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

Abstract

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.

High Prevalence of SARS-CoV-2 Omicron Infection Despite High Seroprevalence, Sweden, 2022

“…We estimated point prevalences in Sweden of 1.4% during March 21–25 and, as previously reported ( 6 ), 1.5% during September 26–29. Those estimated point prevalences were higher than those in our previous national surveys ( 5 ). In another survey of healthcare workers in Stockholm during June 28–29, 2022, we observed asymptomatic SARS-CoV-2 infections in 2.3% of participants ( 7 ).…”

“…Participants were invited from a nationwide probability-based web panel ( 5 , 6 ). Participants received material for sampling and instructions on how to perform self-sampling at home ( Appendix ).…”

Estimating SARS‐CoV‐2 infections and associated changes in COVID‐19 severity and fatality

“…Quantitative estimates provided in this study apply to the case of Italy and may depend on the many country‐specific factors that characterized the response to the COVID‐19 pandemic, such as governmental choices on the adoption of NPIs, or differences in COVID‐19 vaccines uptake. Despite these heterogeneities, estimates of the Italian SARS‐CoV‐2 seroprevalence obtained through the proposed modeling approach are in line with those estimated for high‐income European countries 35,56 . We thus expect the general trends and conclusions of this study may apply also to other high‐income European countries, as well as to other countries that have adopted a similar mitigation approach throughout the pandemic.…”

“…Despite these heterogeneities, estimates of the Italian SARS-CoV-2 seroprevalence obtained through the proposed modeling approach are in line with those estimated for high-income European countries. 35,56 We thus expect the general trends and conclusions of this study may apply also to other high-income European countries, as well as to other countries that have adopted a similar mitigation approach throughout the pandemic. Finally, our results suggest that our approach may represent a valid alternative to assess SARS-CoV-2 infection rates based on routinely collected surveillance data, when serological data are not available.…”