Higher viral load drives infrequent SARS-CoV-2 transmission between asymptomatic residence hall roommates.

BACKGROUND: The COVID-19 pandemic spread to over 200 countries in less than six months. To understand COVID spread, determining transmission rate and defining factors that increase transmission risk are essential. Most cases are asymptomatic, but they have viral loads indistinguishable from symptomatic people and do transmit SARS-CoV-2 virus. However, they are often undetected.

METHODS: Given high residence hall student density, the University of Colorado Boulder established a mandatory weekly screening test program. We analyzed longitudinal data of 6408 students and identified 116 likely transmission events in which a second roommate tested positive within 14 days of the index roommate.

RESULTS: Although the infection rate was lower in single rooms (10%) than in multiple-occupancy rooms (19%), inter-roommate transmission only occurred ~20% of the time. Cases were usually asymptomatic at the time of detection. Notably, individuals who likely transmitted had an average viral load ~6.5-fold higher than individuals who did not (mean Cq 26.2 vs 28.9). Although diagnosed students moved to isolation rooms, there was no difference in time-to-isolation between cases with or without inter-roommate transmission.

CONCLUSIONS: This analysis argues that inter-roommate transmission occurs infrequently in residence halls and provides strong correlative evidence that viral load is proportional to transmission probability.