Seroprevalence of SARS-CoV-2 in Sweden, 26 April to 9 May 2021
At the beginning of April 2021, Sweden experienced a third wave of covid-19, SARS-CoV-2 lineage B.1.1.7 (Alpha) variant dominant1. Vaccination against covid-19 started in December 2020 with the age group 70 years and older as one of the priority groups, and extended to include 65 years and older in early March 2021. In April 2021, 23% of individuals over 65 years and 8% of the Swedish population as a whole were fully vaccinated with one of the Comirnaty, Spikevax or Vaxzevria vaccines2. In this report, we present the results of a point seroprevalence study of SARS-CoV-2 in April / May 2021 that was conducted to estimate seroprevalence at the national level at this time.
Study participants and serological analyzes
Participants were recruited from a national probability-based web panel that is regularly used for health-related questionnaires at the Swedish Public Health Agency3. Among the 4477 registered individuals in the panel, informed consent was received for participation in the study from 3283 individuals. Participation of minors under the age of 16 took place after informed consent from the respective caretaker and minors 16–17 years after informed consent from both themselves and the respective carers. Sampling at home on cartridges with dried blood stain (DBS) was used to collect capillary blood (Capitainer®Solna, Sweden)4. Sampling was performed between April 26 and May 9, 2021. Capillary blood in DBS cassettes was returned to the laboratory and processed for analysis of anti-Spike IgG in a internally SARS-CoV-2 ELISA5. The results of the serological analysis were communicated to the participants electronically and by post. Of the 3283 home sampling kits sent out for the study, 423 were either not returned to the laboratory, did not have sufficient capillary blood in them, or were taken outside the sampling period (Fig. 1). Information on COVID-19 vaccination status and previously confirmed SARS-CoV-2 infection in participants was collected from the National Vaccination Registry and the National Mandatory Communicable Disease Notification System (SmiNet). The participants’ demographics are summarized in Table 1.
Seroprevalence in Sweden’s population
Seroprevalence was calculated as the weighted percentage and corrected for the laboratory method using the Rogan – Gladen method6.7. The weights were based on the sample weights that were adjusted for the survey dropout with logistic regression and calibrated with the GREG estimator for Sweden’s population with respect to age, gender, education and geographical region.8. Estimates are displayed at 95% confidence intervals (CI) using the Clopper-Pearson method. Calculations were made in R v.4.0.2. with survey package v.4.0. The seroprevalence of the study population (n = 2860) can be seen in Table 2.
Differences between age groups and gender were tested by a weighted logistic regression where a p-value <0.05 was considered significant. There was a significantly higher proportion of individuals with SARS-CoV-2 antibodies in the age group 65 years and older compared to the other age groups (p <0.001). No statistical difference in antibody prevalence by sex was found.
A total of 141 participants in the study population were vaccinated with two doses at least two weeks before sampling, and all of these had detectable levels of antibodies to SARS-CoV-2.
Seroprevalence among non-vaccinated individuals
Table 2 also shows seroprevalence among non-vaccinated individuals (n = 1876) where the highest proportion of individuals with antibodies was detected in the age group 11–19 years (30.5%; 95% CI 23.6–38.2). The seroprevalence in the other age groups varied from 18.0 to 20.5%. Seroprevalence levels were similar between the sexes.
Among non-vaccinated participants with previous laboratory-confirmed SARS-CoV-2 exposure (n = 245), the proportion who had detectable levels of antibodies varied with time since infection (Table 3). These estimates were adjusted using the Rogan – Gladen method but unweighted7.9. The highest proportion was seen among the most recently infected individuals (15–90 days before sampling) and was 97.8% (95% CI 92.1–100). In the group that had been infected more than 6 months before sampling, 84.4% (95% CI 68.3–94.5) still had detectable levels of antibodies.
Ethical approval
Approval (registration number 2020-07029) has been obtained from the Ethics Review Authority, Uppsala, Sweden. All experiments were performed in accordance with relevant guidelines and regulations (the Act on Ethical Review of Research Involving People (Act 2003: 460) and ISO / IEC 17025).
Consent to participate
Informed consent was obtained from all individual participants included in the study. Minors participated with the informed consent of the respective caretaker.