Background and Objectives:
The titre of the anti-SARS-CoV-2 antibodies 6 months after the completion of primary vaccination cycle (two doses) shows a relevant decay, as also demonstrated in our previous study in a group of Italian nursing home personnel. We decided to evaluate the trend of antibody response after a third dose of anti-COVID-19 vaccine in the same workers. The possible role of individual factors was also explored.

Methods:
A mixed retrospective-prospective observational study was performed on a sample of workers (healthcare workers (HCWs) and non-HCWs from an Italian nursing home. Antibody titre was evaluated after the third BNT162b2 vaccine dose and compared with the values determined 6 months after the second dose (baseline). Moreover, individual socio-demographic and anamnestic characteristics were collected through a self-administered questionnaire.

Results:
Forty-five nursing home employees vaccinated with three doses of BNT162b2 vaccine were enrolled in this follow up. Results show a significant increase of the neutralizing antibody titre after the third dose compared to the baseline (1221 BAU/ml vs 647 BAU/ml, as median values). A previous COVID-19 diagnosis and being non-HCWs were found to be associated with a higher increase of antibody titre. Analysis on individual characteristics is still ongoing, but preliminary results suggest a possible role of BMI, and age on antibody titre variation after the third vaccine dose.

Conclusion:
The results show a significant increase of antibody titre in the employees of the nursing home after the third dose of BNT162b2 vaccine, supporting the importance of adherence to the proposed vaccine campaigns for the prevention of COVID-19 infection in workers.