Seroprevalence of Anti-SARS-CoV-2 IgG Antibodies in Switzerland

Quick Takes

  • 1 in 10 people in Switzerland are estimated to have detectable antibodies against SARS-CoV-2.
  • Young children are much less likely to seroconvert compared to middle-aged adults.

Study Questions:

What is the estimated weekly change in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) seroprevalence in Geneva, Switzerland?

Methods:

The SEROCoV-POP study is a population-based study of former participants of the Bus Santé study and their household members. The Bus Santé is a study of a yearly sample of the Geneva canton population that includes 500 men and 500 women who fill standardized questionnaires covering risk factors for chronic diseases, sociodemographic, occupational, and educational histories. After inviting 6,229 participants of the Bus Santé study, the authors enrolled 2,834 in SEROCoV-POP, of whom 2,766 had complete data. Enrollees and their household members underwent weekly testing for anti-SARS-CoV-2 immunoglobulin G (IgG) antibodies targeting the spike protein with the Euroimmun enzyme-linked immunosorbent assay (ELISA) (93% sensitivity and 100% specificity), between April 6 and May 9, 2020.

Results:

A total of 1,454 (52.6%) of 2,766 participants were women; and 123 (4.4%) were aged 5–9 years, 332 (12.0%) were aged 10–19 years, 1,096 (39.6%) were aged 20–49 years, 846 (30.6%) were aged 50–64 years, and 369 (13.3%) were aged ≥65 years. The sample had an over-representation of 50-64 years and under-representation of people aged >80 years compared to the Geneva population. Over the course of the study, 219 of 2,766 tested positive for SARS-CoV-2 IgG antibodies. In the first week, the authors estimated a seroprevalence of 4.8% (95% confidence interval [CI], 2.4–8.0; n = 341). The estimate increased to 8.5% (5.9–11.4, n = 469) in the second week, to 10.9% (7.9–14.4, n = 577) in the third week, 6.6% (4.3–9.4, n = 604) in the fourth week, and 10.8% (8.2–13.9, n = 775) in the fifth week. The risk of seropositivity was similar between men and women. Young children aged 5–9 years had (relative risk [RR], 0.32 [95% CI, 0.11–0.63]) and those >65 years (RR, 0.50 [95% CI, 0.28–0.78]) had significantly lower risk of being seropositive than those aged 20–49 years. Younger children were more likely to have a household member that was seropositive (17.0%) compared to those aged ≥65 years (3.0%).

Conclusions:

The study estimates 1 in 10 people have developed detectable antibodies against SARS-CoV-2 in Switzerland.

Perspective:

Similar to other studies in Europe and the United States, SEROCoV-POP revealed a low seroprevalence to SARS-CoV-2 in Switzerland, despite being an area with a relatively high number of cases. Assuming seroprevalence equates to the development of immunity—or the lack of seroprevalence indicating the lack of immunity—then herd immunity is not close to being achieved. However, since the end of the first peak on April 30, coronavirus disease 2019–related deaths in Switzerland have remained low, and cases have not increased dramatically despite easing of measures and reopening their borders. With some studies reporting a rapid loss of antibodies weeks after exposure to SARS-CoV-2, the seroprevalence might be an underestimate of the true exposure. The lower risk of seroconversion in younger children despite a household being more likely to have another seropositive member is consistent with growing evidence suggesting children are less likely to be infected or develop severe disease compared to adults.

Keywords: Antibodies, Chronic Disease, Coronavirus, COVID-19, Enzyme-Linked Immunosorbent Assay, Immunity, Herd, Immunoglobulin G, Middle Aged, Pediatrics, Primary Prevention, Risk Factors, Seroepidemiologic Studies, severe acute respiratory syndrome coronavirus 2, Spike Glycoprotein, Coronavirus


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