 As with the harm principle, those in the liberty tradition are not necessarily opposed to public health measures that prohibit people from harming others. Yet we might wonder whether the liberty tradition’s premium on individual liberty constrains various public health interventions that its proponents wish to endorse. An interesting boundary case concerns whether members of the liberty tradition can endorse vaccine mandates, especially for the measles-mumps-rubella vaccine (Bernstein 2017; Brennan 2018; Flanigan 2014a; Giubilini 2019; Navin 2015). Collectively, parents who delay or refuse vaccination can impose considerable risk on others, collectively. Individually, however, each unvaccinated child does not impose much risk. Given this low risk of harm, it would seem that the most natural argument in defense of mandatory vaccination would be to justify a restriction of individual parental liberty for the sake of some sort of overall benefit. Insofar as libertarians view interference with free and equal persons for promoting overall benefit as illegitimate, however, this argument is not available for them. And self-identified libertarians outside of academia have objected to vaccine mandates as unjustified infringements on individual liberty. (For instance, in 2015, Senator Rand Paul argued against vaccine mandates because “The state doesn’t own your children. Parents own their children.”) But this is a striking conclusion, one that arguably counts against the plausibility of libertarianism rather than against the legitimacy of vaccine mandates (Brennan 2018; Bernstein 2017).