Maternal immune response and neonatal seroprotection from a single dose of a monovalent nonadjuvanted 2009 influenza A(H1N1) vaccine: a single-group trial.

Background: Pregnant women and infants who get influenza are at increased risk for severe illness.

Objective: To evaluate the immunogenicity and transplacental antibody transfer of 2009 pandemic influenza A(H1N1) vaccine administered during pregnancy.

Design: Prospective, multicenter, single-group clinical trial. (ClinicalTrials.gov registration number: NCT01024400)

Setting: Five level-3 perinatal centers in France.

Patients: 107 pregnant women between 22(0/7) and 32(0/7) weeks of gestation.

Intervention: An intramuscular dose of a nonadjuvanted H1N1 vaccine that contained 15 mcg of hemagglutinin.

Measurements: Proportion of women with an influenza antibody titer of 1:40 or greater at days 21 and 42 after vaccination, delivery, and 3 months after delivery. Seroconversion rate, fold increase in the geometric mean titer 21 days after vaccination, and proportion of neonates with an antibody titer of 1:40 or greater at birth were also assessed.

Results: At baseline, 19% of the women had an antibody titer of 1:40 or greater. At day 21, 98% of the women had an antibody titer of 1:40 or greater, the seroconversion rate was 93%, and the fold increase in geometric mean titer was 67.4. At day 42, delivery, and 3 months after delivery, 98%, 92%, and 90% of the women, respectively, had an antibody titer of 1:40 or greater. Ninety-five percent of the cord serum samples obtained from 88 neonates showed an antibody titer of 1:40 or greater. The median neonate-mother antibody titer ratio was 1.4.

Limitations: Only healthy pregnant women were selected. Data on hemagglutination inhibition antibody titers of infants were reported only at birth.

Conclusion: A single dose of a nonadjuvanted influenza A(H1N1) vaccine with 15 mcg of hemagglutinin triggered a strong immune response in pregnant women and a high rate of neonatal seroprotection.

Primary Funding Source: French National Institute of Health and Medical Research.