The COVID-19 outbreak in Stockholm, Sweden, is characterized by a near-absence of governmental interventions and high fatalities in the care home population. This study analyses the outbreak and the social-distancing effectiveness timeline in the general population and the care homes.

A novel distributed-compartmental, time-variant epidemiological model was designed specifically for COVID-19 transmission characteristics, featuring a/pre/symptomatic transmission, a non-linear hospital model, a weakly-coupled sub-model for the care-home population, and parametrized continuous social-distancing functions. The model parameters and the social-distancing timelines are determined by randomization and Monte-Carlo simulations analysing real-world data.

Despite a high initial reproduction number (3·29) and the near-absence of governmental interventions, the model quantitated that the transmission rate in the general population was suppressed by 73%, and in the care homes by 79%. The measures in the care homes took effect 4·8 days delayed; and if applied 4 or 8 days earlier, the fatalities could have been reduced by 63·2% or 89·9%. The infected population is estimated to 16·2% (June 10). An expected underestimation of population immunity by antibody studies is confirmed. The infection fatality ratio extrapolates to 0·61% (peak: 1·34%). The model indicates a seasonal effect which effectively suppressed a new rise. An analysed large-scale public event had no large influence. The asymptomatic ratio was determined to 35%.

The proposed model and methods have proven to analyse a COVID-19 outbreak and to re-construct the social-distancing behaviour with unprecedented accuracy, confirming even minor details indicated by mobility-data analysis, and are applicable to other regions and other emerging infectious diseases of similar transmission characteristics. The self-regulation of the population in Stockholm, influenced by advices by the authorities, was able to suppress a COVID-19 outbreak to a level far beyond that the stringency index of governmental interventions suggests. Proper timing of effective measures in the care homes is important to reduce fatalities.