Background: In August 2022, the vaccination campaign to counter the spread of monkeypox, defined by the WHO as a public health emergency, began in Italy, identifying laboratory personnel, and MSM with specific risk criteria as priority targets. In primary vaccinations, two doses of vaccine were administered intradermally while for booster vaccination, only one dose was administered. Our aim was to describe the adverse events following immunization (AEFIs) with this vaccine (MPXV) through participant-based active surveillance. Methods: From October 2022, the study was conducted in two Italian towns (Bologna and Forlì) population aged over 18 who received MPXV. E-questionnaire (Q1) dissemination occurred through QR-codes during the second vaccine dose appointment (or by e-mail one month after the booster dose). Participants will also be administered online questionnaires by e-mail at different points in time after the second dose: one month (Q2), and three months (Q3). A descriptive analysis was conducted to quantify AEFI incidence, stratifying Results by type and severity of symptoms. Results: Overall, 148 vaccine recipients were recruited in the first two months. Mean age was 36.2 ± 8.6 years. After the first dose, systemic reactions were reported by 32.8% of the participants (mostly asthenia), while local site reactions were reported by 97,1%. The most common local AEFIs were redness, swelling, and hitching at the injection site. Grade 3 or 4 AEFIs were reported - for local AEFIs only - by 16.8% of the participants. Recipients of subcutaneous vaccines (n=12) reported fewer and lower-grade local AEFIs. Conclusions: Our preliminary results suggest that MPXV has a high tolerability profile in terms of short-term common systemic AEFIs. The study will help collect data about the persistence of the frequent local AEFIs following intradermal administration.