Shielding the Fetus From the Coronavirus

Newborns and babies have so far seemed to be largely unaffected by the coronavirus, but three new studies suggest that the virus may reach the fetus in utero.

Even in these studies, the newborns seemed only mildly affected, if at all — which is reassuring, experts said. And the studies are small and inconclusive on whether the virus does truly breach the placenta.

“I don’t look at this and think coronaviruses must cross across the placenta,” said Dr. Carolyn Coyne of the University of Pittsburgh, who studies the placenta as a barrier to viruses. She was not involved in the new work.

Still, the studies merit concern, she said, because if the virus does get through the placental barrier, it may pose a risk to the fetus earlier in gestation, when the fetal brain is most vulnerable.

Pregnant women are often more susceptible to respiratory infections such as influenza and to having more complications for themselves and their babies as a result. It’s still unclear whether pregnant women are more likely to contract the new coronavirus, said Dr. Christina Chambers, a perinatal epidemiologist at the University of California in San Diego.

“We don’t have any knowledge of that at all — that is a complete open question at this point,” she said. It’s also unclear what effect the virus has on the fetus, she added.

The placenta usually blocks harmful viruses and bacteria from reaching the fetus. And it allows in helpful antibodies from the mother that can keep the fetus safe from any germs, before and after birth.

Still, a few viruses do get through to the fetus and can wreak havoc. The most recent example is Zika, which can cause microcephaly and profound neurological damage, especially if contracted in the first and second trimesters.

Neither the new coronavirus, nor its more familiar cousins, has seemed to belong to this more dangerous category. If so, “we would be seeing higher levels of miscarriage and preterm delivery,” Dr. Coyne said.

A study of nine infants in Wuhan, China, published in March in The Lancet, also concluded that the new coronavirus did not seem to cross from mother to fetus.

But in two of the new studies, published yesterday in JAMA, doctors found antibodies in newborns that recognize the virus, suggesting that it does get through to the fetus.

Both studies found high levels of antibodies in the infants called immunoglobulin G, which are known to be ferried from mother to fetus through the placenta. But in three infants, the studies also found evidence of another type of antibody, called immunoglobulin M, that recognize the coronavirus. These antibodies are too large to move across the placenta.

In one of the studies, researchers found high IgM levels in an infant two hours after birth. IgM levels rise over days, so the finding argues against the newborn having been exposed to the virus during delivery.

“The virus could potentially cross the placental barrier, maybe that’s what we’re seeing,” Dr. Coyne said.

A major shortcoming of the new studies, she said, is that the researchers did not test the placenta, cord blood or the amniotic fluid for the virus. Throat swabs of the newborns did not test positive for genetic material from the virus.

“Their evidence for possible vertical transmission was still indirect, based on only serologic data,” said Dr. Wei Zhang, an epidemiologist at Northwestern University who worked on the Lancet study. As such, he said, the data from the JAMA papers “do not prove” vertical transmission.

A third study, published yesterday in JAMA Pediatrics, also suggested the possibility of vertical transmission. In that study, three of 33 newborns born to women infected with the coronavirus showed mild signs of illness. The doctors said they could not rule out transfer of the virus from mother to fetus as the source.

Some answers may be forthcoming from studies now underway. Dr. Chambers said she and her colleagues have begun enrolling pregnant women with suspected or confirmed cases of coronavirus infection into a study that will track them through delivery and also follow their children up to 1 year of age. They also plan to test for the virus in breast milk.

Similar projects have also begun at the University of California, San Francisco, Harvard University and at the Cedars-Sinai Hospital in Los Angeles.

Dr. Coyne said of the virus, “Any damage that it would impart in utero may be difficult to know right now till we go through the full cycle of pregnancy and delivery.”

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