Evaluating the Protective Effects of Influenza Vaccination in Pregnant Women and Their Infants: An Umbrella Review.

Background: Maternal influenza immunization is a primary strategy for protecting mothers and infants under six months, though its comprehensive efficacy and safety profiles undergo continuous evaluation. However, the reliability of current evidence is moderated by varying degrees of primary study overlap across existing reviews. Methods: Five electronic databases-PubMed, Embase, Web of Science, ProQuest, and Scopus-were systematically searched up to August 2024. Study eligibility and quality were assessed using the ROBIS tool. To ensure the integrity of the findings and address potential primary study overlap, the Corrected Covered Area (CCA) formula was applied. Results: Eleven systematic reviews and meta-analyses were evaluated. Maternal influenza vaccination may reduce the risk of laboratory-confirmed influenza (LCI) in both mothers and infants; however, no clear effect was observed on influenza-like illness (ILI). Vaccination is generally associated with a reduced risk of fetal mortality and no consistent evidence suggests a significant increase in congenital anomalies or spontaneous abortion. Maternal influenza vaccination may modestly reduce preterm birth risk, shows no clear effect on small for gestational age, and is associated with reduced low birth weight (LBW). The studies on influenza vaccination in pregnant women showed high overlap for LCI (0.66), infant LCI (0.50), and varying overlap for stillbirth (0.38), congenital anomalies (0.28), spontaneous abortion (0.23), premature birth (0.13), SGA (0.27), and LBW (0.14). Conclusion: Influenza vaccination during pregnancy effectively reduces LCI in mothers and infants without increasing adverse neonatal outcomes, though its impact on ILI remains inconsistent. Due to high study overlap and variable quality, further large-scale research is required to confirm effects on preterm birth and congenital anomalies.