According to this Japanese study, vitamin D supplementation could reduce influenza A incidence in Japanese schoolchildren. Schoolchildren (aged between 6 and 15 years) were divided into a test group (167 children) and a placebo group (167 children). The results showed an incidence of 10.8% in the test group and 18.6% in the placebo group. Among the involved asthma patients, the incidence of asthma attacks was also reduced after vitamin D supplementation. This study showed a significant reduction of influenza A incidence in the test group. Additionally, vitamin D supplementation prevented asthma attacks in asthma patients. More details can be found in the study.
Urashima et al.