Majority of SARS-CoV-2 Transmissions May Occur Prior to Day 2 of COVID-19 Illness

Majority of SARS-CoV-2 Transmissions May Occur Prior to Day 2 of COVID-19 Illness

Klaus Mergener, MD, PhD, FASGE, reviewing Boehmer MM, et al. Lancet 2020 March 31.

This report, co-authored by the scientist who developed the first SARS-CoV-2 polymerase chain reaction test, describes the meticulous analysis of an ultimately contained micro-outbreak in Munich, Germany. It provides unique insights into virus transmission and the COVID-19 illness.  

Virus entry into the cohort could be attributed to a Chinese person who visited Germany to present at a business meeting. A meeting attendee tested positive for SARS-CoV-2 within a few days. All other participants and their subsequent contacts could be identified. High-risk individuals were quarantined, and serial case interviews and sample collections were conducted over several weeks. A total of 17 cases were ultimately identified. Complete sequencing of all viral genomes allowed exact tracking of the infection through the cohort.

Important insights from this study include: 

  • Symptoms could be identified in all but one infected individual.
  • Virus transmission could happen with only limited contact: one transmission pair met only once, sitting back to back for 15 minutes at separate tables in a cafeteria, passing a salt dispenser from one table to the other as their only interaction.
  • The majority of virus transmissions occurred either in the presymptomatic phase or within the first 24 hours of symptom onset.

Small cohort studies, if done well, can sometimes produce valuable insights into a new disease process. This report suggests that the group of completely asymptomatic COVID-19 cases may be smaller than we initially suspected (although symptoms are often minimal). More worrisome for gastroenterologists is the finding of virus transmission during the presymptomatic phase. As the pandemic spreads around the country, we may have to consider all individuals as potentially infectious, whether or not they exhibit symptoms.

Note to readers: At the time we reviewed this paper, its publisher noted that it was not in final form and that subsequent changes might be made.

Klaus Mergener, MD, PhD, FASGE


Böhmer MM, Buchholz U, Corman VM, et al. Outbreak of COVID-19 in Germany resulting from a single travel-associated primary case. Lancet 2020 March 31. (Preprint available:

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