A health worker administers a Japanese encephalitis vaccine to a young woman at a clinic in Vietnam. Photo: T.P. / Tuoi Tre
The child, a 13-year-old living in Hung Dao Commune, was diagnosed about a week ago and is currently receiving intensive treatment, according to the Hanoi Center for Disease Control.
This is the first case of Japanese encephalitis reported in Hanoi so far this year, compared with zero cases recorded during the same period in 2025.
Health authorities said the child had received the full three-dose primary vaccination series but had not received the recommended booster dose.
Doctors said Japanese encephalitis is an acute viral infection transmitted through mosquito bites.
The disease typically peaks during the summer months, when mosquito activity is highest, with children being the most vulnerable group.
Medical experts warned that the disease is particularly concerning because its early symptoms often resemble seasonal influenza or other viral infections, including high fever, headache, nausea, vomiting, and fatigue.
However, the virus can rapidly affect the central nervous system, leading to brain inflammation, cerebral edema, seizures, altered mental status, coma, and multi-organ failure.
The World Health Organization estimates that most infected individuals remain asymptomatic, while about one in every 250 infections becomes severe.
Once the central nervous system is affected, the disease can progress rapidly, potentially leading to acute encephalitis, respiratory failure, or irreversible brain damage.
The fatality rate can reach up to 30 percent, while nearly half of survivors may suffer long-term neurological complications such as epilepsy, movement disorders, memory impairment, or permanent disability.
Before nationwide vaccination programs were introduced, Vietnam reported thousands of cases annually.
Although expanded immunization efforts have significantly reduced case numbers, sporadic infections still occur, mainly among people who are unvaccinated or have missed booster doses.
Health authorities recommend that children receive three primary vaccine doses, followed by booster shots every three to four years until the age of 15 to maintain immunity.
However, many parents focus on the initial doses and overlook booster shots, creating what experts describe as an ‘immunity gap’ that leaves children vulnerable over time.
Dr. Nguyen Tuan Hai, a preventive medicine specialist, said vaccination remains the most effective measure to prevent Japanese encephalitis.
He added that full immunization helps the body build immunity against the virus, significantly reducing the risk of infection and the likelihood of severe complications or death.
In addition to vaccination, residents are advised to prevent mosquito bites by sleeping under mosquito nets, maintaining environmental hygiene, and eliminating mosquito breeding sites, particularly during the peak transmission season from May to September.
