By 4 a.m., hundreds had already lined up in front of the Thua Thien-Hue Center for Disease Control in Phu Nhuan District of the namesake central province. The center had imported 1,100 doses in total, 1,000 of a 6-in-1 vaccine and 100 of a 5-in-1 vaccine.

"I’ve been in front of the center’s gate since 4:30 a.m., but many were already there by then," said Phu Van Binh, who went to register vaccine shots for his seven-year-old daughter.

At 6 a.m., when the center opened its gates, many resorted to climbing the fences to get in, to get past the long lines.

However, vaccine registrations ended one and a half hours later, with the center announcing that all 1,100 doses have been registered and the rest should wait for the next batch of vaccines.

The number of people signing up for on-demand 6-in-1 vaccination, as opposed to government-subsidized 5-in-1 shots, has markedly increased since earlier this year, said Nguyen Dinh Son, vice director of the center.

In previous years, the number of people who signed up for on-demand 6-in-1 vaccination in Thua Thien-Hue only accounted for 5 percent of all vaccinations in the province, with most of the rest opting for government-subsidized vaccines instead.

"We prepared 1,100 on-demand vaccine doses but they were not enough for the people of Thua Thien-Hue and neighboring provinces. We allow registrations today, but not until next week will actual vaccinations take place. We plan to import 1,300 more vaccine doses in June," Son added.

Vaccine shortage however, especially on-demand ones, is a recurring problem that the Ministry of Health has yet to deal with properly, said Nguyen Tien Hong, deputy director of Da Nang’s Department of Health.

In March, pandemonium also ensued at the Da Nang Center for Disease Control in the nearby city when parents tried to secure numbered tokens for 6-in-1 vaccinations for their kids. As it happened in Hue, in just hours from 6 a.m., all vaccine shots available ran out.

Vaccine scares

Some people have claimed that government-subsidized vaccines cause more side-effects on recipients than on-demand ones, said Pham Van Lao, director of the Center for Preventive Medicine of the Central Highlands’ province of Dak Lak, Nhan Dan newspaper reported.

"I’ve heard about how the 5-in-1 vaccine ComBE Five, part of the government-subsidized vaccines, caused several problems... So I wanted to give my child on-demand 6-in-1 vaccines instead," said Nguyen Thi Huong, who lives in Dak Lak’s Cu Kuin District.

The 5-in-1 vaccine ComBE Five, developed by India, has been chosen as a government-subsidized vaccine by the health ministry since last December to replace the South Korea-produced Quinvaxem. The 5-in-1 vaccine protects against diphtheria, whooping cough, tetanus, hepatitis B and Haemophilus influenzae type B. Five million shots are given to about 1.7 Vietnamese children under one-year-old every year.

Quinvaxem, however, became infamous after more than 10 babies died between November 2012 and May 2013 after being injected, and several more also died when the vaccine was reintroduced in October 2013. However, the health ministry has said that the vaccine was not to blame for the deaths.

In January, a two-month-old baby in Hanoi also died a day after being injected with ComBE Five. It was said later that her death was caused by anaphylaxis.

ComBE Five is a DPT vaccine, which belongs to a class of combination vaccines against diphtheria, whooping cough and tetanus. It contains diphtheria and tetanus toxoids, along with killed whole cells of Bordetella pertussis, the bacteria that causes whooping cough. It is the cellular nature of the whooping cough bacterium that triggers more side effects in children injected with ComBE Five, said Nguyen Xuan Tung, head of the Vaccination Management and Biosafety Department of the Preventative Medicine Division.

The 6-in-1vaccine shot, for protection against diphtheria, whooping cough, tetanus, hepatitis B, Haemophilus influenzae type B and polio, uses purified antigens in its whooping cough component which are considered to be safer.

However, the chances for side-effects to appear are slim. Out of a million vaccine doses, only 20 give recipients severe reactions like high fever or convulsions, according to the World Health Organization.

Parents should carefully watch their children for at least 24 hours after a vaccination to watch out for possible side effects, doctors have recommended.