On-Campus Dormitories as Viral Transmission Sinks: Phylodynamic Insights into Student Housing Networks During the COVID-19 Pandemic

University student housing environments are often viewed as hotspots for infectious disease transmission due to their high-density living conditions and high frequency of interpersonal interactions. During the COVID-19 pandemic, concerns arose that on-campus dormitories could serve as amplifiers of viral spread, seeding outbreaks into surrounding off-campus student residences. However, whether on-campus housing acts as a primary driver of transmission or as a recipient of infections introduced from the broader off-campus community remains unresolved. Here, we analyzed 1,431 SARS-CoV-2 genomes collected from students residing on and off campus at the University of North Carolina at Charlotte (UNCC) between September 2020 and May 2022. Using Bayesian phylodynamic and ancestral state reconstruction approaches, we traced viral transmission pathways to determine the directionality of spread between residential settings. Our results indicate that transmission from off-campus housing consistently seeded on-campus dormitory outbreaks. In contrast, viral movement from on-campus to off-campus housing was minimal. These patterns persisted across all major pandemic waves, regardless of shifting mitigation strategies, and suggest that on-campus residences acted as transmission sinks rather than sources of broader student outbreaks. These findings raise the possibility that on-campus residences may be more vulnerable than often considered, functioning as epidemiological islands that primarily receive infections from off-campus sources. Author SummaryUniversity dormitories are often considered high-risk environments for disease spread due to their crowded living conditions and the inter-connectivity of individuals. This risk has led to the perception that university student housing could act as a major driver of outbreaks, raising concerns that on-campus dormitories fuel transmission into surrounding off-campus residences and the broader community. However, the actual transmission dynamics between on and off campus communities are often unclear. To address this, we analyzed over 1,400 SARS-CoV-2 genomes from students living on and off campus at the University of North Carolina at Charlotte between September 2020 and May 2022. Using Bayesian phylodynamic and ancestral state reconstruction approaches, we mapped viral transmission pathways and found that infections consistently flowed from off-campus residences into on-campus dormitories--not the other way around. This pattern persisted across multiple waves of the pandemic, regardless of changing mitigation strategies such as masking, testing, and vaccination policies. Our findings suggest on-campus housing functioned more like transmission "islands"--receiving infections from off-campus networks but rarely exporting them. These results suggest that universities should shift their focus toward managing transmission in off-campus student communities rather than concentrating mitigation efforts solely on dormitory settings. By reframing on-campus housing as a transmission sink rather than a source, our study provides critical insights for improving future outbreak and pandemic response strategies in university environments.