Low and high infection dose transmissions of SARS-CoV-2 in the first COVID-19 clusters in Northern Germany

ObjectivesWe used viral genomics to deeply analyze the first SARS-CoV-2 infection clusters in the metropolitan region of Hamburg, Germany. Epidemiological analysis and contact tracing together with a thorough investigation of virus variant patterns revealed low and high infection dose transmissions to be involved in transmission events.MethodsInfection control measures were applied to follow up contract tracing. Metagenomic RNA- and SARS-CoV-2 amplicon sequencing was performed from 25 clinical samples for sequence analysis and variant calling.ResultsThe index patient acquired SARS-CoV-2 in Italy and after his return to Hamburg transmitted it to 2 out of 132 contacts. Virus genomics and variant pattern clearly confirms the initial local cluster. We identify frequent single nucleotide polymorphisms at positions 241, 3037, 14408, 23403 and 28881 previously described in Italian sequences and now considered as one major genotype in Europe. While the index patient showed a single nucleotide polymorphism only one variant was transmitted to the recipients. Different to the initial cluster, we observed in household clusters occurring at the time in Hamburg also intra-host viral species transmission events.ConclusionsSARS-CoV-2 variant tracing highlights both, low infection dose transmissions suggestive of fomites as route of infection in the initial cluster and high and low infection dose transmissions in family clusters indicative of fomites and droplets as infection routes. This suggests (1) single viral particle infection can be sufficient to initiate SARS-CoV-2 infection and (2) household/family members are exposed to high virus loads and therefore have a high risk to acquire SARS-CoV-2.