The goal of this study is to assess the effectiveness of vitamin D in reducing laboratory-confirmed influenza and in reducing non-influenza viral respiratory tract infections. A cohort of children between the ages of 3 and 17 years from the Thanh Ha Commune, Thanh Liem District, Ha Nam Province, Vietnam will be randomized to either weekly vitamin D supplements or placebo. Participants who develop acute respiratory infection over a 12-month period, will be tested for influenza, the co-primary outcome, and other respiratory viruses, the other co-primary outcome, by RT-PCR.