A double-blind study to evaluate the role of human microbiome and vitamin D in the development of long COVID and PACS in children.

Children worldwide are at risk of SARS-CoV-2 infection because of a lack of approved vaccines for children aged 0-4 years. Moreover, SARS-CoV-2 infected children also suffered with long term sequels of virus infection, which involved multiple organs, such as fatigue, post-exercise malaise, skeletal muscular pains, headache, palpitation and insomnia. In fact, there is limited evidence available on the long-term impact of SARS-CoV-2 infection in children. Recent studies have shown critical-ill COVID-19 patients suffered with low vitamin D concentration and microbiome dysbiosis in their respiratory and gastrointestinal system. Vitamin D has been known to counteract several respiratory virus infections as well as beneficial functions in multiple organs. Also, commensal microbiota in lung and intestinal tracts exert protective functions against virus infections and, through its metabolite and axis links, has anti-inflammatory actions and homeostasis in multiple organs. Hence, in this study, the investigators hypothesis that long COVID or post-acute COVID syndrome (PACS) in children is due to the effect of post-virus infection on the immuno-metabolism change (vitamin D deficiency) and perturbation of gut microbiota (microbiome dysbiosis), therefore our study aims are first, make the comparisons of vitamin D levels and respiratory and gut microbiome between symptomatic and non-symptomatic post-COVID children using cross-sectional study. Next, for interventional study, patients will be divided in two groups to receive supplementation of vitamin D or placebo for 6 months to evaluate the effect of vitamin D on the symptoms relieve and improvement of microbiome dysbiosis in post-acute COVID syndrome (PACS) children. The investigators expect through this study, the investigators can learn more on the pathogenesis and the effect of vitamin D and microbiota in long COVID and PACS in children.

Inclusion Criteria:

1. Children aged 0-18 years
2. The child sought/needed primary or secondary medical care for COVID-19
3. Laboratory (RT-PCR, COVID-19 antigen tests or SARS-CoV-2 antibody testing) or physician confirmed SARS-CoV-2 infection based on classic clinical symptoms and/or ground-glass opacification on CT imaging.
4. 28 days - 3 months from the onset of COVID-19 symptoms
5. Parent's/carer's/guardians consent to participate

Exclusion Criteria:

1. Recruit patients who have used antibiotics, systemic steroids, and immunosuppressants in the previous month.
2. Patients with C1 esterase inhibitor deficiency, lymphocytopenia, thrombocytopenia, severe diseases involving heart, liver, or kidney, metabolic disease, or autoimmune disease.