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RATIONALE, AIMS, AND OBJECTIVES Under conditions of vaccine scarcity, the socially optimal dosing (SOD) strategy administers a lower dose of vaccine to a larger number of people than the individually optimal dosing (IOD) strategy, which administers a higher dose of vaccine to a smaller number of people. In the context of vaccines that generate diminishing returns of effectiveness with each additional dose beyond the first, SOD therefore generates a greater total amount of vaccine-induced protection than IOD and, as such, constitutes the socially optimal strategy. While the clinical and public health arguments in favour of SOD have previously been outlined, this article conducts an ethical analysis of SOD for scarce vaccines through the ethical framework of principlism. METHODS SOD is examined with regard to each principle within the principlism framework-nonmaleficence, beneficence, autonomy, and justice. RESULTS SOD is found to satisfy each of the ethical requirements under examination. Regarding nonmaleficence, SOD induces less iatrogenic harm than IOD since the dose of vaccine administered to each individual is lower in the former than the latter. Furthermore, both the good and bad effects of SOD are foreseen while only the good effects are intended, meaning this strategy simultaneously satisfies the doctrine of double effect. Regarding autonomy, SOD makes vaccine-induced protection available to a greater number of individuals who wish to receive it, thereby respecting their capacity for self-determination and to make independent decisions. Regarding beneficence, SOD renders the good-namely the protection of health-more widely available to the individuals that constitute the population in question. Finally, SOD promotes theories of justice that treat individuals equally and is unlikely to reduce the effectiveness of other distribution policies that allocate scarce vaccines in a just manner. CONCLUSION In conditions of vaccine scarcity, SOD favourably satisfies the ethical framework of principlism.