High COVID-19 incidence among Norwegian travellers returned from Lombardy: implications for travel restrictions

On February 27th, three cases of COVID-19 were reported among Norwegians that had recently returned from Lombardy, Italy. Travellers from the region rapidly became the most common source of imported infections in the earliest stage of the Norwegian COVID-19 epidemic. The situation was exacerbated by the unfortunate temporal overlap between the Norwegian winter holidays and intense epidemic spread of COVID-19 in Northern Italy, resulting in a large number of infected travellers. Here we combined flight data on travels between Norway and Lombardy with patient-level data to determine the fraction of travellers returning to Norway that had been infected with SARS-CoV-2. Travellers returning to Norway from Lombardy contracted COVID-19 at incidence rates up to 0.02 per person-day in the period spanning February 21st and March 1st, with a clear uptick in transmission in the middle of the period. This shows an example of the infection risk in tourist destinations being several fold higher than elsewhere in the region. In Norway, this is also supported by high rates of infections among tourists returning from Austria in February and March, despite a low number of reported cases in the country at the time. The massive COVID-19 prevalence among travellers suggest that mandatory quarantine of returning travellers or suspension of non-essential international flights is essential if the aim is to control or suppress the COVID-19 pandemic.