Pregnant women infected with Covid-19 are not likely to pass the virus to their newborns, reveals study

Pregnant women infected with Covid-19 are not likely to pass the virus to their newborns, reveals study
(Getty Images)

Pregnant women may especially be vulnerable to developing more severe cases of Covid-19 following infection, but not much is known about their anti-coronavirus immune response or how it may affect their child. An analysis now provides new insights that could help improve care for these women and their newborns. Pregnant women who are infected with SARS-CoV-2, the virus that causes Covid-19, during the third trimester are unlikely to pass the infection to their newborns, according to researchers. 

The team studied the occurrence of SARS-CoV-2 infection in the third trimester of pregnancy, evaluating levels of virus in respiratory, blood, and placental tissue samples, the development of maternal antibodies, how well those antibodies passed through the placenta to the fetus (an indicator of potential immune protection from the mother) and examined placental tissue. 

Overall, the authors followed 127 pregnant women who were admitted to Boston hospitals between April 2 and June 13, 2020. Among the 64 pregnant women who tested positive for SARS-CoV-2, no newborns tested positive for the virus. The investigators detected no virus in maternal or cord blood, despite detection in the women’s respiratory system. They also found no signs of the virus in placentas and no evidence of viral transmission to newborns. The researchers suspect that transmission to the fetus may be blocked not only due to the lack of virus in the mothers’ blood but also because the major molecules used by SARS-CoV-2 to enter cells (ACE2 receptor and TMPRSS2 enzyme) are often not physically located together in the placenta.

“This study provides some reassurance that SARS-CoV-2 infections during the third trimester are unlikely to pass through the placenta to the fetus, but more research needs to be done to confirm this finding,” says Dr Diana W Bianchi, director of the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD). 

The study, published in the journal JAMA Network Open, was led by Dr Andrea Edlow, a maternal-fetal medicine specialist at Massachusetts General Hospital, and an assistant professor of obstetrics, gynecology, and reproductive biology at Harvard Medical School. The results reported are limited to women in the third trimester because data on women infected during the first and second trimesters are still being collected and evaluated.

An estimated 49,036 Covid-19 cases among pregnant women in the US and 60 deaths have been reported to the US Centers for Disease Control and Prevention (CDC) as of December 21. Hospitalization data is only available for 40,580 (82.8%) pregnant women and shows that 9,074 were hospitalized due to coronavirus. “ICU admission data were only available for 9,703 (19.8%) women, invasive ventilation data were only available for 7,589 (15.5%) women, and ECMO data were only available for 7,626 (15.6%),” informs the agency.


The analysis

Among those who tested positive for SARS-CoV-2 in the current study, 36% (23/64) were asymptomatic, 34% (22/64) had mild disease, 11% (7/64) had moderate disease, 16% (10/64) had severe disease, and 3% (2/64) had critical disease. The report included, as comparators, 63 pregnant women who tested negative for SARS-CoV-2 and 11 reproductive-age women with Covid-19 who were not pregnant.

The team found that pregnant women who were positive for SARS-COV-2 had detectable levels of virus in respiratory fluids like saliva, nasal and throat secretions, but no virus in the bloodstream or the placenta. 


The team found that pregnant women who were positive for Covid-19 had detectable levels of virus in respiratory fluids like saliva, nasal and throat secretions, but no virus in the bloodstream or the placenta (Getty Images)

The research team did not find significant differences between levels of SARS-CoV-2 antibodies produced by pregnant and non-pregnant women. But while most of the women who tested positive developed antibody responses against SARS-CoV-2 proteins, mother-to-newborn transfer of anti-SARS-CoV-2 antibodies through the placenta was significantly lower than the transfer of anti-influenza antibodies (presumably from maternal flu vaccination). These findings may indicate that SARS-CoV-2 antibodies do not pass through the placenta as easily as other maternal antibodies, say experts. They emphasize that reduced “transplacental transfer of anti-SARS-CoV-2 antibodies may leave neonates at risk for infection.”

The investigators believe theirs is one of the first reports of less-than-expected transfer of SARS-CoV-2 antibodies to the fetus. Low transfer of these antibodies was observed regardless of the woman’s severity of Covid-19 or whether she had an underlying health condition, such as obesity, high blood pressure, or diabetes. The authors note that it will be important to determine why these maternal antibodies are less likely to cross the placenta and whether this reduced antibody transfer renders newborns more vulnerable to SARS-CoV-2 infection, compared to other infections. It will be important to determine how lower levels of maternal SARS-CoV-2 antibodies may affect the health outcomes of preterm babies because coronavirus may increase the risk of preterm labor, they add.

According to Edlow, the transplacental transfer of antibodies to the fetus is typically highest in the third trimester, so it was unexpected to see significantly reduced transfer of SARS-CoV-2 antibodies relative to those against influenza. “Understanding the mechanisms underlying this inefficient transfer of SARS-CoV-2-specific antibodies after third-trimester infection, as well as understanding whether vaccine-generated antibodies have the same or different properties than those from actual infection with the virus, will be critical directions for future research,” she notes.

The authors suggest their findings can immediately inform clinical care and vaccine development and deployment strategies to maximize the benefit for pregnant women and their newborns. 

“Our finding of compromised mother-to-baby transfer of SARS-CoV-2-specific antibodies in third-trimester infections has implications for maternal vaccine administration. Specifically, it highlights that pregnant women are a key population to consider in vaccine rollouts. It also raises questions regarding the optimal timing of vaccine administration to best support maternal and newborn immunity,” says Edlow.


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 Pregnant Women Covid-19 Not Likely Pass Virus Newborns Study