Abstract: Journal of Infection xxx (xxxx) xxx–xxx Contents lists available at ScienceDirect Journal of Infection journal homepage: www.elsevier.com/locate/jinf Letter to the Editor Effect of calcifediol supplementation as add-on therapy on the immune repertoire in recipients of the ChAdOx1 nCoV-19 vaccine: A prospective open-label, placebo-controlled, clinical trial Dear Editor, Coronavirus disease 2019 (COVID-19) has caused formidable global health crisis, and multiple vaccination programs have been rolled out to curb the spread of the pandemic. Genetic disorders like primary immunodeficiency diseases severely compromise the gen­ eration of protective immunity following vaccinations. The spectrum also includes those genetic disorders that have a lesser impact on immunological memory such as Down Syndrome (DS). In a recent study, Valentini et al.1 confirmed that the serological efficacy of COVID-19 vaccination is least affected by DS. The authors included a substantial number of subjects and further observed that the older subjects exhibit lower anti-SARS-CoV-2 antibody titers 6 months after vaccination. Earlier, Yarci-Carrion et al.2 had also reported that DS and non-DS subjects develop similar anti-SARS-CoV-2 titers after the third dose of vaccine that decline with age. However, the same group demonstrated that DS subjects develop milder T cell responses to the standard 2-dose vaccine.3 Valentini et al.1 further denote that the factors modifying immune responses to vaccination in such disorders are less explored. This may include nutritional deficiencies like vitamin D [25(OH)D]. Indeed, reports have shown that hypovi­ taminosis D is quite frequent in subjects with DS.4 It is also known that individuals with low 25(OH)D levels have a higher risk of acquiring severe COVID-19 and stimulation of vitamin D receptor can reduce COVID-19-associated hospitalization and mortality.5,6 However, studies on vitamin D supplementation in COVID-19 vaccination have yielded conflicting results with some reporting improvement, while others observing lack of association between 25(OH)D status and antibody titers post vaccination.7,8 Most of these studies, however, did not assess cellular immune re­ sponses, which are more likely to be influenced by vitamin D. Cal­ cifediol [25(OH)D3], the immediate precursor of 1,25(OH)2D3 leads to a faster achievement of desired levels and biodistribution of cir­ culating 1,25(OH)2D3 as compared to cholecalciferol (vitamin D3).9 There are reports on favorable outcomes of calcifediol supple­ mentation in COVID-19,10 but there are no data on its utility in im­ proving efficacy of COVID-19 vaccines. Therefore, we evaluated the efficacy of calcifediol supplementation on cell-mediated immune responses besides anti-SARS-CoV-2 titers post COVID-19 vaccina­ tion. We carried an open-label, placebo-controlled clinical trial to assess the impact of calcifediol supplementation on the efficacy of ChAdOx1 nCoV-19 vaccine in terms of, protection from break­ through infection and COVID-19 disease, anti-SARS-CoV-2 antibody titers, SARS-CoV-2 specific lymphocyte proliferation and cytokine secretion. Herein 580 adult subjects receiving ChAdOx1 nCoV-19 vaccine were recruited at the Post Graduate Institute of Medical Education and Research (PGIMER), Chandigarh, India. The subjects received either calcifediol (50 µg/day) or placebo, orally for 6 months (Supplementary Fig. 1, Supplementary Table 1) and divided in 3 groups [SARS-CoV-2 unexposed,..