Infection outbreaks are usually recognised when an unexpectedly high number of cases of a clinical illness are observed. With hepatitis B infection, the incubation period is long and a substantial proportion of infected people—especially children and the immunocompromised host—may remain asymptomatic.1^–3 These factors increase the risk of prolonged and unrecognised outbreaks. Denmark has not implemented universal childhood hepatitis B immunisation, and vaccination is not required for health workers performing invasive procedures.

Phylogenetic analysis of hepatitis B virus have been used to investigate recognised outbreaks,4 5 to reject or confirm suspected chains of transmission,6 and to prove unusual reservoirs or routes of infection.7 8 We present the unravelling of an outbreak of asymptomatic hepatitis B on a paediatric ward after the incidental identification of a single case of hepatitis B. The recognition and subsequent halting of the spread of this outbreak were possible owing to the availability of a DNA sequences library compiled from patients with hepatitis B in the locality.9