SARS-CoV-2 outbreak in a tri-national urban area is dominated by a B.1 lineage variant linked to mass gathering events
AbstractBackgroundThe first case of SARS-CoV-2 in Basel, Switzerland was detected on February 26th 2020. We present a phylogenetic study to explore viral introduction and evolution during the exponential early phase of the local COVID-19 outbreak from February 26th until March 23rd.MethodsWe sequenced SARS-CoV-2 naso-oropharyngeal swabs from positive 746 tests that were performed at the University Hospital Basel in the timeframe of our study. We successfully generated 468 high quality genomes from unique patients and called variants with our COVID-19 Pipeline (COVGAP). We analysed viral genetic diversity using PANGOLIN taxonomic lineages. To identify introduction and dissemination events we incorporated global SARS-CoV-2 genomes and inferred a time-calibrated phylogeny. We used epidemiological data to aid interpretation of phylogenetic patterns.FindingsThe early outbreak in Basel was dominated by lineage B.1 (83·6%), detected from March 2nd, although the first lineage identified was B.1.1. Within B.1, a clade defined by the SNP C15324T contains 68·2% of our samples (‘Basel cluster’), including 157 identical sequences at the root of the ‘Basel cluster’, suggesting local spreading events. We infer the origin of the ‘Basel cluster’ defining mutation to mid-February in our tri-national region. The remaining genomes map broadly over the global phylogenetic tree, evidencing several events of introduction from and/or dissemination to other regions of the world via travellers. We also observe family transmission events.InterpretationA single lineage variant dominated the outbreak in the City of Basel while other lineages such as the first (B1.1) did not propagate. We identify mass gathering events and less so travel returners and family transmission as causes for the local outbreak. We highlight the importance of adding specific questions to the epidemiological questionnaires that are collected, to obtain data on attendance of large gathering events and locations as well as travel history to effectively identify routes of transmissions in up-coming outbreaks. This phylogenetic analysis enriches epidemiological and contact tracing data, allowing, even retrospectively, connection of seemingly unconnected events, and can inform public health interventions.