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Introduction: Ethiopia endorsed the African regional measles elimination goal in 2012 and has been implementing measles elimination strategies. Administration of measles vaccine before the age of nine months decreases seroconversion. Ensuring administration of valid doses and monitoring vaccine effectiveness is crucial for achieving measles elimination. The objective of the study was to describe the magnitude of invalid measles dose administration and vaccine effectiveness in Ethiopia. Methods: we analysed the 2016 Ethiopian Demographic and Health Survey (EDHS) immunization coverage data for Ethiopia to determine the age at measles vaccine administration and proportion of measles age invalid doses administered. The national measles surveillance data for children with birthdates that match 12-23 months old children surveyed in the EDHS 2016, were analysed to determine the Proportion of Cases Vaccinated (PCV) with one dose of measles vaccine. We estimated the effectiveness of measles vaccine by using the proportion of measles cases vaccinated (PCV) from measles surveillance data and the measles vaccination coverage among children aged 12-23 months reported in the demographic health survey (DHS) done in 2016 (Percent of Population Vaccinated for measles, PPV). The screening method was used to estimate measles vaccine effectiveness at national level and for regions which reported more than 30 measles cases among children 9-23 months of age in the 2013-2015 period. The correlation between the median age of invalid doses administered, proportion of invalid doses and measles vaccine effectiveness was analysed. Results: at national level, the proportion of invalid measles dose administration was 27.6% for children aged 12-35 months surveyed in the 2016 DHS survey in Ethiopia. Among children reported in the measles case-based surveillance database with birthdates that match the children surveyed in the Ethiopian DHS 2016, the proportion of measles cases vaccinated with a single dose of measles vaccine in the 2013-2015 period was 22.7%. The vaccine effectiveness for single dose measles vaccination was estimated at 75.3%. The measles vaccine effectiveness was low for regions with high proportion of invalid dose administration and lower median age of invalid dose administration. The median age of measles dose administered before the age of nine months was significantly correlated with measles vaccine effectiveness (r=0.971, p=0.001) in the respective regions. Conclusion: the proportion of invalid measles dose administration is very high in Ethiopia and is associated with lower vaccine effectiveness. Further assessment should be carried out to understand the underlying root causes for invalid dose administration, focusing on areas with high proportion of invalid measles doses. The national program should devise strategies to promote timely vaccination as per the national schedule, and to revaccinate those vaccinated before 9 months of age. The ministry of health should also strengthen the platform for immunization in the 2nd year life, to ensure high routine immunization coverage with two doses of measles vaccine to achieve the measles elimination goal in Ethiopia. [ABSTRACT FROM AUTHOR]