Premature births are a leading cause of infant death. They are associated with a host of short- and long-term health problems and developmental deficiencies for babies, their mothers, and their families. During the COVID-19 pandemic, the rate of preterm deliveries increased by more than 11 percent in parts of the US and worldwide. But vaccines were vital in returning the preterm birth rate to levels seen before the pandemic, researchers say.
The team at the University of Wisconsin-Madison analyzed birth records from California, US, to study the impact of maternal COVID-19 infection on premature births. The findings are published today in PNAS. During the peak of the virus’s spread, from July to November 2020, the likelihood that a woman would give birth more than three weeks early was 12.3% — nearly double the usual rate. Having comorbidities such as hypertension, diabetes, and obesity also significantly increased the risk of preterm birth, the research found.
However, as more pregnant women received the vaccine, the excess risk of preterm delivery declined rapidly in communities with high vaccination rates. By summer 2021, the risks were a fraction of what they had been during the pandemic. But it took almost a year longer for that to happen in ZIP codes with lower vaccine uptake.
By the end of 2022, COVID-19 infection had no excess effect on preterm births, the researchers say. Their analysis is one of the first to provide causal estimates of how vaccines change pregnancy outcomes.
Nobles and her colleagues looked at the full range of factors that can affect preterm birth, including the mother’s age, the type of delivery, and whether she has had previous miscarriages or a history of a chromosome abnormality called Down syndrome. They also included information on the mother’s socioeconomic status and their education and training level.
Their results show that if a mother was vaccinated during her second trimester, the chances of having a premature birth dropped by a third. Those who had been vaccinated in their first trimester and those who had been vaccinated twice in their second trimester both saw an even more significant drop.
Nobles says the results support the current recommendation that all pregnant people receive two vaccine doses, one in their second trimester and a booster. “The fact that vaccines reduced the COVID-19-related risk of preterm birth should provide further encouragement for every pregnant person to get vaccinated,” she says. “This is not just a good idea for moms and dads; it’s critical for the health of all babies.” Grants from the National Institutes of Health, the National Science Foundation, and the UW-Madison Department of Sociology supported the study. Vaccines are the best way to prevent infections like COVID-19 that can have life-long consequences, such as early births and mental health issues, for both their mothers and their infants. The American College of Obstetricians and Gynecologists recommends that all pregnant people receive the COVID-19 vaccine.