Measles infection is of immense clinical and public health importance and has a worldwide distribution. Its burden, however, lies disproportionately in the under-fives living in resource limited settings. Vaccination has been the mainstay in the prevention and control of the disease as it helps to build herd immunity which confers protection to both the vaccinated and unvaccinated. Although the vaccine has been effective, its success has been inadequate. In Nigeria, measles vaccine failure has compounded the already existing problems of low vaccination coverage and uptake. Consequently, endemic transmission of measles and its epidemics are increasingly being reported in vaccinated under-fives. Measuring the specific neutralising Immunoglobulin G (IgG) antibodies is a reliable method of assessing herd immunity in vaccinated subjects. This study aimed at determining the seroprevalence of protective measles-specific IgG antibodies in vaccinated under-fives. It further sought to determine the relationship, if any, between the measles immunity and time since vaccination as well as the socio-demographic factors. The study was cross-sectional, descriptive and conducted at the Institute of Child Health of the Ahmadu Bello University Teaching Hospital (ABUTH), Zaria. The subjects were 139 apparently healthy children aged 12 to 59 months who had received a single dose of measles vaccine at nine months at the study centre. Ethical approval was obtained from the ethics committee of ABUTH and consent from the caregivers of each subject. Data on details of vaccination and the sociodemographics of the subjects were collected with the aid of an interviewer-administered questionnaire. Blood samples were drawn and analysed for measles-specific IgG antibodies using commercial ELISA kit obtained from IBL International (2014), Hamburg, Germany. The data were analysed using Statistical Package for Social Sciences (SPSS) version 23 (IBM USA, 2015) and the appropriate statistical tests. The findings were presented in prose, tables and charts. The mean (±SD) age of the subjects was 28.41months (±12.43), with a male to female ratio of 1.4:1. The serum levels of measles-specific IgG titre values ranged between 48 to 4777mIU/ml, with a median of 219mIU/ml. The seroprevalence of protective levels of measles-specific IgG antibodies was 56%. There was no statistically significant relationship between having protective levels of the antibodies and time categories since vaccination (p= 0.559). Subjects belonging to families with five or fewer members were more likely to have protective levels of antibodies (p = 0.002). Additionally, subjects whose fathers engage in unskilled occupation were more likely to have protective levels of the measles-specific IgG antibodies (p = 0.006). Family size and father’s occupation were independently related to having protective levels of measles-specific IgG antibodies. The seroprevalence of the protective levels of measles-specific IgG antibodies recorded in this study was below the 95% herd immunity required to interrupt measles transmission. The protective levels of measles-specific IgG antibodies did not have significant relationship with time since vaccination. Family size and father’s occupation were independent predictors of having protective levels of measles-specific IgG antibodies in the vaccinated under-fives. The researcher recommends the conduct of similar but larger multi-facility studies across Zaria for necessary interventions and further studies to investigate the reasons for the low seroprevalence of protective levels of the measles-specific IgG antibodies. In addition, booster vaccination targeting all underfives who were vaccinated at the age of 9 months should be provided.