Understanding the Intersection of Behavioral Risk and Social Determinants of Health and the Impact on an Outbreak of Human Immunodeficiency Virus Among Persons Who Inject Drugs in Philadelphia

Background. In 2018, Philadelphia identified an outbreak of new human immunodeficiency virus (HIV) infections among persons who inject drugs (PWID). Although conventional HIV surveillance systems capture individual-level behavioral risk, they are not able to capture the social and environmental factors contributing to rapid transmission. Methods. HIV surveillance data were used to assess demographic, clinical, and behavioral factors for PWID with HIV diagnosed during 2017 and 2018. Social factors such as homelessness, disruption of encampments, and trends in sexual behaviors, drug use and syringe availability among PWID were captured through National HIV Behavioral Surveillance, routine hepatitis and sexually transmitted infection surveillance, and shelter and homeless outreach data. Results. In 2018, there were 71 new infections among PWID, an increase of 115% since 2016. During this time, opioid overdose deaths peaked at 59 deaths per 100 000 persons, 85% of which involved the use of fentanyl. While overall reported homelessness increased, rates of those living unsheltered rose by 13%. The Philadelphia Department of Public Health identified increased injection frequency, encampment closures, and lack of syringe access as promoters of continued HIV transmission. Conclusion. The use of conventional surveillance methods only is inadequate for determining HIV risk during outbreaks. Incorporation of individual and aggregate level data on social and environmental determinants is necessary to develop effective outbreak response interventions.