Effectiveness of Previous Mumps Vaccination during the 2014-2015 Outbreak in Lebanon

Abstract

Introduction: In Lebanon, MMR was introduced at 12 months and 4-5 years in 1996. In 2014, the 2nd MMR dose was shifted to 18 months. Despite a national MMR coverage estimated at 79%, a six-fold increase in mumps national incidence was observed in December 2014. The objective of this study is to determine mumps vaccine effectiveness (VE) among Lebanese population to guide immunization policies. Methods: Clinical and confirmed mumps cases reported to the Epidemiological Surveillance Program between 2014W46 and 2015W11 were eligible if they were Lebanese and aged 1.5 to 19 years-old. Controls were randomly selected controls using phonebook of the same area and matched 1:1 on age and locality. Information was collected by structured phone interviews. Mumps vaccination status was based on documented valid dates for MMR doses. Data were entered using Epidata 3 and analyzed using Stata13. VE ((1-OR) × 100) of one and two doses and ORs (95%CI) for acquiring mumps were estimated using conditional logistic regression. Results: 91 cases and 91 controls were included. Only 36% of cases had vaccination cards, compared to 71% of controls (p<0.001). 94% of cases were not vaccinated compared to 51% of controls (p<0.001). Vaccine effectiveness was estimated 60% (CI=-27%: 88%) for one dose and 88% (CI=60: 96%) for 2 doses. Conclusion: Two-doses of MMR vaccine were estimated as 88% effective against mumps, similar to results found in the literature. This outbreak can be explained by suboptimal MMR2 coverage. Efforts should focus on achieving high MMR coverage and raising population's awareness about preserving documentation of vaccination.