This is a tertiary care hospital-based prospective molecular epidemiology study in Montreal, Canada. When nosocomial transmission was suspected by local infection control teams' investigations, SARS-CoV-2 viral genomic sequencing was performed locally for all putative outbreak cases and contemporary controls. Molecular and conventional epidemiology data were confronted in real time to improve understanding of COVID-19 transmission and reinforce or adapt prevention measures.

This study will include infected healthcare workers (HCW) and patients for whom a nosocomial infection in CHUM is suspected. The CHUM laboratory is responsible for COVID-19 diagnostic testing in all eligible HCW and patients. After initial diagnosis on standard PCR testing, samples are maintained at -80. Viral RNA is extracted and next-generation sequencing of the viral genome is performed using Nanopore long read sequencing platform. Preliminary data suggest that SARS-CoV-2 has a "molecular clock" of 2 single nucleotide polymorphism (SNP) per month. The analysis and between-isolate comparaison of those SNP among epidemiologically related cases will allow to support or refute the hypothesis of person to person nosocomial transmission. All infected patients will be matched 1:1 with contemporary community cases to provide a viral genome back catalog and better contextualize molecular epidemiology analyses. Molecular clusters will be reported to infection prevention and control teams and resulting interventions will be monitored.

Inclusion Criteria:

• Hospitalized in the institution during the study period
• Healthcare workers in the institution during the study period

Exclusion Criteria:

• None

Patients and healthcare workers in the institution during the study period.

Viral genomic sequences from all participants are to be deposited on GenBank.