To respond to the burgeoning SARSCoV-2 epidemic, we propose to leverage the infrastructure of a new binational cohort of 600 people who inject drugs (PWID), as well as additional UCSD investigators who have expertise in analysis of social networks, microbiome, and genetic sequencing data to address the proposed aims: Aim 1. To determine the prevalence and correlates of subclinical and symptomatic SARSCoV-2 infection among PWID within the SD/TJ border region. Aim 2. To determine the prevalence and correlates of SARSCoV-2 shedding in nasal secretions and fecal specimens among PWID. Aim 3. To determine network features associated with cross-border transmission of subclinical SARSCoV-2 infection among PWID. Aim 4: To use molecular epidemiology to determine the global migration of SARSCoV-2 between PWID in San Diego and Tijuana and factors associated with local and global dispersal. We will use social network surveys, microbiome analysis and full length SARSCoV-2 sequencing to characterize PWIDs’ social network determinants of SARSCoV-2 infection, the influence of the microbiome on shedding, and the viral genetic migrations within SD, TJ and beyond. Accurate estimates of subclinical infection of SARSCoV-2 and its risk factors are needed to inform prevention, treatment, and public policy, especially in a cross-border context where border closures have already been implemented as a costly measure to curb transmission.