Objective: In Slovakia, thanks to a highly effective vaccination programme, no domestic cases of measles have been reported since 1999. However, there are several outbreaks of measles currently hitting some countries in Europe. Difficulties in reaching the goal of measles elimination make it necessary to monitor the status of the population susceptibility to prevent similar outbreaks in the future. We hypothesize that immunity wanes overtime, which can substantially impact the population susceptibility. This work introduces a model that estimates a proportion of individuals susceptible to measles in the Slovak population in 2015.
Methods: Our analysis is based on an age-cohort model that incorporates waning immunity, vaccination schedule and changes in demographic structure. The inputs of the model are data on the vaccination coverage, last seroprevalence survey in 2002 and age structure of the population.
Results: In a short-term horizon, waning immunity does not affect the estimated proportion of the susceptible population. However, in a long-term horizon, the antibody titers can fall below the level of protection, which would result in a substantial transfer of initially immune individuals to the compartment of the susceptible ones. Incorporating of waning immunity in the cohort model has indicated that the most susceptible cohorts are not-vaccinated youngest children and cohorts born between 1969 and 1986.
Conclusions: Applying the model to the current situation shows that people aged 30-45 years and unvaccinated infants represent the most susceptible groups. Model partially replaces missing seroprevalence survey, but, because the parameters of model and phenomenon of waning immunity are not exactly known, we suggest reintroducing the regular national serosurveys in order to empirically determine the level of susceptibility for measles in Slovakia.
Keywords: cohort model; measles; susceptibility; vaccination; waning immunity.
Copyright© by the National Institute of Public Health, Prague 2017