At least one of the persons experienced anaphylaxis at the Vietnam Military Medical University in Hanoi on June 12, according to the research team running the trials. The person has since recovered and been discharged from the hospital, the team said.

Nanocovax, produced by Nanogen, is currently the most promising candidate among four domestic Covid-19 vaccines being developed by Vietnam. The others are Covivac by the Institute of Vaccines and Medical Biologicals (IVAC) and two other vaccines by the Vaccine and Biological Production Company No. 1 (Vabiotech) and the Center for Research and Production of Vaccines and Biologicals.

The third phase of Nanocovax's human trials would require around 13,000 volunteers split into two groups: one injected with the 25 mcg doses and the other receiving placebos.

Throughout the trials, Nanocovax has proven to be mostly safe, having minor side effects such as fever and pain at the site of injection.

Once the trials are completed, Nanocovax could be approved for emergency use immediately. Production could begin as soon as the vaccine is approved, at 20-30 million doses a year, and more could be produced with greater investments.

Vietnam has inoculated over 1.5 million people with the Covid-19 vaccine produced by British-Swedish firm AstraZeneca starting March. The country aims to secure 150 million vaccine doses to cover 70 percent of its population.