Prevalence, incidence and longevity of antibodies against SARS-CoV-2 among primary healthcare providers in Belgium: a prospective cohort study with 12 months of follow-up by Adriaenssens, N, Scholtes, B, Bruyndonckx, R, Van Ngoc, P, Verbakel, JYJ, De Sutter, A, Heytens, S, Van Den Bruel, A, Desombere, I, Van Damme, P, Goossens, H, Buret, L, Duysburgh, E, Coenen, S