🇬🇧 Policy Relevant Effects in Infectious Disease Studies

Abstract

When facing infectious diseases, healthcare professionals must make decisions under uncertainty. Certain considerations are frequently recurring in this process. Knowing whether a vaccine’s effect wanes is important for determining the timing of seasonal flu shots or assigning COVID-19 boosters. Understanding whether a vaccine has heterogeneous effects on different subtypes, such as Malaria variants, has implications for the development of new treatments. Whether vaccinating a subgroup of individuals is sufficient to reach herd immunity, due to spill-over effects, shapes public health policies. Although the previous plain English sentences might seem clear, I will argue that terms like “waning”, “heterogeneous effects”, and “spill-over” are ambiguously interpreted. I will then propose ways to clarify these terms based on theory and methods for causal inference. Using Malaria, HIV, influenza, and COVID-19 as illustrative examples, I will show that these clarifications matter in practice. In particular, I will leverage the clarifications to define new treatment effects in vaccine settings, which sometimes justify existing analytic approaches and also motivate new methods that are feasible to implement.