For studies examining risk factors of sexually transmitted infections (STIs), confounding can stem from the characteristics of the partners of study subjects, and persist after adjustment for the subjects' individual-level characteristics. Two conditions can produce confounding by the subjects' partners: C1) partner choice is assortative by the risk factor examined and, C2) sexual activity is associated with the risk factor. We term this bias the assortativity bias. The objective of this paper is to illustrate the potential impact of the assortativity bias in studies examining STI risk factors, using smoking and Human papillomavirus (HPV) as an example. We developed an HPV transmission-dynamic mathematical model in which we nested a crosssectional study assessing the smoking-HPV association. In our base-case, we assumed 1) no effect of smoking on HPV, and 2) conditions C1-2 hold for smoking (based on empirical data).The assortativity bias caused an overestimation of the odds ratio (OR) in the simulated study after perfect adjustment for the subjects' individual-level characteristics (adjusted OR=1.51 instead of 1.00). The bias was amplified by a lower basic reproductive number (R 0 ), greater mixing assortativity and greater sexual activity among smokers. To mitigate the assortativity bias studies should adjust for the characteristics of study partners.
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Word count=200Infectious diseases are transmissible, which implies that the contact network of an individual is a crucial determinant of his risk of infection (1). Thus, an individual's risk of infection not only depends on individual-level risk factors, such as his age and gender, but on network-level risk factors. A classic example of this particularity of infectious diseases is herd immunity: vaccinating a portion of the population reduces the chance of non-vaccinated individuals being exposed to the infectious agent (2). Hence, an individual's risk of infection following the introduction of vaccination depends on his/her vaccine status (individual-level risk factor),