Vaccine serocoverage under the expanded program on immunization among hill tribe children in Thailand: A cross-sectional study.

Background: Expanded programs on immunization (EPIs) are country-specific vaccine programs designed and implemented to prevent childhood diseases globally, including in Thailand. Hill tribe children in Thailand live in remote areas with underdeveloped education systems and low economic status. This study aimed to assess serocoverage under the EPI and access to vaccination clinics.

Methods: A cross-sectional study was performed to assess serocoverage after childhood vaccination among hill tribe children who lived in 34 selected villages in Chiang Rai Province, Thailand. A validated questionnaire was administered, and 3-mL blood specimens were collected. Antibodies against hepatitis B surface antigen (anti-HBs), hepatitis B core antigen (anti-HBc), measles, Japanese encephalitis virus (JEV), and tetanus were detected. Chi-square tests were performed to detect the different proportion of patients with antibodies with different characteristics.

Results: Half of the hill tribe children aged 1-18 years did not have medical evidence (logbook) of immunization. More than 98.0% of the children who had medical evidence received the recommended immunizations. Only half of the children had anti-HBs (51.1%), and 22.3% had antibodies against JEV. The majority were found to be positive for antibodies against measles (83.3%) and tetanus (91.4%). Sex (p-value = 0.028), tribe (p-value < 0.001), age (p-value < 0.001), and parents' monthly income (p-value = 0.008) were associated with a lack of medical evidence.

Conclusions: Existing immunization programs for hill tribe children in Thailand should be urgently evaluated and monitored for effectiveness.