Researchers Propose Criteria for Confirming Vertical Transmission of SARS-CoV-2

By Will Boggs MD

August 12, 2020

NEW YORK (Reuters Health) - Specific testing for SARS-CoV-2 within certain times and using placental tissue is necessary to confirm transmission of SARS-CoV-2 from infected pregnant mothers to their offspring, according to an expert commentary.

"Based upon the pathology criteria that we suggest in our article - demonstrating viral infection of fetal cells of the placenta using molecular pathology techniques - there are now a few neonates reported who we believe to be infected following intrauterine transplacental passage of SARS-CoV-2 from a pregnant mother with COVID-19," said Dr. David A. Schwartz of the Medical College of Georgia, in Atlanta.

"In the case of newborn infants that test positive for COVID-19 where transplacental infection is not occurring, there are likely other mechanisms of infection occurring, including intrapartum and postpartum transmission," he told Reuters Health by email.

Theoretically, viruses can be transmitted vertically from mother to infant through three different mechanisms: intrauterine, via hematogenous (more commonly with viruses) or ascending (almost always with bacteria) routes; intrapartum, when the fetus passes through an infected birth canal during vaginal delivery; and postpartum (through contamination via respiratory excretions, skin-to-skin contact and breast milk).

Dr. Schwartz and colleagues propose that the diagnosis of early-onset neonatal COVID-19 infection be limited to neonates with positive RT-PCR testing for SARS-CoV-2 within the initial 72 hours of life. Those testing positive within the initial 24 hours of life should be considered to have very early-onset COVID-19.

Neonates testing positive within the first 72 hours of life are most suspicious for having acquired COVID-19 from vertical transmission prior to or around the period of delivery, the authors report in Archives of Pathology and Laboratory Medicine.

The experts also propose that identification of virus in chorionic villous tissue of the placenta (using in situ nucleic acid hybridization methods or immunohistochemistry techniques) is prima facie evidence of intrauterine fetal infection.

"Following delivery to a pregnant woman with COVID-19, newborn infants who test positive for the virus can have their placentas tested for evidence of SARS-CoV-2 viral antigen or nucleic acid on the fetal side of the placenta, such as the cells of the chorionic villi," Dr. Schwartz said. "In order to do this, the placentas from infected mothers need to be saved following delivery, placed in formalin preservative, and submitted to the pathology department for molecular pathology testing."

"The risk factors remain unknown as to why some pregnant women have newborn infants that test positive for COVID-19, but most do not," he said. "As a result, it cannot currently be predicted which pregnant women with COVID-19 will have newborn infants who also are infected."

"Additional information on how frequently infected mothers transmit the virus to the fetus prior to delivery is needed, as well as understanding the mechanisms for transplacental transmission," Dr. Schwartz said. "Research is also required to evaluate the frequency of other mechanisms of vertical infection, including intrapartum and postpartum transmission."

"Fortunately," he said, "the probability of a pregnant woman with COVID-19 having a newborn infant who is infected remains low."

SOURCE: Archives of Pathology and Laboratory Medicine, online July 24, 2020