Persons with SARS-CoV2 can be contagious with few or no symptoms. They can infect others in private, during education or work without knowing it. Few so-called super-propagators can thus initiate clusters of infections and chains of transmission. Isolation of new cases and quarantine of their contacts (forward contact tracing) often does not uncover such situations. Adding detailed backward investigations of events and places with elevated risk of transmission can increase the identification of potentially infected persons. These can then be quarantined, and chains of transmission be interrupted. We describe the principles and challenges of cluster investigation, epidemiological methods and IT tools that we deve loped at the Centre for contact tracing, Vaud. Knowledge of this method is useful in general clinical practice during a pandemic.