In a recent paper in this journal, Jason Brennan correctly notes that libertarians struggle to justify a policy of compulsory vaccination. The most straightforward argument that justifies compulsory vaccination is that such a policy promotes welfare. But libertarians cannot make this argument because they claim that the state is justified only in protecting negative rights, not in promoting welfare. I consider two representative libertarian attempts to justify compulsory vaccination, and I argue that such arguments are unsuccessful. They either fail to show that the state is justified in implementing the policy or overgeneralise. I suggest that Brennan's solution is especially well motivated insofar as it addresses the shortcomings of these arguments. Brennan argues that we violate the rights of others by participating in an activity that imposes an unacceptable collective risk of harm. Going unvaccinated is an activity that imposes an unacceptable collective risk of harm, and thus amounts to a rights violation. So, the state can implement a policy of compulsory vaccination I object, however, that Brennan's delineation of acceptable and unacceptable risk implicitly rests on classical liberal rather than libertarian principles; he justifies compulsory vaccination on the grounds that it promotes welfare. I also object that Brennan's argument would entail significant departures from libertarian institutional arrangements. This leaves libertarians with a choice: they can develop new arguments to demonstrate that their position is compatible with compulsory vaccination, or they can accept that their view entails the impermissibility of compulsory vaccination, and argue that this is not an unpalatable implication of their view.