Incidence of SARS-CoV-2 infection in health care workers from Northern Italy based on antibody status: immune protection from secondary infection- A retrospective observational case-controlled study

Objective: The protection from SARS-CoV-2 infection induced by SARS-CoV-2 anti-S1 and anti-S2 IgG antibody positivity resulting from natural infection was evaluated. Methods: The frequency of SARS-CoV-2 infection (as determined by virus RNA detection) was evaluated in a group of 1,460 seropositive and a control group of 8,150 seronegative healthcare workers in three Centres of Northern Italy in the period June-November 2020. Neutralizing serum titers were analyzed in seropositive subjects with or without secondary SARS-CoV-2 infection. Results: During the 6-month survey, 1.78% seropositive subjects developed secondary SARS-CoV-2 infection while 6.63% seronegative controls developed primary infection (odds ratio: 0.26; 95% confidence interval: 0.17-0.38). Secondary infection was associated with low or absent serum neutralizing titer (p