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Introduction: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV2), the causative agent of coronavirus disease 2019 (COVID19), has led to a global pandemic. Currently, the diagnosis of COVID-19 is confirmed by detecting SARS-CoV-2 via RealTime-RT-PCR in rhino-pharyngeal swab. Serological test is required to evaluate a previous exposure to the virus, as well as antibody response. The aim of this multicentric study is to analyse the antibody response following COVID-19 vaccination in healthcare workers and guests of nursing homes, and to determine differences between subjects with or without a confirmed previous infection. Methods: Our study included 657 subjects, recruited from guests and healthcare workers of nursing homes in the Piedmont region. 10 days after the completion of the vaccination cycle, a blood sample was taken from the subjects to measure IgG against SARS-CoV-2 spike protein. Serological testing was performed using an enzyme immunoassay kit (EUROIMMUN AntiSARS-CoV-2 QuantiVac ELISA). We performed statistical analysis with R software. Results: We recruited 657 subjects aged between 19 to 106. Among them, 404 had a previous infection and 253 didn’t have a known previous infection. The serological examination resulted positive in 655 subjects and negative in 2 (X IgG 1095 RU/ml). Samples with titer> 1200 RU/ml will be further diluted to identify the titer and differences from the baseline situation. Conclusions: This study analyses neutralizing antibodies, which should prevent the virus from binding to target cells via spike protein. According to literature, immunological response to vaccine presents a great variability of antibodies level in the study population. However, no significantly correlations were found between the available variables. Preliminary data show the presence of a very intense antibody response after active vaccine immunization both in subjects with previous infection and in the rest of the study population. Key messages: Covid-19 vaccine response in a population at risk. Differences between subjects with or without a confirmed previous infection. [ABSTRACT FROM AUTHOR]