COVID-19(9) Result(7) pandemic(7) severe COVID-19(6) Vitamin(6) infections(6) Patient(6) COVID-19 patient(5) coronavirus(5) cause(5) control group(4) COVID-19 group(4) Hospitalized(4) two group(4) Comorbidity(4) COVID-19 pathogenesis(4) Immune homeostasis(4) retinoic acid(4) indication(4) was measured(4) serum(4) severe COVID-19 patient(4) Immunopathogenesis(4) no significant difference(4) significantly lower(4) Respiratory failure(4) Gender(4) statistically significant(4) RSV(4) vitamin A(4)