Knowledge of the protection afforded by vaccines might, in some circumstances, modify a vaccinated individual's behaviour, potentially increasing exposure to pathogens and hindering effectiveness. Although vaccine studies typically do not explicitly account for this possibility in their analyses, we argue that natural direct effects might represent appropriate causal estimands when an objective is to quantify the effect of vaccination on disease while blocking its influence on behaviour. There are, however, complications of a practical nature for the estimation of natural direct effects in this context. Here, we discuss some of these issues, including exposure-outcome and mediator-outcome confounding by healthcare seeking behaviour, and possible approaches to facilitate estimates of these effects. This work highlights the importance of data collection on behaviour, of assessing whether vaccination induces riskier behaviour, and of understanding the potential effects of interventions on vaccination that could turn off vaccine's influence on behaviour.

翻译：在某些情况下，对疫苗保护效力的认知可能会改变接种者的行为，从而增加其接触病原体的风险并降低疫苗的实际效果。尽管疫苗研究通常未在分析中明确考虑这种可能性，但我们认为，当研究目标是在阻断疫苗对行为影响的条件下量化接种对疾病的效应时，自然直接效应可能构成恰当的因果估计量。然而，在此背景下估计自然直接效应存在实际层面的复杂性。本文讨论了其中部分问题，包括就医行为导致的暴露-结局与中介-结局混杂，以及促进这些效应估计的可能方法。本研究强调了收集行为数据、评估疫苗接种是否诱发风险行为，以及理解可能消除疫苗对行为影响的接种干预措施之潜在效应的重要性。