The human placenta and fetus are unlikely to be infected with COVID-19 due to minimal expression of SARS-CoV-2-related proteins and receptors in the placenta, a new study says.

The placenta can shield the fetus against pathogens causing infections in the mother. However, the fetal organ does not offer the same protection against all viruses. The Rubella virus, herpesvirus, and Zika virus are known to be capable of crossing the placenta and infecting the fetus.

Researchers from the National Institutes of Health and Wayne State University, among others, published their findings on Tuesday in the journal eLife.

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The SARS-CoV-2 virus is believed to enter the body through airways, and adheres to the angiotensin-converting enzyme 2 (ACE2) receptors in the nasal passage, lungs and gastrointestinal tract. These areas of the body have a high expression of ACE2 receptors which promote virus entry and spread.

In the new findings, researchers noted that the placenta has a minimal expression of the ACE2 receptor and an enzyme the virus uses to enter a cell, called TMPRSS2.

“These results consistently indicate that the human placental tissues negligibly co-express ACE2 and TMPRSS2,” they wrote.

This finding offers a possible explanation as to why SARS-CoV-2 is rarely found in fetuses or infants of women infected with the virus.

Researchers maintained that vertical transmission of SARS-CoV-2 has not been proven. Vertical transmission refers to the passage of a pathogen from mother to fetus.

Nevertheless, there is a possibility that the virus could infect the placenta through alternate entry routes while interacting with other proteins, “however, further research is required to test their participation in the pathogenesis of COVID-19,” researchers wrote.

Last week, researchers from Italy said they studied 31 women with COVID-19 who delivered babies in March and April. They found signs of the virus in several samples of umbilical cord blood, the placenta and, in one case, breast milk.

However, the study leader, Dr. Claudio Fenizia, an immunology specialist at the University of Milan, said the possibility of fetal infection seems relatively rare.

Dr. Anton Pozniak, a conference leader and virus expert at the Chelsea and Westminster Hospital in London, said the implications of the Italian research "have to be worked out."

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Children under age 3 rarely get seriously ill from coronavirus, and "I would suspect that even if there was transmission to babies, it was not harmful," he said.

The Associated Press contributed to this report.