Background: Coronavirus (COVID-19) disease portends a poor prognosis in patients with type 1 diabetes (T1D) . As a consequence, the booster dose of Covid-vaccination should be prioritized in these patients. Nonetheless, concerns exist about vaccine-induced dysglycemia. Objectives: Aim of this study was to assess the short-term effects of booster dose of Covid-vaccination on glycaemic control assessed by flash glucose monitoring (FGM) in people with T1D. Methods: In this observational cross-sectional study we investigated changes in daily insulin requirement (IR) and glycaemic control between 7 days before and 7 days after the third dose of vaccination with BioNTech Pfizer among 30 individuals with T1D on multiple daily insulin injections wearing a flash glucose monitoring (FGM) device. The following parameters of glycaemic variability were analysed: mean glucose, time in range (TIR) , time above range (TAR) , time below range (TBR) and coefficient of variation (CV) . Results: No significant differences were found for mean glycemia, TIR, TAR and TBR over the course of the vaccination from 7 days prior to receiving the third-dose vaccination until 7 days after. Nonetheless, CV and IR were significantly higher (CV, p-value = 0.001; IR, p-value = 0.05) in the week after the vaccination compared to the week earlier. The median value of CV was 35.5% [33-37] before and 36.9% [34-39] after the booster dose, whereas the median value of IR changed from 0.55 UI/Kg/day to 0.61 UI/Kg/day. Conclusions: Our study suggests that the booster dose of Covid-vaccination impact on glycaemic variability and insulin requirement in people with T1D, probably due to the pro-inflammatory cytokines and immune responses. While this observation should be investigated in larger studies, potential glycaemic aberrations in response to Covid-immunization should be considered by health care professionals and glucose monitoring intensified within the days around the vaccination. Disclosure S.Pieralice: None. L.Monte: None. G.Defeudis: None. G.Tabacco: None. N.Napoli: Advisory Panel; Novo Nordisk, Consultant; Lilly Diabetes. P.Pozzilli: Advisory Panel; Dompé, Consultant; Abbott, Speaker’s Bureau; Dompé, Lilly Diabetes, Medtronic, Sanofi.