An Investigation of Pertussis Outbreak in a Low Vaccination Coverage Area, Mae Lan District, Pattani Province, Thailand, September–December 2023

On 26 Oct 2023, a pertussis cluster was reported in Muang Tia Subdistrict. We investigated to identify the source, determine contributing factors, and estimate diphtheria-tetanus-pertussis (DTP) vaccine coverage. We reviewed medical records and performed active case findings in three villages. Suspected cases included close contacts with upper respiratory tract infection (URI) symptoms, residents with a week-long cough and a typical pertussis symptom, or those in URI clusters. Confirmed cases were individuals with positive Bordetella Pertussis result by polymerase chain reaction. Age-appropriate DTP coverage was estimated. Post-hoc analysis on sensitivity of national (two-week cough with a typical symptom) and modified (any cough duration with a typical symptom or close contact with a URI symptom) pertussis definition was conducted. We found eleven confirmed and four suspected cases (attack rate 0.6%; 15/2,394) with one hospitalized with pneumonia. Cases’ age ranged from 11 months to 53 years old (median 5 years). The attack rate among children aged 0–1 year was 2.9%. Thirty-six percent of confirmed cases had a cough lasting at least two weeks. The national definition had a sensitivity of 9.1% (95% CI 0.2%–41.3%), while the modified definition’s sensitivity was 81.8% (95% CI 48.2%–97.7%). DTP coverage in children aged 0–1 year was 25.0% (95% CI: 10.4%–39.6%). Pertussis positivity among household and community contacts was 40.0% and 50.0%, respectively. This pertussis outbreak was driven by low vaccine coverage and community transmission. Enhancing vaccine coverage to 90% and using modified definitions are recommended for outbreak control in low-vaccination areas.