In his paper ‘A libertarian case for mandatory vaccination’, Jason Brennan argues that even libertarians, who are very averse to coercive measures, should support mandatory vaccination to combat the harmful disease outbreaks that can be caused by non-vaccination. He argues that libertarians should accept the clean hands principle, which would justify mandatory vaccination. The principle states that there is a (sometimes enforceable) moral obligation not to participate in collectively harmful activities. Once libertarians accept the principle, they will be compelled to (...) support mandatory vaccination. In my paper, I argue that the cases Brennan uses to justify this principle are disanalogous to the case of non-vaccination and that they are not compelling to libertarians. The cases Brennan offers can be explained by a libertarian using the individual sufficiency principle: which states that if an individual’s action is sufficient to cause harm, then there is a (sometimes enforceable) moral obligation not to carry out that action. I argue that this principle is more appropriate to Brennan’s examples, and more appealing to the libertarian, than the clean hands principle. In order to get libertarians to accept the clean hands principle, I present a modified version of one of Brennan’s cases that is analogous to the case of non-vaccination. Using this case, I argue that whether the clean hands principle will justify mandatory vaccination is dependent on whether the herd immunity rate in a given population is approaching a threshold after which a collective risk of harm will be imposed onto others. (shrink)