Risk factors for persisting measles susceptibility: a case-control study among unvaccinated orthodox Protestants.

Background: Measles is an infectious disease providing lifelong immunity. Epidemics periodically occur among unvaccinated orthodox Protestants in the Netherlands. During the 2013/2014 epidemic, 17% of the reported patients was over 14 years old. Apparently, they did not catch measles during the previous 1999/2000 epidemic and remained susceptible. We wanted to identify risk factors for this so-called persisting measles susceptibility, and thus risk factors for acquiring measles at older age with increased risk of complications.

Methods: A case-control study was performed among unvaccinated orthodox Protestants born between 1988 and 1998; cases had measles in 2013/2014, controls during or before 1999/2000. Associations between demographic, geographical and religion-related determinants and persisting measles susceptibility were determined using univariate and multivariable logistic regression. Analyses were stratified in two age-groups: infants/toddlers and primary school-aged children during the 1999/2000 measles epidemic.

Results: In total, 204 cases and 563 controls were included. Risk factors for persisting measles susceptibility for infants/toddlers in 1999/2000 were belonging to a moderately conservative church, absence of older siblings and residency outside low vaccination coverage (LVC)-municipalities. Risk factors for primary school-aged children were residency outside LVC-municipalities and attendance of non-orthodox Protestant primary school.

Conclusion: Unvaccinated orthodox Protestant adolescents and adults who resided outside the LVC-municipalities, did not attend an orthodox Protestant primary school, had no older siblings and belonged to a moderately conservative church were at risk for persisting measles susceptibility and, thus, for acquiring measles at older age with increased risk of complications. For this subgroup of orthodox Protestants targeted information on vaccination is recommended.