and would soon offer guidance.

Epidemiologist Truong Huu Khanh, a consultant with the infectious and neurological diseases department at the city's Children's Hospital No. 1, said many countries have allowed mixing of Moderna and Pfizer.

Nguyen Hien Minh, deputy head of the vaccination unit at the University Medical Center HCMC, said for now, the World Health Organization and the U.S. Centers for Disease Control and Prevention have not gathered enough research data on the safety and efficacy of the vaccine when replacing Moderna with Pfizer at the second administration.

They both recommended people get the same vaccine for both doses and wait if they could not get the same the second time round.

However, Canada and the U.K. have officially recommended mixing the two vaccines and either of them could be first or second.

To date, these countries have not recorded any serious adverse effects related to the Moderna-Pfizer mix.