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Objectives: To estimate the dose-dependent effects of maternal vitamin D3 (VD) supplementation during pregnancy and lactation on the risk of first nasal pneumococcal colonization (PnC) and the dynamics of nasal pneumococcal (Pn) carriage in infants from birth to 6 months (mos) of age in Dhaka, Bangladesh. Methods: Infants born to mothers who received weekly VD doses in one of 4 pre/post-partum dose combinations or placebo as part of the Maternal Vitamin D Supplementation during Pregnancy and Lactation to Prevent Acute Respiratory Illness (MDARI) study, were monitored from 0-6 mos for signs of an acute respiratory infection (ARI). A nasal swab was performed to obtain a sample of nasal epithelial tissue if the infant met pre-set ARI criteria. Nasal PnC was detected by qPCR. The effect of VD on the risk of first observed PnC was estimated by an interval-censored survival model. A multi-state model was used to characterize the dynamics of Pn carriage using transition probabilities of state changes. Results: Of 1060 infants in MDARI, PnC was observed in 90% during the first 6 mos of life with 69% of swabs being positive (n = 3792). There was no significant difference between any VD group and placebo on the risk of initial nasal PnC (Table 1). Overall, the predicted median time to colonization was 7 weeks. Transitioning between positive and negative episodes was common: among 949 infants who were ever positive, 238 (25%) were observed to revert to negative during the first 6 mos of life, of whom 99 (40%) had at least one additional positive swab. Across all groups, an infant was predicted to spend 69 days negative and 120 days positive during the first 6 mos; yet, no Pn dynamics differed significantly across treatment groups (P >0.05). PnC and Pn dynamics were associated with season of birth, parental education and number of siblings, but were not associated with infant characteristics including anthropometric parameters. Conclusions: These results do not provide evidence supporting an effect of VD on infants' risk of first nasal PnC nor the dynamics of carriage in a setting where early infant PnC is highly probable. In contrast to in vitro data that demonstrated that VD promoted immune responses against pneumococcus, the present study does not support the hypothesis that improvements in VD status in early infancy reduces the risk of Pn colonization. Funding Sources: Bill & Melinda Gates Foundation. Supporting Tables Images and/or Graphs:

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