Students from a school in the south-central province of Phu Yen have been struck by inexplicable bouts of fainting spells during the past two months.
Doctors suspect they suffer from a psychological disorder.
Chau Lot, principal of the Son Thanh Secondary and High School in Tay Hoa District, said the phenomenon first appeared in a handful of students and eventually spread.
All of the cases occurred among the seventh-grade, eighth-grade, and eleventh-grade classes, which ran in the afternoon, he said.
Since January 17, between 12 and 40 students faint every day at school, mainly between 1 p.m. and 3 p.m., local news website VnExpress reported.
The school management has moved their classes to the morning and even allowed the students to remain at home. Despite their efforts, the problem continued.
The students' parents say their children are healthy and do not suffer from fainting spells while at home.
According to the school, the students all suffered from respiratory difficulties, fatigue, and convulsions. They are given to fits of screaming before they collapse.
Dr. Nguyen Hung, director of Phu Yen's Preventive Health Center, told the news website that the students recovered after massages, adding that the students were suffering from symptoms of mass hysteria.
Mass hysteria is caused by either a physical culprit, or psychological stress and anxiety, according to an article by Professor Gary Small from the University of California. It often afflicts children and teenagers, especially girls.
Hung said that mass hysteria has struck a number of Vietnamese schools, like those in the central cities of Hue and Da Nang. The outbreaks tended to last for a shorter amount of time and manifested on a smaller scale than they have been in Phu Yen.
Hung suggested that the school provide a more relaxing study environment as well as frequent exercise. He also recommended the school provide them with consultations from professional psychologists.
According to Prof. Small, the illness usually manifests for a few days and tends to disappear when the afflicted crowd disperses.