BACKGROUND During the COVID-19 pandemic the implementation of a range of measures to suppress transmission, such as social distancing and home confinement resulted in limited sunlight exposure and physical inactivity in people under age 18 years, which can elevate the risk of vitamin D deficiency and insufficiency. The aim of this study was to systemically evaluate the effect of the COVID-19 pandemic on serum vitamin D levels in people under age 18 years. MATERIAL AND METHODS Following the PRISMA recommendations, we searched PubMed, Embase, and the Cochrane Database for trials from inception to November 3, 2021. All trials assessing the effects of the COVID-19 pandemic on serum vitamin D levels in people under age 18 years were included and analyzed. Mean differences (MDs) of serum 25-hydroxyvitamin D (25[OH] D) levels before and during the COVID-19 pandemic were calculated and pooled using a random-effects model. Risk differences were used to assess changes in the proportions of people under age 18 years with vitamin D deficiency. RESULTS Our analysis included 5 studies comprising 4141 people under age 18 years. The combined result MD of serum 25(OH)D levels before and during the COVID-19 pandemic as 3.28 ng/mL, 95% CI=0.95-5.62 ng/mL, P<0.01] indicated serum 25(OH)D levels were significantly lower during the COVID-19 pandemic. The decreased serum 25(OH)D level was not observed among infants (age under 1 year) (P=0.28). CONCLUSIONS During the COVID-19 pandemic, the serum vitamin D levels of people under age 18 years were significantly lower and vitamin D supplementation for people under age 18 years might reduce the risk of COVID-19. More research is needed to validate the present findings.