Seroprevalence of anti-SARS-CoV-2 antibodies six months into the vaccination campaign in Geneva, Switzerland

  1. Silvia Stringhini
  2. María-Eugenia Zaballa
  3. Nick Pullen
  4. Javier Perez-Saez
  5. Carlos de Mestral
  6. Andrea Loizeau
  7. Julien Lamour
  8. Francesco Pennacchio
  9. Ania Wisniak
  10. Roxane Dumont
  11. Hélène Baysson
  12. Viviane Richard
  13. Elsa Lorthe
  14. Claire Semaani
  15. Jean-François Balavoine
  16. Didier Pittet
  17. Nicolas Vuilleumier
  18. François Chappuis
  19. Omar Kherad
  20. Andrew S. Azman
  21. Klara Posfay-Barbe
  22. Laurent Kaiser
  23. Idris Guessous
  24. the Specchio-COVID19 study group
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Abstract

Background

Up-to-date seroprevalence estimates are critical to describe the SARS-CoV-2 immune landscape in the population and guide public health measures. We aimed to estimate the seroprevalence of anti-SARS-CoV-2 antibodies 15 months into the COVID-19 pandemic and six months into the vaccination campaign.

Methods

We conducted a population-based cross-sectional serosurvey between June 1 and July 7, 2021, recruiting participants from age- and sex-stratified random samples of the general population. We tested participants for anti-SARS-CoV-2 antibodies targeting the spike (S) or nucleocapsid (N) proteins (Roche Elecsys immunoassays). We estimated the anti-SARS-CoV-2 antibodies seroprevalence following vaccination and/or infection (anti-S antibodies), or infection only (anti-N antibodies).

Results

We included 3355 individuals, of which 1814 (54.1%) were women, 697 (20.8%) were aged <18 years and 449 (13.4%) were aged ≥65 years, 2161 (64.4%) tested positive for anti-S antibodies, and 906 (27.0%) tested positive for anti-N antibodies. The total seroprevalence of anti-SARS-CoV-2 antibodies was 66.1% (95% credible interval, 64.1-68.0). We estimated that 29.9% (28.0-31.9) of the population developed antibodies after infection; the rest having developed antibodies only via vaccination. Seroprevalence estimates were similar across sexes, but differed markedly across age groups, being lowest among children aged 0-5 years (20.8% [15.5-26.7]) and highest among older adults aged ≥75 years (93.1% [89.6-96.0]). Seroprevalence of antibodies developed via infection and/or vaccination was higher among participants with a higher educational level.

Conclusions

Most adults have developed anti-SARS-CoV-2 antibodies, while most teenagers and children remain vulnerable to infection. As the SARS-CoV-2 Delta variant spreads and vaccination rates stagnate, efforts are needed to address vaccine hesitancy, particularly among younger individuals and socioeconomically disadvantaged groups, and to minimize spread among children.

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