Vitamin D Reduces Risk of Respiratory Tract Infection

COVID-19 infection shows similar properties to the Severe Acute Respiratory Syndrome (SARS) and Middle Eastern Respiratory Syndrome (MERS)(2). However, from an epidemiology perspective, COVID-19 is far more contagious than SARS-CoV and MERS-CoV, with confirmed cases far exceeding the latter two combined(2, 56). COVID-19 involves respiratory tract illness ranging from mild, moderate to severe forms(57).
Research has shown that vitamin D deficiency is a predisposing factor in respiratory tract infection (RTI)(58). RTI affect the majority of the world population. It is also the one of the main causes for hospital admission as it can progress into moderate to severe pneumonia(59, 60). One of the main predisposing factors for developing RTI is partial or full impairment of the immune system. Vitamin D is recognised to regulate innate and adaptive immune system by stimulating the production of antimicrobial peptides in the event of viral or bacterial infections(28). Several studies have concluded a positive correlation between low serum level of vitamin D with a higher incidence and severity of lower respiratory infection in both adults and children(61-65). There are also studies which showed that vitamin D supplement helps to lower the rate of development of lower respiratory tract infection, especially in winter when there is a scarcity of sunlight(66, 67). However, there are some studies which showed no beneficial effects of short-term bolus doses of vitamin D supplement on the incidence and resolution of pneumonia(68-70). The mixed findings may be explained by different ages at study enrolment and severity of pneumonia, with children at the younger extreme of age having not fully developed immune system associated with poorer outcomes(61). A large European study has also shown that correcting serum level of 25(OH)D has beneficial effects on prevention of RTI and reducing disease severity in adults(71). A randomised controlled trial in Japan looking at the effects of vitamin D supplement on prevention of seasonal influenza A showed that taking 1200 international units (IU) of vitamin D per day has a 58% relative risk reduction of influenza A incidence in school children compared to the placebo group(72).
Epidemiological and observational studies have linked vitamin D deficiency with the spread of RTI and other infectious diseases(63, 73-76). It has been seen that vitamin D deficiency is common among children below the age of five, which coincides with the prevalence of lower respiratory tract infection (LRTI), respiratory syncytial virus (RSV), and related human metapneumovirus (hMPV) infections(77). Reviews of serum vitamin D levels among the paediatric population in America shows a correlation between low serum vitamin D with LRTI and RSV severity(78). Another large scale study of the paediatric population in Middle Eastern countries further showed that children who have a low serum level of vitamin D are usually tested positive for RSV infection(79). There are studies which suggested that vitamin D deficiency is an important predisposing factor for developing community-acquired pneumonia and stroke-associated pneumonia (SAP)(80). A low serum level of vitamin D is associated with poorer disease progression in all types of acquired pneumonia including viral pneumonia, streptococcal pneumonia, and Legionella pneumonia(81). Granulomatosis with polyangiitis is an autoimmune disease characterised by vasculitis that primarily affects the respiratory tract and the kidneys. Vitamin D deficiency has been shown to be an important risk factor for RTI in this group of patients(82). The effects of vitamin D on lung immunity and respiratory diseases are well established(83).