Behavioral factors associated with SARS-CoV-2 infection. Results from a web-based case-control survey in the Capital Region of Denmark

OBJECTIVE: To study the association between behavioural factors and incidence rates of SARS-CoV-2 infection. DESIGN: Case–control web-based questionnaire study. SETTING: Questionnaire data were collected in the Capital Region of Denmark in December 2020 when limited restrictions were in place, while the number of daily SARS-CoV-2 cases increased rapidly. PARTICIPANTS: 8913 cases of laboratory-confirmed SARS-CoV-2 infection were compared with two groups of controls: (1) 34 063 individuals with a negative SARS-CoV-2 test from the same date (negative controls, NCs) and 2) 25 989 individuals who had never been tested for a SARS-CoV-2 infection (untested controls, UC). Controls were matched on sex, age, test date and municipality. EXPOSURE: Activities during the 14 days prior to being tested positive for SARS-CoV-2 or during the same period for matched controls and precautions taken during the entire pandemic. MAIN OUTCOMES AND MEASURES: SARS-CoV-2 infection incidence rate ratios (IRR). RESULTS: Response rate was 41.4% (n=93 121). Using public transportation, grocery shopping (IRR: NC: 0.52; UC: 0.63) and outdoor sports activities (NC: 0.75; UC: 0.96) were not associated with increased rate of SARS-CoV-2 infection. Most precautions, for example, using hand sanitizer (NC: 0.79; UC: 0.98), physical distancing (NC: 0.79; UC: 0.82) and avoiding handshakes (NC: 0.74; UC: 0.77), were associated with a lower rate of infection. Activities associated with many close contacts, especially indoors, increased rate of infection. Except for working from home, all types of occupation were linked to increased rate of infection. CONCLUSIONS: In a community setting with moderate restrictions, activities such as using public transportation and grocery shopping with the relevant precautions were not associated with an increased rate of SARS-CoV-2 infection. Exposures and activities where safety measures are difficult to maintain might be important risk factors for infection. These findings may help public health authorities tailor their strategies for limiting the spread of SARS-CoV-2.