In 2015 Grant and colleagues analyzed the supplementation of vitamin D to pregnant women and to their infants up to 6 months of age, comparing two regimens: 1000 IU to the mothers and 400 IU to the infants vs. 200–800 IU, and found a lower proportion of children made a primary care visit for respiratory infections up to 18 months of age in the higher dose group [163]. A 2018 Chinese study tested the efficacy of vitamin D in preventing influenza A, comparing a low dose scheme (400 IU/day) vs. a high dose one (1200 IU/day) for 4 months, and reported less frequent infections in the high dose group [167].