Recent models indicate seasonal influenza transmission in Brazil begins each year in the semiarid state of Ceará−before vaccine campaigns begin. To assess the extent and maternal−child health consequences of this misalignment, we examined the burden of severe acute respiratory infections (SARI) and influenza from 2013-2018. Of 3,297 SARI cases, 145 (4%) occurred in pregnancy. Vaccine coverage was >80%; however, campaigns often occurred during or after peak influenza activity. Mean birth weights and gestational age nadired 30-40 weeks following peak influenza and SARI activity. We identified 61 babies of mothers with gestational SARI; they weighed 10% less at birth (P = 0.019) and were more often premature (OR: 2.944 ; 95% CI: 1.100 − 7.879) relative to controls (n=122). Mistiming of influenza vaccination adversely impacts pregnancy and birth outcomes in Ceará, with critical implications for the first 1000 days of life and influenza transmission dynamics nationally.

R espiratory infections are a leading cause of disease and death worldwide (1,2), especially among young children and older adults. However, the adverse effects of respiratory infections on pregnant women and fetal development are understudied, particularly in low-and middle-income countries. Respiratory infections in pregnant women can negatively affect birth outcomes, early childhood growth, and neurodevelopment (3).Infl uenza epidemics are associated with excess rates of pneumonia, related hospitalizations, and death (4). Pregnant women and their infants are at heightened risk for severe infl uenza (5,6). In 2020, Regan et al. ( 7) conducted a retrospective cohort study of pregnant women from Australia, Canada, Israel, and the United States; results showed hospitalizations for acute respiratory or febrile illnesses were associated with low birthweight but not small-for-gestational-age births. A prospective cohort study of pregnant women in India, Peru, and Thailand showed infl uenza during pregnancy is associated with late pregnancy loss and reduced mean birthweight (8). A meta-analysis ( 9) found that during the 2009 pandemic of infl uenza A(H1N1)pdm09, the risk for infl uenza hospitalization was 2-fold higher for women who were pregnant than those who were not. Children born to mothers infected during pregnancy face potential adverse consequences for physical and neurocognitive development. These consequences resemble the growth and developmental challenges described in children born to undernourished mothers in global settings with high rates of pneumonia and diarrhea (10-14).In 2018, Almeida et al. (15) revealed that 12 of 27 states in Brazil demonstrate annual seasonal infl uenza activity. States along the coast generally have seasonal infl uenza patterns, whereas states in the North and Central West regions exhibit no readily identifi able seasonality, probably because landlocked states might have more complex and diffi cult to detect transmission patterns. In the semiarid state of Ceará, which has a population of ≈8.8 million persons, peak seasonal infl uenza