Point prevalence of SARS-CoV-2 infection in Sweden at six time points during 2020.

BACKGROUND: In order to estimate the prevalence and understand the spread of SARS-CoV-2 in Sweden, the Public Health Agency of Sweden, with support from the Swedish Armed Forces, conducted a series of point prevalence surveys between March and December 2020. METHODS: Sampling material and instructions on how to perform self-sampling of the upper respiratory tract were delivered to the homes of the participants. Samples were analysed by real-time PCR, and the participants completed questionnaires regarding symptoms. FINDINGS: The first survey in the Stockholm region in March 2020 included 707 participants and showed a SARS-CoV-2 prevalence of 2.5%. The following five surveys, performed on a national level, with between 2461 and 2983 participants, showed SARS-CoV-2 prevalences of 0.9% (April), 0.3% (May), 0.0% (August), 0.0% (September), and 0.7% (December). All positive cases who responded to questionnaires reported experiencing symptoms that occurred from 2 weeks before the date of sampling up to and including the date of sampling. INTERPRETATION: None of the individuals shown to be PCR-positive were asymptomatic at the time of sampling or in the 14 days prior to sampling. This is in contrast to many other surveys in which a substantial proportion of positive cases have been reported to be asymptomatic. Our surveys demonstrate a decreasing ratio between notified cases and the observed prevalence throughout the year, in line with increasing testing capacity and the consecutive inclusion of all symptomatic individuals in the case definition for testing.

Point prevalence of SARS-CoV-2 infection in Sweden at six time points during 2020 (preprint)

Background: To estimate the prevalence and understand the dissemination of SARS-CoV-2 in Sweden, the Public Health Agency of Sweden, with support from the Swedish Armed Forces, conducted a series of point prevalence surveys between March and December 2020. Methods: Sampling material and instructions on how to perform self-sampling of the upper respiratory tract were delivered to the homes of the participants. Samples were analysed by real-time PCR, and the participants completed questionnaires regarding symptoms. Findings: The first survey in the Stockholm region in March, including 707 participants, showed a SARS-CoV-2 prevalence of 2.5%. The following five surveys, performed on a national level, with between 2,461 and 2,983 participants, showed SARS-CoV-2 prevalences of 0.9% (April), 0.3% (May), 0.0% (August), 0.0% (September), and 0.7% (December).All positive cases responding to the questionnaires regarding experienced symptoms had experienced symptoms two weeks before sampling up until the day of sampling. Interpretation None of the individuals shown to be PCR-positive were asymptomatic at the time of sampling, including 14 days prior to sampling. This is in contrast to many other surveys, where a substantial proportion of positive cases are reported to be asymptomatic.In line with an increasing testing capacity and consecutive inclusion of all symptomatic individuals in the case definition for testing, our surveys demonstrate a decreasing ratio between notified cases and shown prevalence throughout the year.