Background There are conflicting data concerning the impact of antenatal influenza vaccination on birth outcomes including low birthweight (LBW), preterm birth, small for gestational age (SGA), and stillbirth. Methods We conducted a retrospective observational cohort study of infants born to women residing in Mitchells Plain, Cape Town. Infants were born at 4 health facilities during May 28 – December 31, 2015 and April 15 – December 31, 2016. We performed crude and multivariable logistic regression, propensity score (PS) matching logistic regression, and inverse probability of treatment weighted (IPTW) regression to assess vaccine effectiveness (VE) against LBW, preterm birth, SGA, and stillbirth adjusting for measured confounders. Results Maternal vaccination status, antenatal history, and ≥1 birth outcome(s) were available for 4084/5333 (76.6%) pregnancies, 2109 (51.6%) vaccinated, and 1975 (48.4%) unvaccinated. The proportion LBW was lower in vaccinated (6.9%) vs. unvaccinated (12.5%) in multivariable [VE 0.27 (95% CI 0.07‐0.42)], PS [VE 0.30 (95% CI 0.09‐0.51)], and IPTW [VE 0.24 (95% CI 0.04‐0.45)]. Preterm birth was less frequent in vaccinated (8.6%) than unvaccinated (16.4%) in multivariable [VE 0.26 (0.09‐0.40)], PS [VE 0.25 (95% CI 0.09‐0.41)], and IPTW [VE 0.34 (95% CI 0.18‐0.51)]. The proportion SGA was lower in vaccinated (6.0%) than unvaccinated (8.8%) but not in adjusted models. There were few stillbirths in our study population, 30/4084 (0.7%). Conclusions Using multiple analytic approaches, we found that influenza vaccination was associated with lower prevalence of LBW (24‐30%) and preterm birth (25‐34%) in Cape Town during 2015‐2016.

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Influenza vaccination of pregnant women reduces influenza-associated illness in the women and their infants, but effectiveness estimates against influenza-associated hospitalization are limited and are lacking from settings with high maternal HIV-infection prevalence. We assessed the effect of maternal vaccination in HIV-uninfected women and women living with HIV in preventing influenza-associated hospitalizations in infants aged <6-months and in the women. During 2015-2018, influenza vaccination campaigns targeting pregnant women were augmented at selected antenatal clinics in two South African provinces; these were coupled with ongoing prospective hospital-based surveillance for acute respiratory or febrile illness in infants <6-months-old and cardio-respiratory illness among pregnant or post-partum women. Vaccine effectiveness (VE) was assessed using a test-negative case-control study. Overall, 71 influenza-positive and 371 influenza-negative infants were included in the analysis; mothers of 26.8% of influenza-positive infants were vaccinated during pregnancy compared with 35.6% of influenza-negative infants, corresponding to an adjusted VE (aVE) of 29.0% (95%CI: -33.6, 62.3). When limited to vaccine matched strains, aVE was 65.2% (95%CI: 11.7, 86.3). For maternal hospitalizations, 56 influenza-positive and 345 influenza-negative women were included in the analysis, with 28.6% of influenza-positive women being vaccinated compared with 38.3% of influenza-negatives, for an aVE of 46.9% (95%CI: -2.8, 72.5). Analysis restricted to HIV-uninfected women resulted in 82.8% (95%CI: 40.7, 95.0)) aVE. No significant aVE (-32.5%; 95%CI: -208.7, 43.1) was detected among women living with HIV. Influenza vaccination during pregnancy prevented influenza-associated hospitalizations among young infants when infected with influenza vaccine strains and among women without HIV, although no effect was detected in women living with HIV.

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