Retrospective Observational Study on the Risk in Developing Long-COVID Syndrome and in Contracting SARS-CoV-2 Infection After COVID-19 Vaccination in Relation to Vitamin D3 Supplementation

Inclusion Criteria (For aim I): * age ≥18 * outpatient with first visit on Endocrinology Department performed from 01/01/2020 to 31/12/2022 * previous SARS CoV-2 infection * clinical,anamnestic and therapeutic data 6 months post SARS CoV-2 infection * data on cholecalciferol supplementation * signature of informed consent Exclusion Criteria (For aim I): * age ≤ 18 * pregnant or breastfeeding women * patients suffering from primary and/or acquired immunodeficiency and/or severe impairment of general clinical conditions * patients treated with drugs that interfere with vitamin D metabolism * first vaccination anti Sars COVID performed before the acute infection * first vaccination performed within 6 months of the acute infection * patients unable to sign the informed consent Inclusion Criteria (For aim 2) \-- age ≥18 * outpatient with first visit on Endocrinology Department performed from 01/01/2020 to 31/12/2022 * vaccination with BNT162b2 Biontec mRNA Pfizer/BioNtech * clinical, anamnestic and therapeutic data 6 months post SARS CoV-2 infection * data on cholecalciferol supplementation * signature of informed consent Exclusion Criteria (For aim 2): * age ≤ 18 * pregnant or breastfeeding women * patients suffering from primary and/or acquired immunodeficiency and/or severe impairment of general clinical conditions * patients treated with drugs that interfere with vitamin D metabolism * additional vaccination performed before 6-12 months * acute infection before the vaccination * patients unable to sign the informed consent