Kaiser Permanente researchers add to evidence that inoculation in pregnancy protects baby in first months of life
A new Kaiser Permanente study in the journal Pediatrics finds infants up to 6 months old were hospitalized less often with COVID-19 if their mother was vaccinated against SARS-CoV-2 while pregnant.
The analysis follows up on a 2022 analysis from the same study team with similar results. This new study covers a later period in the COVID-19 pandemic when multiple variants were circulating, and so solidifies the evidence in favor of maternal vaccination, said lead author Karen Jacobson, MD, MPH, a research scientist with the Kaiser Permanente Division of Research.
“Infants under 6 months can’t be vaccinated against COVID-19, so maternal vaccination is the only way we can protect infants against this virus,” Jacobson said.
COVID is one of the leading causes of hospitalization in infants before 6 months, and they are the age group with the highest hospitalizations for COVID-19 after the very elderly, she noted. “Infants who are hospitalized with COVID-19 are often subjected to invasive measures, like lumbar punctures and systemic antibiotics, in the course of hospitalization. Maternal vaccination not only protects the mother from COVID-19, but we found it can also prevent the infant from getting sick.”
At the same time, the study found that neither previous COVID-19 infections nor being vaccinated against COVID-19 before pregnancy appeared to protect the baby. This conclusion makes sense given the mechanism of how vaccination of the mother protects the fetus: through antibodies that cross the placenta, Jacobson said. When more time has passed since vaccination, fewer antibodies are available to transfer to the fetus.
The analysis included 78,644 infants born at Kaiser Permanente Northern California between July 2021 and June 2023. Of these, 3,648 (4.6%) had a COVID-19 infection and 76 (0.1%) had a hospitalization because of COVID-19 before the age of 6 months. The researchers reviewed patient charts to confirm that the main reason for hospitalization was SARS-CoV-2 infection.
The researchers found that third trimester vaccination conveyed more protection to the baby than vaccination in any time during pregnancy, 64.6% effectiveness compared with 52.9% for vaccinations given earlier in pregnancy. However, Jacobson urged pregnant people not to interpret these findings to say that they should delay vaccination until late pregnancy because its main goal is to reduce the risk of COVID-19 in the mother. Like all adults, pregnant people should stay up to date with COVID-19 vaccines as they become available.
“Our 2022 study was not powered enough to estimate the effectiveness of maternal vaccination against infant hospitalization, but this new larger study fills in that gap,” said senior author Ousseny Zerbo, PhD, a DOR research scientist.
Zerbo was lead author of the 2022 study, published in Nature Communications, that analyzed more than 30,000 pregnancies in Kaiser Permanente Northern California between December 2020 and May 2022. That study found strong protection against a positive COVID-19 test in infants of vaccinated mothers, particularly in the delta variant period, and less so when omicron was dominant.
Additional co-authors were Nicola Klein, MD, PhD, Maqdooda Merchant, MSc, MA, and Bruce Fireman, MA, all of the Kaiser Permanente Vaccine Study Center.
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About the Kaiser Permanente Division of Research
The Kaiser Permanente Division of Research conducts, publishes, and disseminates epidemiologic and health services research to improve the health and medical care of Kaiser Permanente members and society at large. KPDOR seeks to understand the determinants of illness and well-being and to improve the quality and cost-effectiveness of health care. Currently, DOR’s 720-plus staff, including 73 research and staff scientists, are working on nearly 630 epidemiological and health services research projects. For more information, visit divisionofresearch.kp.org.
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