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).