Presentation (CTI)

OBJECTIVES: Susceptible-Exposed-Infectious-Recovered (SEIR) models are commonly used to model how infection diseases such as measles propagate through a population. Using historical data to parameterise and validate an SEIR model, this study aims to accurately simulate the outbreak of measles in a paediatric population in an isolated town, to subsequently assess the impact that varied levels of vaccination coverage have on disease propagation.
METHODS: Using the Hagelloch dataset, a well-studied historic dataset detailing a measles outbreak in an unvaccinated paediatric population in Hagelloch, Germany in 1861, varying stochastic SEIR models were tested for goodness-of-fit, with the best model differentiating infection risk by school group (preschool, primary, secondary). The model then applied a risk reduction relative to the MMR vaccine efficacy to varying proportions of the cohort to simulate disease propagation with this altered cohort. Infection rates, both total and disaggregated by vaccinated/non-vaccinated, were recorded as well as the amplitude of the peak infections and trajectory of the major breakout.
RESULTS: In the original case with no vaccinations, the entire cohort were infected by roughly five weeks after the initial case. With increasing vaccination coverage, greater proportions of the cohort avoided infection entirely, with 17% of the cohort avoiding infection at 25% coverage, and 38% avoiding infection at 50% coverage. By 90% coverage, more people avoided infection (91%) than were vaccinated, indicating mild herd immunity. With no coverage, peak simultaneous infections hit 49% of the population, with 32% and 16% for the 25% and 50% coverage respectively. By 90% there was no notable peak infection point, with simultaneous infections never greater than 3%.
CONCLUSIONS: The outputs of the model indicate that whilst herd immunity is possible, it requires significantly high rates of coverage, likely greater than 90%. Conversely, lower coverage rates increase infection risk to the vaccinated cohort, leading to heightened infection rates in this population.