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Pregnant and breastfeeding women who receive an mRNA COVID-19 vaccine develop a strong immune response and produce antibodies that can transfer to the fetus through the placenta and to newborns through breast milk, according to a prospective cohort study published March 25 in the American Journal of Obstetrics and Gynecology.
The findings revealed that the antibody response to vaccination in this cohort was greater than that from a COVID-19 infection during pregnancy. Though the researchers detected SARS-CoV-2 antibodies in umbilical cord blood and breast milk, it’s not yet known how much protection these antibodies might provide to newborns.
“The presence of neutralizing antibody transfer in nearly all cords, and improved transfer with increased time from vaccination, points to the promise of mRNA vaccine-induced delivery of immunity to neonates,” wrote Kathryn J. Gray, MD, PhD, of Harvard Medical School and Brigham and Women’s Hospital’s department of obstetrics and gynecology, and colleagues. “Transfer would perhaps be optimized if vaccination is administered earlier during gestation, though this needs to be directly examined in future studies.”
The researchers tracked 84 pregnant women, 31 lactating women, and 16 nonpregnant women who received the COVID-19 vaccine. The titers of IgG, IgA, and IgM antibodies against the SARS-CoV-2 spike, receptor binding domain (RBD), and S1 and S2 components of the spike were measured in the 131 participants’ blood and in the lactating women’s breast milk four times: at baseline, when they received their second vaccine dose, at 2-6 weeks after their second dose, and at delivery for the 13 women who delivered during the study period.
The study population included health care workers and was predominantly White and non-Hispanic. In addition, two pregnant women, two lactating women, and one nonpregnant woman in the study had a previous SARS-CoV-2 infection.
Most of the pregnant women received the vaccine in their second (46%) or third (40%) trimester. The women across all three groups — pregnant, lactating, and nonpregnant — experienced similar side effects from the each dose of the vaccine, including fever/chills in 32% of the pregnant women and half the nonpregnant women after the second dose.
Titers induced by the vaccine were similar across the pregnant, lactating, and nonpregnant women, and titers did not differ based on the trimester when women received the vaccine. The researchers then compared the titers from the vaccine recipients to titers of 37 pregnant women drawn 4-12 weeks after a natural SARS-CoV-2 infection. Vaccine-induced titers were significantly greater than those measured in the women who had a natural infection during pregnancy (P < .001).
The researchers identified IgG, IgA, and IgM antibodies in the breast milk samples, including a boost in IgG antibodies after the second vaccine dose from baseline. “However, whether these antibodies were transferred efficiently to infants remained unclear,” the authors noted.
The researchers found vaccine-induced antibodies in all 10 umbilical cord blood samples tested, all but one of which had been exposed to two doses of the vaccine.
“The cord with the lowest spike- and RBD-specific IgG belonged to a mother who delivered between the first and second vaccine doses and had received her first vaccine dose 17 days prior to delivery, suggesting that 2 doses may be essential to optimize humoral immune transfer to the neonate,” the authors wrote. “Based on what is known about other vaccines, the amount of maternal IgG transferred across the placenta to the cord is likely to differ by trimester of vaccination.”
Although umbilical cord sera had lower titers of neutralizing antibodies than found in maternal sera, the difference was not significant (median interquartile range 52.3 vs. 104.7, P = .05). The two cord blood samples without neutralizing antibodies came from a woman who had not had the second dose and a woman who received the second dose 1 week before delivery.
“These data provide a compelling argument that COVID-19 mRNA vaccines induce similar humoral immunity in pregnant and lactating women as in the nonpregnant population,” the authors wrote. “These data do not elucidate potential risks to the fetus.”
While the study provides evidence about the immune response induced by the COVID-19 mRNA vaccines during pregnant, it leaves other questions unanswered, said Kevin A. Ault, MD, professor of ob.gyn at The University of Kansas Medical Center in Kansas City.
“The important thing about these findings is that the COVID vaccines are immunogenic in pregnant women. There may be a benefit to the newborns because antibodies are passed on through the placenta,” Ault said in an interview. “The main questions that remain are safety of the vaccine during pregnancy and effectiveness of the vaccine during pregnancy.”
He said he expects to see more studies on the safety and effectiveness of COVID-19 vaccines during pregnancy. Despite more than 73,600 infections and 80 deaths from COVID-19 in people who were pregnant, none of the initial COVID-19 vaccine trials included pregnant or lactating participants.
“This is an important initial study to confirm the antibody generation from mRNA vaccination in pregnant women, and the passage of antibody via cord blood and breast milk,” said Linda Eckert, MD, a professor of ob.gyn at The University of Washington, Seattle, who specializes in maternal immunization. “Further studies are important to look at the timing of vaccination in pregnancy and whether it influences the level of antibody passed to the fetus.”
Though this study is not a safety study, it “does not show increased expected vaccine reactions, such as aches, pains, and fever, in pregnant versus nonpregnant patients,” Eckert said in an interview. “It is not able to evaluate pregnancy outcome data, but it does allow pregnant women being vaccinated with the mRNA vaccines to know that the vaccine is generating protection for them, and the protection is being passed to the fetus in utero via cordblood and to the infant via breast milk.”
The research was funded by the National Institutes of Health along with the Gates Foundation, the Massachusetts Consortium on Pathogen Readiness (MassCPR), the Musk Foundation, the Ragon Institute of MGH and MIT, and Massachusetts General Hospital and Brigham and Women’s Hospital.
Lead author Gray has consulted for Illumina, BillionToOne, and Aetion, and three other authors have financial or scientific/medical advising connections to Alba Therapeutics, NextCure, Viome, Systems Seromyx, and Mirvie. Ault and Eckert had no disclosures.