Cohort-based surveillance of SARS-CoV2 transmission mirrors infection rates at the population level: a one-year longitudinal study

Background More than one year into the COVID-19 pandemic, important data gaps remain on longitudinal prevalence of SARS-CoV-2 infection at the population level and in defined risk groups, efficacy of specific lockdown measures, and on (cost-)effective surveillance. Methods The ELISA (Luebeck Longitudinal Investigation of SARS-CoV-2 Infection) study invited adult inhabitants (n=~300,000) from the Luebeck area (Northern Germany) and enrolled 3051 participants (~1%); 1929 population-matched and 1645 with high-exposure based on profession. The one-year study period (03/2020-02/2021) covered massive influx of tourism in the summer, rise of infection rates in the fall/winter 2020/2021, and two lockdowns. Participants were screened seven times for SARS-CoV-2 infection using PCR and antibody testing and monitored with an app-based questionnaire (n=~91,000). Results Cohort (56% female; mean age: 45.6 years) retention was 75%-98%; 92 persons (3.5%) were antibody- and/or PCR-positive. Seropositivity was almost 2-fold higher in men and increased risk detected in several high-exposure groups (highest for nurses, followed by police, army, firemen, and students). In May 2020, 92% of the infections were missed by PCR testing; by February 2021, only 29% remained undiagnosed. Contact to COVID-19-affected was the most relevant risk factor. Other factors, such as frequent use of public transportation, shopping, close contacts at work, and extensive tourism in the summer did not impact infection rates. Conclusions We i) provide a model for effective, regional surveillance; ii) identify infection risk factors informing public health measures; iii) demonstrate that easing of lockdown measures appears safe at times of low prevalence in the presence of continuous monitoring.