Evaluating causal effects in the presence of interference is challenging in network-based studies of hard-to-reach populations. Like many such populations, people who inject drugs (PWID) are embedded in social networks and often exert influence on others in their network. In our setting, the study design is observational with a non-randomized network-based HIV prevention intervention. Information is available on each participant and their connections that confer possible HIV risk through injection and sexual behaviors. We considered two inverse probability weighted (IPW) estimators to quantify the population-level effects of non-randomized interventions on subsequent health outcomes. We demonstrated that these two IPW estimators are consistent, asymptotically normal, and derived a closed-form estimator for the asymptotic variance, while allowing for overlapping interference sets (groups of individuals in which the interference is assumed possible). A simulation study was conducted to evaluate the finite-sample performance of the estimators. We analyzed data from the Transmission Reduction Intervention Project, which ascertained a network of PWID and their contacts in Athens, Greece, from 2013 to 2015. We evaluated the effects of community alerts on HIV risk behavior in this observed network, where the links between participants were defined by using substances or having unprotected sex together. In the study, community alerts were distributed to inform people of recent HIV infections among individuals in close proximity in the observed network. The estimates of the risk differences for spillover using either IPW estimator demonstrated a protective effect. The results suggest that HIV risk behavior can be mitigated by exposure to a community alert when an increased risk of HIV is detected in the network.

翻译：与许多此类人群一样,注射毒品者(PWID)被嵌入社交网络,并经常影响其网络中的其他人。在我们所处的环境中,研究设计是非随机网络式的艾滋病毒预防干预措施的观测设计,同时允许发生重叠干扰(假设有可能发生干扰的人群),进行了模拟研究,以评价估算者的有限性IPSP性表现。我们分析了来自“传播减少干预项目”的数据,该项目确定了PWID网络及其在雅典、希腊的近距离接触,使用了最新的艾滋病毒抗逆转录病毒风险网络。我们评估了艾滋病毒抗逆转录病毒感染者在2013年至2015年之间观察到的网络联系。我们评估了艾滋病毒抗逆转录病毒感染者在2013年至2015年之间观察到的网络风险。