A particularly strong reason to vaccinate against transmittable diseases, based on considerations of harm, is to contribute to the realization of population-level herd immunity. We argue, however, that herd immunity alone is insufficient for deriving a strong harm-based moral obligation to vaccinate in all circumstances, since the obligation significantly weakens well above and well below the herd immunity threshold. The paper offers two additional harm-based arguments that, together with the herd immunity argument, consolidates our moral obligation. First, we argue that individuals should themselves aim not to expose others to risk of harm, and that this consideration becomes stronger the more non-vaccinated people there are, i.e., the further we are below herd immunity. Second, we elaborate on two pragmatic reasons to vaccinate beyond the realization of herd immunity, pertaining to instability of vaccination rates and population heterogeneity, and argue that vaccinating above the threshold should serve as a precautionary measure for buttressing herd immunity. We also show that considerations of harm have normative primacy in establishing this obligation over considerations of fairness. Although perfectly sound, considerations of fairness are, at worst secondary, or at best complementary to considerations of harm.