Stackelberg game models have recently seen considerable practical and academic success in security applications, with defender as the leader, and attacker the follower. The key conceptual insight of Stackelberg security games is that defense needs to be proactive, optimally accounting for attacker’s response to a defensive posture. We propose that this insight has relevance in another important application domain: vaccination. Vaccination therapies are important tools in the battle against infectious diseases such as HIV and influenza. However, many viruses, including HIV, can rapidly escape the therapeutic effect through a sequence of mutations. We propose to design vaccines, or, equivalently, antibody sequences, that make such evasion difficult. Formally, we model the interaction between a vaccine and a virus as a Stackelberg game in which the vaccine designer chooses an antibody, and the virus chooses a minimal sequence of mutations to escape it. Our crucial observation is that...