More than four years after the virus that causes COVID first appeared, scientists continue to discover new ways that the illness threatens pregnant people and babies—as well as additional evidence that vaccination offers significant protection.
A new study finds that babies born to women who got COVID while they were pregnant were three times more likely to develop serious breathing problems than infants whose mothers didn’t have the disease during pregnancy, even if the infants weren’t infected themselves. Seventeen percent of the babies in the study who were exposed to COVID before birth developed respiratory distress, compared with only about 5 to 6 percent of newborns in the general population before the pandemic. The findings were published on Wednesday in Nature Communications.
Respiratory distress is a serious and sometimes life-threatening complication that can lead babies to be hospitalized in intensive care units, where they’re given extra oxygen or even placed on a ventilator. In the study, babies with respiratory distress were sick for an average of 24 days. None died. The researchers followed the infants for six months and don’t know if any of the babies had longer-term complications.
Respiratory distress is most often seen in premature babies whose lungs aren’t fully developed, says Karin Nielsen, the study’s senior author and a professor of pediatrics who specializes in infectious diseases at the David Geffen School of Medicine at the University of California, Los Angeles.
In this study, however, even full-term infants were more likely to develop breathing problems if their mother had COVID while pregnant.
“It’s just one more thing to worry about with this mysterious virus,” says Sean O’Leary, chair of the American Academy of Pediatrics’ Committee on Infectious Diseases, who was not involved in the new study. “Four years in, we’re still new things about this virus.”
Notably, COVID vaccination in the mothers conferred significant protection to their baby, the researchers found. Infants whose mother received at least one dose of an mRNA vaccine were 67 percent less likely to experience serious breathing problems than those whose mother was unvaccinated.
Relatively few pregnant women in the study, which involved babies born from April 2020 to August 2022, received two doses of a COVID vaccine before giving birth. Given the evidence from other studies showing the benefits of vaccination in pregnant people and babies, the authors of the new paper say that two doses or more likely provide even better protection against newborn respiratory distress.
The American College of Obstetricians and Gynecologists “strongly recommends” that pregnant people get vaccinated against COVID. According to a Centers for Disease Control and Prevention study of women in the U.S. who were pregnant last winter, 65 percent had received at least one COVID vaccine dose; 59 percent had received both doses in a primary series; and 27 percent had received a bivalent booster.
Research has shown that people who are vaccinated while pregnant can pass on antibodies to their infants that protect the babies for at least six months, says Kevin Ault, a professor and chair of obstetrics and gynecology at Western Michigan University Homer Stryker M.D. School of Medicine, who was not involved in that work or the new study.
The authors of the paper acknowledge that it had limitations that could have affected their results. The study was small: it included just 34 infants with respiratory distress, all of whom survived. The high survival rate may reflect the fact that the mothers and babies were treated at a large university hospital in Los Angeles with access to specialists and advanced medical equipment.
On the other hand, mothers in this study may have been slightly sicker than average, says Olivia Man, an internal medicine resident at the Mayo Clinic and the study’s first author. Several mother-baby pairs were transferred to the university medical center from smaller community hospitals because of the need for more advanced care.
Despite these limitations, the study’s rigorous design makes its findings significant, Ault says. Beyond observing what happened to babies with respiratory distress, the authors also sought to explain how and why the condition developed.
A molecular analysis of proteins in the babies’ blood found abnormalities in the mechanisms that regulate hairlike structures in the respiratory tract called cilia. When working normally, cilia beat together in a wavelike motion to sweep microbes and inhaled particles out of airways. The molecular analysis also revealed high levels of antibodies that were associated with wheezing, allergies and breathing problems, Nielsen says.
Altogether, the study suggests that babies who are exposed to COVID in the womb experience an “inflammatory cascade,” a dominolike effect in which one type of immune protein activates others, Nielsen says. While a well-regulated immune response can keep people healthy by neutralizing viruses, a hyperactive immune system can cause more harm than good.
During chronic inflammation, “the immune system goes above and beyond what’s necessary,” Nielsen says. Although very few fetuses become infected with COVID, “the babies are reacting to the mother’s illness,” Nielsen says. “Maternal COVID infection can create a hostile environment in the uterus.”
Many studies have documented that COVID vaccines are safe before and during pregnancy, especially when weighed against the tremendous harm COVID itself can cause. Pregnant people with COVID have an increased risk of severe disease and hospitalization. When pregnant people get the disease, both they and their babies are more likely to die.
Studies have shown that when people develop COVID while pregnant, their newborns are twice as likely to be born prematurely and three times as likely to need intensive care. Research also suggests that maternal COVID infection may increase the risk that male infants will be diagnosed with a neurodevelopmental disorder such as autism.
“We saw waves of premature and low-birth-weight babies after every COVID surge [from] 2020 to 2022,” says Linda Yancey, director of infection prevention at the Memorial Hermann Health System in Houston, who was not involved in the new study. “We are still seeing patients, including pregnant ones, being hospitalized this winter. COVID has not gone away.”