The Ham Tan District traffic police said the people were from the central Quang Tri, Ha Tinh and Nghe An provinces, and were trying to return home surreptitiously from Ho Chi Minh City, where they had been living.
They had been told to gather outside a supermarket in Dong Nai's Bien Hoa Town that borders HCMC on Sunday, and picked up in two cars.
At Long Khanh Station in Dong Nai, they were transferred to the truck, which had permission to pass through Covid checkpoints on National Highway 1.
At around 10 p.m., on reaching Binh Thuan, the truck stopped near a restaurant two kilometers away from a checkpoint in Tan Duc Commune.
Traffic police officers patrolling the highway became suspicious and ordered the driver Le Van Tuan, 39, to open the doors, and found 14 adults and a seven-year-old boy inside.
"The truck had not turned on its refrigeration," a police officer at the scene said.
Tuan said the two cars had been rented.
The 15 passengers, who all had negative coronavirus test certificates, claimed they did not know they would be transported in a refrigerated truck.
They have been quarantined in Ham Tan District, and an investigation is ongoing.
Nguyen Minh, deputy chairman of the Binh Thuan People's Committee, said the 15 people had violated coronavirus regulations, but considering that they are poor migrants who could no longer afford lives without jobs in HCMC amid prolonged lockdown, the province would take them to their hometowns.
The police fined the company owning the truck VND15 million ($661.38), and the drivers of the two cars that transported the people VND7.5 million each for violating coronavirus prevention measures.
Since April Binh Thuan has fined dozens of people and organizations for trying to transport people without complying with coronavirus restrictions.