SARS-CoV-2 Vaccine Booster Elicits Robust Prolonged Maternal Antibody Responses and Passive Transfer Via The Placenta And Breastmilk

Abstract

Background: Infection during pregnancy can result in adverse outcomes for both pregnant persons and offspring. Maternal vaccination is an effective mechanism to protect both mother and neonate into post-partum. However, our understanding of passive transfer of antibodies elicited by maternal SARS-CoV-2 mRNA vaccination during pregnancy remains incomplete.

Objective: We aimed to evaluate the antibody responses engendered by maternal SARS-CoV-2 vaccination following initial and booster doses in maternal circulation and breastmilk to better understand passive immunization of the newborn.

Study design: We collected longitudinal blood samples from 121 pregnant women who received SARS-CoV-2 mRNA vaccines spanning from early gestation to delivery followed by collection of blood samples and breastmilk between delivery and 12 months post-partum. During the study, 70% of the participants also received a booster post-partum. Paired maternal plasma, breastmilk, umbilical cord plasma, and newborn plasma samples were tested via enzyme-linked immunosorbent assays (ELISA) to evaluate SARS-CoV-2 specific IgG antibody levels.

Results: Vaccine-elicited maternal antibodies were detected in both cord blood and newborn blood, albeit at lower levels than maternal circulation, demonstrating transplacental passive immunization. Booster vaccination significantly increased spike specific IgG antibody titers in maternal plasma and breastmilk. Finally, SARS-CoV-2 specific IgG antibodies in newborn blood correlated negatively with days post initial maternal vaccine dose.

Conclusion: Vaccine-induced maternal SARS-CoV-2 antibodies were passively transferred to the offspring in utero via the placenta and after birth via breastfeeding. Maternal booster vaccination, regardless of gestational age at maternal vaccination, significantly increased antibody levels in breastmilk and maternal plasma, indicating the importance of this additional dose to maximize passive protection against SARS-CoV-2 infection for neonates and infants until vaccination eligibility.

Figure 1:. SARS-CoV-2 initial vaccination regimen and the booster result in robust RBD-specific IgG antibody response in maternal plasma.

(A) Experimental design to investigate the impact of maternal SARS-CoV-2 vaccination on passive transmission of RBD-specific IgG antibodies by assessing antibody titers in maternal plasma, UCB, newborn plasma, and breastmilk. (B) RBD-specific IgG antibody titers in maternal plasma relative to days post first vaccination (n=370 samples). (C) IgG isotypes titers in maternal circulation 163.53 ± 14.28 days post first vaccine and pre-booster, (n=15). (D) RBD-specific IgG antibody titers 50.59 ± 4.46 days before and 55.74 ± 4.14 days after booster dose (n=77 pairs). (E) RBD-specific IgG antibody titers in maternal plasma relative to days post booster dose (n=112). (F) IgG isotype levels 84.07 ± 12.34 days before and 58.47 ± 8.98 days after the booster dose (n=15 pairs). (G) Isotype analysis of maternal plasma 58.13 ± 8.87 days post booster (n=16). (H-I) Average RBD-specific IgG antibody titers post-partum in women who have not received the booster (H) (Prepreg, n=0; T1, n=26; T2, n=57; T3, n=64; Postpartum, n=40) and (I) who have received the booster (Prepreg, n=30; T1, n=16; T2, n=43; T3, n=30; Postpartum, n=12) classified by trimester of initial vaccination. Bar graphs show median values with the standard error of the mean (SEM). * indicates a significant difference between the antibody levels at that timepoint when comparing pre- and post-booster groups (panel H vs. panel I). * p < 0.03 ** p < 0.002, *** p < 0.0002, **** p<0.0001.

Figure 2:. SARS-CoV-2-specific IgG antibody titers in breastmilk correlate with those in maternal circulation.

(A) RBD-specific IgG levels in breastmilk after the first and second vaccine doses (n=179). (B) IgG isotype levels in breastmilk 174.33 ± 10.08 days after the first and second vaccine doses, (IgG1, n=26; IgG2, n=22; IgG3, n=24; IgG4, n=22). (C) Breastmilk IgG levels 37.84 ± 3.80 days prior to and 55.32 ± 5.30 days after booster (n=45 pairs). (D) RBD-specific IgG antibodies in breastmilk after maternal booster vaccination (n=123). (E) Levels of RBD specific IgG isotypes in breastmilk 57.50 ± 8.17 days before (n=28) and 117.23 ± 11.32 days after the booster dose (n=44). (F) IgG isotype detection of SARS-CoV-2 RBD specific antibodies in breastmilk 115.10 ± 11.46 days after booster dose (n=43). (G-H) Average IgG titers post-partum in women who had not received the booster dose (G) (Prepreg, n=0; T1, n=15; T2, n=52; T3, n=51; Postpartum, n=39) and (H) who had been boosted (Prepreg, n=26; T1, n=14; T2, n=38; T3, n=21; Postpartum, n=12) classified by trimester of initial vaccination. * indicates a significant difference between the antibody levels at that timepoint when comparing pre- and post-booster (panel G vs H). Data are median values ± SEM. (I) Correlation between RBD-specific IgG levels in breastmilk and maternal plasma post-partum (Delivery, n=24; 6 weeks (6wpp), n=60; 3 months (3mopp), n=59; 6 months (6mopp), n=48; 9 months (9mopp), n=32 and 12 months (12m), n=19). * p < 0.03, ** p < 0.002, *** p < 0.0002, **** p<0.0001.

Figure 3:. Maternal IgG antibodies generated in response to vaccination are detected in umbilical cord plasma.

(A) RBD-specific IgG titers in maternal circulation and umbilical cord plasma at delivery (n=45 pairs). (B) Correlation of peak levels of RBD-specific IgG antibodies in maternal circulation pre-delivery and UCB (n=41). (C) Comparison of antibody levels in UCB and maternal circulation (M) at delivery, by trimester of initial maternal vaccination (T1=10, T2=19, T3=14). (D) IgG isotype analysis of UCB (n=14 pairs). (E) Correlation between UCB and maternal RBD-specific IgG titers at delivery (n=45). (F) Correlation between UCB and peak RBD specific IgG in maternal circulation before delivery (n=41). (G) RBD-specific IgG titers in UCB relative to days since maternal first vaccine dose (n=48). * p < 0.03, ** p < 0.002, *** p < 0.0002, **** p<0.0001.

Figure 4:. Passively transferred antibodies are detected in newborn circulation.

(A) Comparison (n=38 pairs) and (B) correlation (n=38) between UCB and newborn blood RBD-specific antibody titers. (C) Overall comparison between maternal RBD-specific IgG antibodies at delivery and newborn RBD-specific IgG titers, independent of trimester of initial vaccination, (n=35) and (D) RBD-specific IgG titers in maternal plasma (M) at delivery and newborn plasma stratified by trimester of initial maternal vaccination (T1, n=6; T2, n=20; T3, n=7). (E) Correlation (n=35) of RBD-specific IgG titers in newborn and maternal plasma at delivery. (F) IgG isotype analysis in newborn plasma (n=14 pairs). (G) RBD-specific IgG titers in newborn plasma relative to days post maternal vaccination (n=40). * p < 0.03, ** p < 0.002, *** p < 0.0002, **** p<0.0001.