The spread of the Delta variant of SARS-CoV-2 led to a reduction in the efficacy of the primary schedule of anti-SARS-CoV-2 vaccination, making a booster dose necessary to prompt the neutralizing immune response. This retrospective cohort study aims at evaluating risk factors associated with SARS-CoV-2 infection after the booster dose of Pfizer-BioNTech BNT162b2 mRNA vaccine in healthcare workers (HCWs) of a large Teaching Hospital in Rome. HCWs who completed the primary course with the BNT162b2 vaccine and received the booster dose at the Hospital Vaccination Center were included, while those who received a vaccine other than BNT162b2 or carried out the booster vaccination in a vaccination center outside the Hospital were excluded. Data about vaccinations of HCWs and nasal swabs positive for SARS-CoV-2 were extracted from the digital archives of the Hospital from October 2021 to May 2022. Risk of infection was estimated using the Nelson-Aalen cumulative risk function. In total, 6,103 HCWs were observed. The cumulative risk of becoming infected by SARS-CoV-2 increased with time (9.0% at 8 weeks, 25% at 16 weeks and 45% at 32 weeks) and was significantly higher for women compared to men (at 32 weeks 48% vs 41%) and younger classes of patients compared to elderly (at 32 weeks 55% vs 30%); no significant differences were detected according to occupational category and risk profile. Multivariate analysis by Cox regression confirmed the differences detected by the univariate analysis. This study describes a higher incidence of infections after the booster dose of COVID-19 vaccine among women and young people; moreover, the probability of being infected increases over time, as a results of the reduction in the neutralizing potency and titer of the antibodies: this poses the need for continuous monitoring of vaccine-induced immunogenicity, to establish the correct timing of vaccination with more effective vaccination schedules.