Association between COVID-19 Infection/Outcomes and Vitamin D Deficiency: A Systematic Review and Meta-Analysis

Background: Protecting from coronavirus disease which is reported to become more contagious with its mutation, reducing the risk of severity of the disease, and consequently the mortality rate are current vital problems. The main purpose of this study is to indicate the possible effect of low serum vitamin D (25(OH)D<20 ng/mL and 50 nmol/L) on the Covid-19 infection and outcomes.Methods: This study is a systematic review and meta-analysis of the effects of vitamin D deficiency on Covid-19 infection and outcomes. To accomplish our objectives, we searched on Google Scholar, PubMed, Scopus, Web of Science, and ScienceDirect databases without any language restrictions for articles between 01.01.2020 and 15.12.2020. We included all the available research articles and studies. We excluded article types that were review, reply, and letter along with research articles that had inappropriate or inadequate results for the quantitative analysis. We extracted the data using a standardized data format from studies that gave the number of cases related to vitamin D level on Covid-19 infection and/or outcomes as a cross-tabulated table. To show the pure effect of vitamin D deficiency on the examined outcomes, we did not use the estimates of Odds Ratio (OR) value obtained from a statistical model in the included studies. We calculated OR values by using the cross-tabulated data and performed three meta-analyses to combine the OR values by paying attention to laboratory measurement units for vitamin D and the measured serum 25 (OH) D level.Findings: 23 eligible studies involving the relationship between vitamin D and Covid-19 infection/outcomes were identified (n=206861). We applied three meta-analyses called D-CIMA (assessed in eight studies, n=202561), D-CSMA (assessed in ten studies, n=2332) and D-CMMA (assessed in six studies, n=1397) for Covid-19 infection, severity, and mortality, respectively. According to obtained result from D-CIMA, one which has serum 25 (OH) D level below 20ng/mL or 50nmol/L are 1.64 times (95%CI=[1.32-2.04], p<0.001, [[EQUATION]] =85.4%) more likely to get Covid-19 infection. In D-CSMA, we found that people with the serum 25 (OH) D level below 20ng/mL or 50nmol/L have 2.58 times (95% CI=[1.28-5.19], p=0.008, [[EQUATION]] =91.5%) more likely to risk having severe Covid-19. We obtained from D-CMMA that low vitamin D level has no effect on Covid-19 mortality (OR=2.42 95% CI=[0.73-8.04], p=0.148, [[EQUATION]] =92.3%).Interpretation: Vitamin D deficiency increases the risk of Covid-19 infection and the potential for the severity of the disease. Therefore, vitamin D supplements should be added to prevention and treatment protocols for Covid-19 disease. There are insufficient data to prove its effect on mortality.Funding Statement: There was no funding source for this study.Declaration of Interests: The authors declare there is no conflict of interests.