 Methods: Our study comprises two studies. Study 01 will be undertaken in infants while Study 02 for preschool children. At first we need to undertake one feasibility study to verify whether the oral dose of 400 μg 13C10 or 13C3- retinyl acetate isotopic VA that we propose to use in this study would work in infants as we observed in adults, secondly, whether PENTA valent vaccines induce inflammation in infants plasma CRP as maker that closely mimics natural infection. We need such infants with inflammatory condition in Study 01 to answer the question of whether the isotope dilution technique will still work in children who undergo inflammation. Information of the reliability of the retinol isotope dilution technique in infectious conditions is important as national dietary assessment surveys often observe children with chronic or latent infection. To minimize the taking of multiple blood samples from each infantchild in both studies, a super kid study design will be adopted that ensure no more than 2 venous blood samples from each infantchild, even though multiple time points at least 5 subjects time-point over a 28-day study period will be available for mathematical modelling. In Study 01 a total of 115 infants 9-11 mo will receive an oral dose of 13C10- retinyl acetate 400 μg dissolved in 0.5 mL of sunflower oil and then blood samples will be taken from 6h to 16th day at 9 different time-points. On day 16, a single dose PENTA vaccine will be provided to the randomly selected 50 of infants. On day 17, another type of stable isotopic VA e.g. 13C3- retinyl acetate 400 μg will be orally supplemented to all infants. Blood sample will be collected from day 16 to day 28 at 11 different time-points for each of the vaccine and non-vaccine groups. In Study 02 50 children 3-5y in each of the adequate dietary VA intake group and higher dietary VA intake group will be enrolled to receive an oral dose of 13C10- retinyl acetate 400 μg. Blood sample will be collected from 6h to day 28 at 11 time-points for each of the two groups. Specific activity of 13C10-VA and 13C3-VA from the blood samples will be measured by liquid chromatography-tandem mass spectrometry LCMSMS. Plasma biomarkers of VA toxicity e.g. bone metabolism, liver fibrogenesis and VA metabolites will be measured in the children of Study 02. 