Background: Pregnant women and young infants are at increased risk for severe influenza-illness. Influenza vaccination during pregnancy may confer indirect protection against influenzaillness to infants < 6 months of age, for whom there is no licensed influenza vaccine. A co-primary objective of our study was to evaluate the efficacy of trivalent inactivated influenza vaccine (IIV3) when given during pregnancy against PCR-confirmed influenzaillness (PCI) in their infants up until 6 months of age. Methods & Materials: We conducted a double-blind, randomized, placebo-controlled trial in Soweto, South Africa during the 2011 and 2012 influenza seasons. 2116 HIV-uninfected women were randomized to receive IIV3 or placebo intramuscularly between 20-36 weeks gestation. Infants were screened weekly for influenza-like-illness (ILI) after birth. When ILI was suspected nasopharyngeal aspirates were collected for influenza A and B testing by real-time PCR. Influenza A viruses were subtyped for A/H1N1pdm09 and H3N2 and influenza B isolates were tested for Yamagata and Victoria lineages. Results: Pregnancy outcomes were similar between the IIV3 and placebo-group, including 8 miscarriages and 22 stillbirths. 2048 babies were born alive, 1025 and 1023 to mothers who received IIV3 and placebo, respectively. The rates of preterm deliveries (10%) and low birth weight (12.4%) were also similar between the study groups. 706 and 698 illness visits were completed in infants of mothers who received IIV3 and placebo, respectively; of which 665 (64.9%) and 656 (64.1%) illness episodes fulfilled ILI criteria. In infants born to IIV3-recipients, 19 PCI cases were detected compared to 37 in infants born to placebo-recipients, resulting in vaccine efficacy of 48.8% (95%CI: 11.5% to 70.3%) against PCI in infants. Excluding the non-vaccine influenza B/Yamagata cases from the analysis vaccine efficacy (VE) did not change (VE: 48.1%; 95%CI: -0.9% to 73.3%). In twelve (21.4%) of the 56 PCI cases among infants (including 11 whose mothers received placebo), PCI was also identified concurrently in the mother. Conclusion: Vaccination of African pregnant women with IIV3 was safe and reduced PCI in their infants by 49% up until 6 months of age. Immunization of pregnant women with IIV3 is warranted for the protection of their young infants.
Madhi, S., Cutland, C., Jones, S., Hugo, A., Treurnicht, F. K., Kuwanda, L., … Nunes, M. S. C. (2014). Randomized, placebo-controlled trial on safety and efficacy of inactivated influenza vaccination of pregnant women in preventing illness in their infants. International Journal of Infectious Diseases, 21, 32. https://doi.org/10.1016/j.ijid.2014.03.480