SARS-CoV-2 outbreaks on Danish mink farms and mitigating public health interventions

Schmidt, Torben Dall; Mitze, Timo

BACKGROUND: First severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections on Danish mink farms were reported in June 2020 and thereupon spread geographically. We provide population-level evidence on excess human incidence rates in Danish municipalities affected by disease outbreaks on mink farms and evaluate the effectiveness of two non-pharmaceutical interventions, i.e. culling of infected mink and local lockdowns. METHODS: We use information on SARS-CoV-2 outbreaks on mink farms in 94 Danish municipalities together with data on human SARS-CoV-2 cases and tested persons in weeks 24 to 51 of 2020. Difference-in-difference estimations and panel event studies for weekly human incidence rates are applied to 1) identify epidemiological trends of human SARS-CoV-2 infections associated disease outbreaks on mink farms, and 2) quantify the mitigating effects from the two non-pharmaceutical interventions. RESULTS: SARS-CoV-2 outbreaks on mink farms in a municipality associate with increase in weekly human incidence rates by about 75%; spatial spillover effects to neighbouring municipalities are also observed. Local lockdowns reduce human incidence rates, while culling of mink appears to be more effective in combination with a lockdown. The temporal lag between an outbreak on a mink farm and a significant increase in human incidence rates is estimated to be 1-3 weeks; lockdowns and culling of mink neutralizes this effect 4-8 weeks after the initial outbreak. CONCLUSION: SARS-CoV-2 infections among farmed mink in Denmark significantly link to local human infection trends. Strict animal and human disease surveillance in regions with mink farming should be pursued internationally to mitigate future epidemic developments.