Recovered mothers transfer SARS-CoV-2 antibodies to newborns

A Hong Kong study demonstrates vertical transfer of antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) to babies whose mothers have recovered from coronavirus disease 2019 (COVID-19).

“There is emerging evidence showing that if a woman is infected with SARS-CoV-2 during pregnancy, it is possible for her baby to pick up immunity to the virus in the womb. In this study, we confirmed the mother-to-child transfer of anti–SARS-CoV-2 antibodies and discovered that the median transplacental transfer ratio of anti–SARS-CoV-2 immunoglobulin G [IgG] was 1.3, which means newborns have 30 percent higher levels of anti–SARS-CoV-2 IgG concentrations than their mothers,” reported key study investigator, Professor Liona Poon of the Department of Obstetrics and Gynaecology, Faculty of Medicine, Chinese University of Hong Kong.

The study recruited 20 pregnant women with SARS-CoV-2 infection between March 2020 and January 2021. Their babies were delivered by 31 January 2021. In total, 15 women were seropositive at the time of delivery, with two having an active infection and 13 having recovered from infection. Of the 13 neonates born to mothers with recovered infection, 12 tested positive for IgG. All neonatal nasopharyngeal swab samples were negative for SARS-CoV-2. [Ultrasound Obstet Gynecol 2021;doi:10.1002/uog.23639]

The IgG concentrations in cord and maternal sera at delivery were highly correlated. At the same time, IgG concentrations in cord and maternal sera were negatively correlated with infection-to-delivery interval. “Our study has demonstrated that in pregnant women who have recovered from COVID-19, the higher the viral load during infection and the shorter the infection-to-delivery interval, the higher the anti–SARS-CoV-2 IgG concentrations in maternal sera at delivery,” explained the researchers.

Despite the high viral load leading to higher concentrations of anti–SARS-CoV-2 antibodies in mothers, it affected the transplacental transfer of IgG to the foetuses. “Antibody transfer from the mother to the foetus depends on maternal antibody levels, antibody receptors in the placenta and antibody glycosylation profile. As in other infections, we observed that in maternal SARS-CoV-2 infection, neonatal antibody levels are proportional to maternal antibody levels. However, maternal SARS-CoV-2 viral load restricted the transplacental IgG transfer,” said Poon.

Possible explanations include alteration of antibody receptors in the placenta or glycosylation profiles of the antibody, as shown by other researchers. Perturbed Fc glycosylation and elevated IgG and FCGR3A expression in trophoblast cells, especially in the third trimester, can compromise transplacental IgG transfer in infected pregnancy. [Cell 2021;184:628-642 e10]

“Our finding that transfer ratio decreases with increasing viral load is concordant with the observation of an earlier study that maternal SARS-CoV-2 infection reduces transplacental transfer of SARS-CoV-2 receptor binding domain, spike and nucleocapsid-specific antibodies,” commented the researchers. [Cell 2021;184:628-42 e10] “However, we did not find that IgG transfer was defective during third-trimester infection. Instead, the transfer ratio was constant across gestational age at diagnosis.”

“Our study raises questions regarding the vertical transfer of vaccine-induced anti–SARS-CoV-2 IgG. There is an urgent need to generate clinical data on efficacy and safety of various COVID-19 vaccines in pregnant women to determine vaccination timing and technology of maximum benefit to pregnant women and their babies,” concluded the researchers.