Limited transfer of COVID-19 antibodies from mothers to infants during pregnancy

While pregnant mothers with COVID-19 can mount a robust antibody response against the virus, transfer of antibodies from mothers to child across the placenta is less efficient than expected, according to a study presented during the SMFM 2021 Meeting.

Among 32 paired mother-infant dyads analysed, neutralising antibodies against SARS-CoV-2 was present in 94 percent of maternal blood samples but only 25 percent of cord blood samples. 

In the prospective study, the researchers collected blood samples of 32 women who were tested positive for COVID-19 during pregnancy and cord blood samples of their offspring at delivery. [SMFM 2021, abstract LB01]

“A recent study analysed maternal antibody response to infection, but our study is the first to look at the maternal immune response and neutralizing antibodies,” said presenting author Dr Naima Joseph of Emory University in Atlanta, Georgia, US.

All samples from mothers contained IgG antibodies and 94 percent had IgM antibodies, while neutralising antibodies were present in 94 percent of these.

By comparison, analysis of cord blood samples revealed that IgG and IgM were detected in 91 percent and 9 percent, respectively, and 25 percent had neutralizing antibodies.

“There is a maternal antibody response that is robust following infection,” said Joseph, noting the high titres of antibodies found in maternal samples.

“What was interesting about this study is that even if a woman was asymptomatic, she still developed high levels of COVID-19 IgG and neutralizing antibodies,” she pointed out.

For IgG, titres were significantly higher in mothers with symptomatic vs asymptomatic infection (p=0.03). However, there were no significant differences in IgM and neutralizing antibody titres between symptomatic and asymptomatic women.

Similarly, IgG titres were higher in mothers who delivered >28 days after infection than those with shorter period of latency (p=0.05), while no such differences were seen for IgM or neutralizing antibodies.

When analysing cord blood of infants, however, antibody titres were significantly lower compared with their corresponding maternal samples (p<0.001), and this was independent of symptoms or latency period between infection to delivery.

“A major way that infants are protected from infection is from the antibodies that they receive in utero, so whether a woman was asymptomatic or not, we would have expected to see a higher percentage of antibodies transferred from mother to infant, especially neutralizing antibodies,” said study co-investigator Dr Martina Badell from Emory University School of Medicine, Atlanta, Georgia, US.

The lower than expected transfer of SARS-CoV-2 antibodies from mothers to infants suggests that infants might be left unprotected — and hold implication for vaccinations of pregnant women. 

“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,” said Badell.

Although the WHO* has suggested to withhold COVID-19 vaccines from pregnant people due to a lack of evidence in this particular population, the SMFM and ACOG** subsequently issued a joint statement that FDA-approved COVID-19 vaccines “should not be withheld from pregnant individuals who choose to receive the vaccine.”

*WHO: World Health Organization
**ACOG: American College of Obstetricians and Gynecologists