Maternal and Neonatal Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Immunoglobulin G Levels After the Pfizer-BioNTech Booster Dose for Coronavirus Disease 2019 (COVID-19) Vaccination During the Second Trimester of Pregnancy

OBJECTIVE: To evaluate maternal and neonatal severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) immunoglobulin G (IgG) antibody levels at birth after a third (booster) dose of the Pfizer-BioNTech messenger RNA (Pfizer) coronavirus disease 2019 (COVID-19) vaccine during the second trimester of pregnancy, and compare them with those in women who received two vaccine doses during the second trimester. METHODS: We conducted a prospective cohort study of women admitted to the delivery ward at a single center who received the third Pfizer COVID-19 vaccine dose (booster group) at 17-30 weeks of pregnancy and who did not have previous SARS-CoV-2 infection. Maternal and neonatal antibody levels were measured on admission for delivery and in the umbilical cord blood after birth. Antibody levels for the booster group were compared with those in a historical control group of pregnant women who received their second vaccine dose (two-dose group) within the same gestational age window. RESULTS: Between October 2021 and February 2022, antibody levels were measured in 121 women and 109 neonates at a mean +/- SD of 15.3 +/- 3.9 weeks after booster vaccination. Neonatal titers measured two times higher than maternal titers, with inverse correlation between maternal and neonatal titers at birth and time interval from third vaccination. The two-dose group included 121 women and 107 neonates, with antibody levels measured at a mean +/- SD of 14.6 +/- 2.6 weeks after the second dose. Median [interquartile range] maternal antibody titers were higher in the booster group (4,485 [2,569-9,702] AU/mL) compared with the two-dose group (1,122 [735-1,872] AU/mL) (P<.001). Furthermore, neonatal antibody titers were higher in the booster group (8,773 [5,143-18,830] AU/mL) compared with the two-dose group (3,280 [2,087-5,754] AU/mL) (P<.001). CONCLUSION: Maternal and neonatal SARS-CoV-2 IgG antibody titers after second-trimester maternal Pfizer COVID-19 vaccination were significantly higher after the booster dose compared with the two-dose vaccination series. Although there is uncertainty as to whether antibody levels correlate with protection, these data support the importance of booster vaccination during pregnancy to restore maternal and neonatal protection against COVID-19.