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.