Metformin use before COVID-19 vaccination and the risks of COVID-19 incidence, medical utilization, and all-cause mortality in patients with type 2 diabetes mellitus

•Type-2 diabetes mellitus aggravates COVID-19 complications and severity.•Metformin use before COVID-19 vaccination could not reduce disease incidence.•Metformin use decreased COVID-19 severity in fully vaccinated T2D cohort.•Metformin use reduced mortality following COVID-19 in fully vaccinated T2D patients.•Metformin use before COVID-19 vaccination may be useful in patients with T2D.