London, Feb. 2, 2021 (AltAfrica)-A mother’s COVID-19 antibodies can cross the placenta, potentially protecting her foetus from infection, researchers reported  in JAMA Paediatrics.

However, a second, unpublished study suggests that the maternal-infant antibody transfer is lower than expected.

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Several weeks after the mother’s infection begins, antibody levels in umbilical cord blood “can be as high as – or even higher – than the levels in maternal blood,” said study leaders Dr. Karen Puopolo and Dr. Scott Hensley of the University of Pennsylvania.

In their study, out of 83 women who had COVID-19 antibodies when they gave birth, 72 of their newborns also had antibodies but no evidence of direct infection.

“It is reassuring that maternal infection, whether symptomatic or asymptomatic, results in sufficient antibody production for an efficient transplacental antibody transfer to new-borns of infected mothers,” said Dr. Flor Munoz of Baylor College of Medicine in an editorial published with the study.

It is possible mother’s COVID-19 antibodies created via a vaccine could have the same effect, Munoz noted.

Transfer of substances in the mother’s blood to the foetus starts around 17 weeks of gestation, she said, which suggests that “maternal vaccination starting in the early second trimester might be optimal to achieve the highest levels of antibodies in the new-born.” 

The authors said their findings show that maternally derived SARS-CoV-2 antibodies may help protect infants from infection, as well as inform both neonatal care guidance and the design of vaccine trials in pregnant women.

“Further studies are needed to determine if SARS-CoV-2 antibodies are protective against new-born infection; if so, at what concentration; and whether the transplacental kinetics of vaccine-elicited antibodies are similar to naturally acquired antibodies,” the researchers wrote.

Lower-than-expected transfer

The second study, the results of which were presented yesterday at the Society for Maternal-Fetal Medicine’s (SMFM’s) annual meeting, involved analysis of maternal and cord blood samples from 32 women who tested positive for COVID-19 during pregnancy.

All maternal samples contained IgG antibodies, and 94% had neutralizing antibodies, while 91% of the cord blood samples contained IgG, and 25% had neutralizing antibodies.

When the researchers stratified the study sample based on time from positive SARS-CoV-2 test to delivery, there were no significant differences over time in the levels of IgG and IgM in the maternal or cord blood samples. Neither were there significant differences in testing when the mothers were asymptomatic versus symptomatic, although all cord blood samples with detectable IgM antibodies were from women with symptoms.

Coauthor Naima Joseph, MD, MPH, a maternal-fetal medicine fellow at Emory University, told CIDRAP News in an email that her team expected to see higher ratios of antibody transfer because other pathogens, such as those that cause pertussis and flu, have ratios higher than 3.

“For us, seeing <1 is not as high as what we know the placenta can do,” she wrote. “We are not saying that this confers reduced protection to the neonate, as that is not known. But we are seeing less than what has been seen in other infections.”

Coauthor Martina Badell, MD, also of Emory, said in an SMFM news release, “The next step is to understand why antibody transfer is different in COVID-19 infection from other infections and whether the transfer of these antibodies increases when we vaccinate a pregnant woman.”

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