mportantly, our study is the first to utilise high daily doses for an extended period of one full year. Thus, we covered all four seasons, which was important in Sweden with a known seasonal variation in 25-hydroxyvitamin D3 levels.25 Two previous RCTs were performed during the winter season—when vitamin D levels are low—but only during 422 and 6 months,20 respectively. Previous RCTs have been conducted during shorter periods; 42 days,23 6 weeks26 and 12 weeks,21 respectively. Interestingly, we observed a clear time-dependent effect suggesting that a long-term supplementation approach (>6 months) may be necessary to affect immunity. To expand on the results of a previous study in healthy individuals where no difference between the intervention and placebo groups was observed,21 we chose a study population with frequent RTIs and at least 42 days with infection during the year prior to inclusion. Notably, patients in the study represent a selected group of individuals with frequent RTI, although the immune disorders that they represent (sIgAD, IgG-subclass deficiency and patients with no defined immune disorder) are generally mild in character and dominated by mucosal RTIs. We also included a small number of CVID-patients, which can be considered to be a more severe immune disorder, but all these patients are treated with IgG replacement therapy and thus well controlled. Hence, the results from this study cannot directly be applied to the general healthy population. Nevertheless, the results provide solid support for additional interventional studies of vitamin D3, especially in groups consuming large amounts of antibiotics.