<p>Primary data.</p>

The presence of maternal cells in the offspring at birth, a phenomenon known as maternal microchimerism, has been previously associated with decreased odds of malaria and respiratory infections in early childhood suggesting a role in immunological responses to infections. Here, we assess the effect of cord blood maternal microchimerism on symptomatic non-malarial infections in Tanzanian children. We conducted a secondary analysis using a nested birth cohort of 52 children from Muheza, Tanzania, with previously measured cord blood maternal microchimerism and longitudinal records on infections in the first four years of life. The associations between maternal microchimerism and symptomatic lower and upper respiratory tract infections, diarrhea, and non-malarial fever were estimated using generalized estimating equation models. In total, 29% of the 52 children in the study screened positive for cord blood maternal microchimerism. Detected versus non-detected maternal microchimerism was associated with 58% lower odds of non-malarial fever (fully adjusted odds ratio (OR): 0.42 [95% CI: 0.18-0.98]) and 28% lower odds of respiratory tract infection (OR: 0.72 [95% CI: 0.53-0.96]). Lower and upper respiratory tract infections contributed equally to the observed association with any symptomatic respiratory tract infections (ORs respectively: 0.81 [95% CI: 0.50-1.31] and 0.71 [95% CI: 0.50-1.01]). We did not find any association between maternal microchimerism and odds of diarrhea (OR: 1.63 [95% CI: 0.85-3.13]). Detectable cord blood maternal microchimerism was associated with lower odds of non-malarial fever and symptomatic respiratory infections in Tanzanian infants. These findings emphasize that MMc may play an underrecognized role in protection from infection during early childhood.