Efficacy of Maternal Influenza Vaccination Against All-Cause Lower Respiratory Tract Infection Hospitalizations in Young Infants: Results From a Randomized Controlled Trial

BACKGROUND: Influenza immunization of pregnant women protects their young infants against laboratory-confirmed influenza infection. Influenza infection might predispose to subsequent bacterial infections that cause severe pneumonia. In a secondary analysis of a randomized clinical trial (RCT), we evaluated the effect of maternal vaccination on infant hospitalizations for all-cause acute lower respiratory tract infection (ALRI). METHODS: Infants born to women who participated in a double-blind placebo-controlled RCT in 2011 and 2012 on the efficacy of trivalent inactivated influenza vaccine (IIV) during pregnancy were followed during the first 6 months of life. RESULTS: The study included 1026 infants born to IIV recipients and 1023 born to placebo recipients. There were 52 ALRI hospitalizations (median age, 72 days). The incidence (per 1000 infant-months) of ALRI hospitalizations was lower in infants born to IIV recipients (3.4 [95% confidence interval {CI}, 2.2–5.4]; 19 cases) compared with placebo recipients (6.0 [95% CI, 4.3–8.5]; 33 cases) with a vaccine efficacy of 43.1% (P = .050). Thirty of the ALRI hospitalizations occurred during the first 90 days of life, 9 in the IIV group (3.0 [95% CI, 1.6–5.9]) and 21 in the placebo group (7.2 [95% CI, 4.7–11.0]) (incidence rate ratio, 0.43 [95% CI, .19–.93]) for a vaccine efficacy of 57.5% (P = .032). The incidence of ALRI hospitalizations was similar in the IIV and placebo group for infants >3 months of age. Forty-four of the hospitalized infants were tested for influenza virus infection and 1 tested positive. CONCLUSIONS: Using an RCT as a vaccine probe, influenza vaccination during pregnancy decreased all-cause ALRI hospitalization during the first 3 months of life, suggesting possible protection against subsequent bacterial infections that influenza infection might predispose to. CLINICAL TRIAL REGISTRATION: NCT01306669.