Seroprevalence trends of anti-SARS-CoV-2 antibodies and associated risk factors: a population-based study
Authors: Tancredi Stefano , Chiolero Arnaud , Wagner Cornelia , Haller Moa Lina , Chocano-Bedoya Patricia , Ortega Natalia , Rodondi Nicolas , Kaufmann Laurent , Lorthe Elsa , Baysson Hélène , Stringhini Silvia , Michel Gisela , Lüdi Chantal , Harju Erika , Frank Irene , Imboden Medea , Witzig Melissa , Keidel Dirk , Probst-Hensch Nicole , Amati Rebecca , Albanese Emiliano , Corna Laurie , Crivelli Luca , Vincentini Julia , Gonseth Nusslé Semira , Bochud Murielle , D’Acremont Valérie , Kohler Philipp , Kahlert Christian R. , Cusini Alexia , Frei Anja , Puhan Milo A. , Geigges Marco , Kaufmann Marco , Fehr Jan , Cullati Stéphane ,   the Corona Immunitas Research Group, Amendola Antonio , Anagnostopoulos Alexia , Anker Daniela , Annoni Anna Maria , Aschmann Hélène , Azman Andrew , Bal Antoine , Ballouz Tala , Bezani Kleona , Blattmann Annette , Bleich Patrick , Bodenmann Patrick , Buttaroni Peter , Butty Audrey , Camerini Anne Linda , Chocano-Bedoya Patricia Orializ , Collombet Prune , Santos Diana Sofia Da Costa , Deschamps Agathe , D’Ippolito Paola , Domenghino Anja , Dubos Richard , Dumont Roxane , Duperrex Olivier , Dupraz Julien , Egger Malik , El-May Emna , Merjani Nacira El , Engler Nathalie , Epure Adina Mihaela , Erksam Lukas , Estoppey Sandrine , Fadda Marta , Faivre Vincent , Felappi Andrea , Fiordelli Maddalena , Flahault Antoine , Fornerod Luc , Corti Cristina Fragoso , Francioli Natalie , Frangville Marion , Frank Irène , Franscella Giovanni , Graindorge Clément , Guessous Idris , Harnal Séverine , Jendly Emilie , Jeong Ayoung , Kaiser Laurent , Kessler Simone , Krähenbühl Christine , Kriemler Susi , Lamour Julien , Levati Sara , Lescuyer Pierre , Loizeau Andrea , Luedi Chantal , Magnin Jean-Luc , Martinez Chantal , Masserey Eric , Menges Dominik , Morese Rosalba , Mösli Nicolai , Noël Natacha , Paris Daniel Henry , Pasquier Jérôme , Pennacchio Francesco , Pfister Stefan , Piumatti Giovanni , Poulain Géraldine , Pugin Caroline , Puhan Milo , Pullen Nick , Radtke Thomas , Rasi Manuela , Richard Aude , Richard Viviane , Robert Claude-François , Rodondi Pierre-Yves , Sabatini Serena , Samir Khadija , Zozaya Javier Sanchis , Schlüter Virginie , Schmid Alexia , Schneider Valentine , Schüpbach Maria , Schwab Nathalie , Semaani Claire , Speierer Alexandre , Steiner-Dubuis Amélie , Testini Stéphanie , Thabard Julien , Tonolla Mauro , Troillet Nicolas , Ulyte Agne , Vassaux Sophie , Vermes Thomas , Villers Jennifer , von Wyl Viktor , Wenger Rylana , West Erin , Wisniak Ania , Zaballa María-Eugenia , Zens Kyra , Zuppinger Claire 
Published: March 2023

Abstract
Purpose
We aimed to assess the seroprevalence trends of SARS-CoV-2 antibodies in several Swiss cantons between May 2020 and September 2021 and investigate risk factors for seropositivity and their changes over time.

Methods
We conducted repeated population-based serological studies in different Swiss regions using a common methodology. We defined three study periods: May–October 2020 (period 1, prior to vaccination), November 2020–mid-May 2021 (period 2, first months of the vaccination campaign), and mid-May–September 2021 (period 3, a large share of the population vaccinated). We measured anti-spike IgG. Participants provided information on sociodemographic and socioeconomic characteristics, health status, and adherence to preventive measures. We estimated seroprevalence with a Bayesian logistic regression model and the association between risk factors and seropositivity with Poisson models.

Results
We included 13,291 participants aged 20 and older from 11 Swiss cantons. Seroprevalence was 3.7% (95% CI 2.1–4.9) in period 1, 16.2% (95% CI 14.4–17.5) in period 2, and 72.0% (95% CI 70.3–73.8) in period 3, with regional variations. In period 1, younger age (20–64) was the only factor associated with higher seropositivity. In period 3, being aged ≥ 65 years, with a high income, retired, overweight or obese or with other comorbidities, was associated with higher seropositivity. These associations disappeared after adjusting for vaccination status. Seropositivity was lower in participants with lower adherence to preventive measures, due to a lower vaccination uptake.

Conclusions
Seroprevalence sharply increased over time, also thanks to vaccination, with some regional variations. After the vaccination campaign, no differences between subgroups were observed.