This paper argues that mandatory, government-enforced vaccination can be justified even within a libertarian political framework. If so, this implies that the case for mandatory vaccination is very strong indeed as it can be justified even within a framework that, at first glance, loads the philosophical dice against that conclusion. I argue that people who refuse vaccinations violate the ‘clean hands principle’, a (in this case, enforceable) moral principle that prohibits people from participating in the collective imposition of unjust harm or risk of harm. In a libertarian framework, individuals may be forced to accept certain vaccines not because they have an enforceable duty to serve the common, and not because cost-benefit analysis recommends it, but because anti-vaxxers are wrongfully imposing undue harm upon others.