SARS-CoV-2 antibody dynamics and transmission from community-wide serological testing in the Italian municipality of Vo’

Dorigatti, Ilaria; Lavezzo, Enrico; Manuto, Laura; Ciavarella, Constanze; Pacenti, Monia; Boldrin, Caterina; Cattai, Margherita; Saluzzo, Francesca; Franchin, Elisa; Del Vecchio, Claudia; Caldart, Federico; Castelli, Gioele; Nicoletti, Michele; Nieddu, Eleonora; Salvadoretti, Elisa; Labella, Beatrice; Fava, Ludovico; Guglielmo, Simone; Fascina, Mariateresa; Grazioli, Marco; Alvisi, Gualtiero; Vanuzzo, Maria Cristina; Zupo, Tiziano; Calandrin, Reginetta; Lisi, Vittoria; Rossi, Lucia; Castagliuolo, Ignazio; Merigliano, Stefano; Unwin, H. Juliette T.; Plebani, Mario; Padoan, Andrea; Brazzale, Alessandra R.; Toppo, Stefano; Ferguson, Neil M.; Donnelly, Christl A.; Crisanti, Andrea

In February and March 2020, two mass swab testing campaigns were conducted in Vo’, Italy. In May 2020, we tested 86% of the Vo’ population with three immuno-assays detecting antibodies against the spike and nucleocapsid antigens, a neutralisation assay and Polymerase Chain Reaction (PCR). Subjects testing positive to PCR in February/March or a serological assay in May were tested again in November. Here we report on the results of the analysis of the May and November surveys. We estimate a seroprevalence of 3.5% (95% Credible Interval (CrI): 2.8–4.3%) in May. In November, 98.8% (95% Confidence Interval (CI): 93.7–100.0%) of sera which tested positive in May still reacted against at least one antigen; 18.6% (95% CI: 11.0–28.5%) showed an increase of antibody or neutralisation reactivity from May. Analysis of the serostatus of the members of 1,118 households indicates a 26.0% (95% CrI: 17.2–36.9%) Susceptible-Infectious Transmission Probability. Contact tracing had limited impact on epidemic suppression.