Peste des petits ruminants (PPR) is an acute infectious disease of small ruminants affecting flocks throughout large parts of Africa, Asia and the Middle East. The Global Strategy for Control and Eradication (GSCE), targeting global eradication by 2030, recommends vaccination of small ruminants in endemic regions to attain 70% post-vaccination immunity. We develop a deterministic, age-and sex-structured, demographic model of flock dynamics and conduct a global sensitivity analysis to identify the most important demographic parameters influencing post-vaccination immunity in pastoral small ruminant populations in sub-Saharan Africa. The model is parameterised using empirical demographic data extracted from the literature to explore post-vaccination immunity levels in different flock profiles. We show that under the GSCE recommendation, immunity decays below 70% within 12 months for all profiles. We highlight the importance of tailoring vaccination to local demographic contexts and the need for widely available, standardised demographic data to characterise small ruminant populations.