Vaccination programmes against infectious diseases aim to protect individuals from serious illness but also offer collective protection once a sufficient number of people have been immunized. This so-called 'herd immunity' is important for individuals who, for health reasons, cannot be immunized or who respond less well to vaccines. For these individuals, it is pivotal that others establish group protection. However, herd immunity can be compromised when people deliberately decide not to be immunized and benefit from the herd's protection. These agents are often referred to as free riders: their omissions are deemed to be unfair to those who do contribute to the collective's health. This article addresses the unfairness of such 'free riding'. An argument by Garett Cullity is examined, which asserts that the unfairness of moral free riding lies neither in one's intentions, nor in one's reluctance to embrace a public good. This argument offers a strong basis for justifiably arguing that free riding is unfair. However, it is then argued that other considerations also need to be taken into account before simply holding free riding against non-compliers. © The Author 2012. Published by Oxford University Press.