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No cases of vertical transmission or placental infection were seen in a prospective cohort study involving over 60 pregnant women with COVID-19.

Placental transfer of antibodies against SARS-CoV-2, however, was less efficient than expected (lower than vaccine-induced influenza antibodies), suggesting that infants remained susceptible to illness, reported Andrea Edlow, MD, MSc, of Massachusetts General Hospital in Boston, and colleagues.

"These data fill a substantial knowledge gap, point to the rarity of vertical transmission, and establish compromised SARS-CoV-2-specific immunity in the neonate," Edlow and colleagues wrote in .

The researchers added that with only a few cases of vertical transmission reported in U.S.-based studies, the mechanisms of fetal and placental protection from SARS-CoV-2 -- and how those impact neonatal immunity -- are "a critical area of investigation." Edlow's group suggested that the rarity of vertical transmission may be related to low levels of virus detected in the mother's bloodstream.

In an , Denise Jamieson, MD, MPH, of Emory University School of Medicine in Atlanta, and Sonja Rasmussen, MD, MS, of the University of Florida College of Public Health in Gainesville, said the findings may bring us closer to understanding the characteristics of SARS-CoV-2 as well as other viruses in pregnancy.

While this research supports previous findings that intrauterine transmission is possible but not common, the editorialists stated that the other major finding is the inefficient transfer of maternal SARS-CoV-2 antibodies.

"Although it is not known whether the inefficient transplacental transfer of antibodies observed by Edlow et al will also extend to antibodies elicited by future SARS-CoV-2 vaccines, it underscores the susceptibility of infants, particularly since it is unlikely that young infants will be eligible for SARS-CoV-2 vaccination," Jamieson and Rasmussen wrote.

The study from Edlow's group included 127 pregnant women who presented at three tertiary care centers (Massachusetts General, Brigham and Women's Hospital, and Beth Israel Deaconess Medical Center) in Boston. Patients who received a positive polymerase chain reaction (PCR) test for SARS-CoV-2 (n=64) were recruited from April to June 2020, and were followed up until July 10. Infants born to mothers who tested positive for SARS-CoV-2 infection were tested by nasopharyngeal swab 24 hours after birth.

Edlow and colleagues tested for viral load in mother and baby, transplacental passage of anti-SARS-CoV-2 antibody, and cases of fetoplacental infection. The researchers enrolled 63 pregnant women without COVID-19 illness as a comparison group, as well as nonpregnant women of reproductive age with confirmed COVID-19.

Among the pregnant group testing positive for COVID-19, mean age was 34 years old, 36% were asymptomatic, 34% had mild illness, 11% had moderate disease, 16% had severe disease, and 3% had critical illness. Most patients (86%) were diagnosed with COVID-19 in the third trimester, but nine tested positive during the second trimester.

In an analysis of viral load in 107 participants, the researchers found no detectable virus in either the maternal or umbilical cord blood.

The researchers performed antibody quantification for 77 mother-infant pairs, including 37 mothers with confirmed COVID-19, and 40 mothers who tested negative. Among the mothers who tested positive, 65% had detectable anti-SARS-CoV-2 receptor binding domain (RBD) immuno globin G (IgG) and 70% had detectable anti-nucleocapsid IgG. Among babies born to positive mothers, 62% had detectable anti-RBD IgG and 59% had detectable anti-nucleocapsid IgG. The transfer of influenza HA–specific antibody was higher than that of SARS-CoV-2, regardless of whether mothers tested positive for COVID-19.

Pathologic exams were performed on 88 placentas (44 of which were from mothers who tested positive), and there were no SARS-CoV-2 RNA found.

Discussing the study's limitations, Edlow and colleagues stated that recruiting participants for the control group as a convenience sample resulted in demographic differences between the cases and controls in their study. Additionally, the researchers stated that the study only examined transplacental antibody transfer during the third trimester, due to the timing of the pandemic in Boston.

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