At least 1 000 individuals, 18 years old and older, are randomly sampled using random digit dialing from the population of Saint Petersburg, Russia. Random sampling is performed by a survey company on the list of mobile phone numbers with designated geography prefixes of St. Petersburg. The baseline questionnaire for computer-assisted telephone interview includes travel history, medical history, and socio-economic status of the respondents. Sampled individuals are then invited to the clinic for blood sampling, their refusal to participate is recorded. Collected blood samples are used for SARS-CoV-2-specific antibodies testing. Data and sample collection spans four weeks and is conducted every two weeks. Participants complete additional questionnaires in the clinic providing information on medical history, history of allergies, chronic disease, smoking, and medication taken regularly. Contact tracing data and environmental conditions of the household are reсorded.
