LAUSR

We gratefully acknowledge Speeckaert and colleagues for the interest in the results of our study. Different viewpoints on the potential immune-modulating properties of vitamin D have been recently published, but our study is the first providing data supporting the role of vitamin D in the prevention of severe acute respiratory syndrome coronavirus 2 infection in patients with a preexisting neurological condition. On the other hand, we recognize that the lack of information on the real vitamin status of the patients is a major limitation as supplementation data could only partly account for it. Unfortunately, the strict lockdown strategies imposed by the governments along with emergency faced in our hospitals did not enable to design a study including blood drawings in the community setting. Indeed, to clarify a potential preventive role of vitamin D status, future studies must consider a high-quality and standardized assessment of serum 25-hydroxyvitamin D alongside other important confounders such as dietary habits and exposure to sunlight. Two recent studies have indeed considered the issue of serum 25-hydroxyvitamin D in coronavirus disease 2019 patients. One study found significant negative correlations between the average vitamin D levels of different European countries and the national prevalence of coronavirus disease 2019 cases. Importantly, another age-stratified study in Swiss patients found significantly lower vitamin D levels at the individual level in severe acute respiratory syndrome coronavirus 2 C-reactive protein-positive versus C-reactive protein-negative cases. On the other hand, the analytical performance of the different assays has been object of criticism and has been demonstrated to be an important source of heterogeneity in the results, for example, of studies addressing the difference in 25-hydroxyvitamin D between patients with Parkinson’s disease and matched controls. Nonetheless, we would like to emphasize that the useful considerations provided by Speeckaert and colleagues do apply to all future studies in this area and not only to trials addressing the value of vitamin D supplementation in patients with Parkinson’s disease in the prevention of coronavirus disease 2019.