Understanding the transmission dynamics of a large-scale measles outbreak in Southern Vietnam

Objectives: Southern Vietnam experienced a large measles outbreak of over 26,000 cases during 2018-2020. We aimed to understand and quantify the measles spread in space-time dependence and the transmissibility during the outbreak.

Methods: Measles surveillance reported cases between January 2018 and June 2020, vaccination coverage, and population data at provinicial level were used. To illustrate the spatio-temporal pattern of disease spread, we employed the endemic-epidemic multivariate time series model decomposing measles risk additively into autoregressive, spatio-temporal, and endemic components. Likelihood-based estimation procedures were performed to determine the time-varying reproductive number R_e of measles.

Results: Our analysis showed that the incidence of measles was associated with vaccination coverage heterogeneity and spatial interaction between provincial units. The risk of infections was dominated by between-province transmission (36.1% to 78.8%), followed by local endogenous transmission (4.1% to 61.5%). In contrast, the endemic behavior had a relatively small contribution (2.4% to 33.4%) across provinces. In the exponential phase of the epidemic, R_e was above the threshold with a maximum value of 2.34 (95% CI: 2.20-2.46).

Conclusion: Local vaccination coverage and human mobility are important factors contributing to the measles dynamics in Southern Vietnam, and the high risk of inter-provincial transmission is of most concern. Strengthening the disease surveillance is recommended, and further research is essential to understand the relative contribution of population immunity and control measures in measles epidemics.