Background and objective:
The pandemic caused by the SARS-CoV-2 virus still being a major public health issue, causing significant death tolls and straining national health systems. Characterized by innovative technologies (mRNA for Spikevax and Comirnaty, viral vector for Jcovden and Vaxevria), new vaccine products have been developed, approved, distributed and, starting from December 2020, given primarily to Healthcare Workers (HCWs) in countries such as Italy and USA. The aim of this systematic review is to define the term of protection offered by the administration of EMA (European Medicines Agency)-approved COVID-19 vaccine booster dose for healthcare professionals, considering the Vaccine Efficacy/Effectiveness (VE) against SARS-CoV-2 infections.

Methods:
A systematic research of the literature was carried out using the PICO (Population, Intervention, Comparison, Outcomes) model and following the recommendations of the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines.

Results:
Starting from 1241 obtained, 4 studies met the inclusion criteria. All studies dealt with only first booster dose. Considering the different viral variants reported, VE against infection was estimated, with Comirnaty booster dose, between 50% (95% C.I. 42–56%) and 85,6% (95% C.I. 79,2–90,1%) and, with Spikevax booster dose, between 56% (95% C.I. 45–65%) and 96% (95% C.I. 82–99%).

Conclusions:
The results of this systematic review show high VE levels among HCWs in the prevention of SARS-CoV-2 outcomes of infections, which however rapidly decline. Hence, the periodic administration of additional booster vaccine doses, could maintain the percentages previously recorded.