Predicting Factors for Malaria Reintroduction and A Multi-factorial Approach to Prevent Malaria Outbreaks in the Malaria-free Areas

Thailand’s National Malaria Elimination Strategy, 2017–2026, aims to reach zero indigenous transmission by 2024. During 2016–2021, Thailand had successfully reduced its malaria burden by more than 80%. However, a resurgence of malaria in 2022 saw an increase in the incidence in 33 provinces. To identify the predictors of malaria epidemic re-occurring in malaria-free areas, secondary data of malaria-reintroduced villages in 2022 were obtained from the Malaria Information System. A descriptive cross-sectional study was conducted to compare characteristics, prevention, and response measures between villages with and without sustained local transmission after malaria reintroduction. A retrospective cohort study was conducted to determine the associations between sustained local transmission and potential predictors. Among the transmission foci in 2022, 336 villages had previously been malaria-free, of which 73 (21.7%) reported sustained local transmission. A multi-level logistic regression model, considering villages clustered within provinces, found that villages located in a district which contained active foci (adjusted odds ratio (AOR) 1.03, 95% confidence interval (CI) 1.01–1.05) and having a higher proportion of non-Thai cases (AOR 12.3, 95% CI 5.69–26.6) were significantly associated with sustained local transmission whereas coverage of malaria control within 7 days was protective (AOR 0.20, 95% CI 0.09–0.44). Areas with high migrant populations were associated with a higher risk of malaria reintroduction. Proactive case search should target these populations to quickly detect reintroduced cases and conduct timely control to prevent further local transmission.