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Mothers with COVID-19 produced a robust antibody response, but transfer of antibodies across the placenta to their infants was less efficient than expected, researchers found.

In an analysis of pregnant women who had COVID-19, neutralizing activity -- which measures the potency of antibody response -- was detected in 94% of maternal blood samples and only 25% of cord blood, according to Naima Joseph, MD, MPH, of Emory University in Atlanta, and colleagues.

The overall cord-to-maternal anti-receptor binding (RBD) immunoglobulin (Ig)G ratio was 81%, Joseph reported at the Society for Maternal-Fetal Medicine annual meeting.

"There is a maternal antibody response that is robust following infection," Joseph said in her presentation. But while she emphasized that anti-RBD domain IgG titers were detected in umbilical cord samples, she said it was unexpected that the efficiency of transfer was less than 1.

She told 麻豆传媒 it was surprising to see inefficient transfer of SARS-C0V-2 antibodies to infants because rates are higher with other viral infections: "That gives us more clues that there's something inherent to this virus that's different from others, or maybe there is something about natural infection versus vaccine that will affect transplacental antibody transfer," she said.

Joseph added that the study investigators have now expanded protocols to include vaccinated pregnant patients, in an attempt to assess how vaccination may affect passive immunity.

She noted that although the team's study corroborates recent findings about maternal antibody response to COVID-19 infection, it is the first to evaluate maternal immune response and neutralizing antibodies. The research is the first to focus on areas hit hard by the pandemic, and the study cohort, which consisted primarily of Black women, provides critical information to support equitable vaccine research and allocation, Joseph said.

The researchers characterized maternal humoral response and neutralizing potency against COVID-19, as well as in utero transfer of SARS-CoV-2 spike receptor binding domain antibodies.

The team collected maternal and umbilical cord blood samples from 32 mother-infant pairs at delivery. All the mothers tested positive for COVID-19 at some point during their pregnancy, and were either recognized via universal labor and delivery screening or by showing symptoms prenatally.

Of the 32 mothers who tested positive for COVID-19, 47% were asymptomatic. Most of the mothers were diagnosed with COVID-19 illness during the third trimester. Around three-quarters of study participants were non-Hispanic Black; 84% had Medicaid insurance; and 72% had at least one comorbidity including obesity, hypertension, or pulmonary disease.

All mothers had detectable IgG antibodies and 91% had detectable IgM antibodies. The researchers also found that the majority of infants (80%) had detectable SARS-CoV-2 IgG, and 13% developed IgM antibodies.

Maternal IgG titers were higher following symptomatic infection, but there was no difference in IgM or neutralizing response between symptomatic and asymptomatic patients. The three cord blood samples with detectable IgM antibodies were all from symptomatic mothers.

Study limitations, Joseph said, included the small sample size; the fact that there was no enrolled control group; and generalizability, since the majority of participants were non-Hispanic Black and had underlying conditions.

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