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COVID-19 vaccination of mother during pregnancy protects infant, though protection wanes

Kaiser Permanente analysis finds less effectiveness during period after omicron variant became dominant

Being vaccinated against COVID-19 during pregnancy provides protection for the baby through its first several months of life, a Kaiser Permanente analysis finds. Protection was stronger against the delta variant than the more recent omicron variant. The study was published in Nature Communications.

The researchers looked at health records for more than 30,000 babies born to members of Kaiser Permanente Northern California between December 2020 and May 2022 and compared the likelihood of positive COVID-19 tests for babies of mothers who received 2 or more doses of COVID-19 vaccine during pregnancy with babies of mothers who were unvaccinated.

Ousseny Zerbo, PhD

During their first 2 months of life, protection (reduced risk of a positive COVID-19 test) was 84% for infants of vaccinated mothers during the delta period and 21% during omicron.

“Our analysis supports the continued value of vaccination during pregnancy in protecting not only the mother, but the child as well,” said lead author Ousseny Zerbo, PhD, a research scientist with the Kaiser Permanente Division of Research. “Because babies under 6 months cannot currently be vaccinated against COVID-19, receiving this protection through their mothers in utero is very important.”

Previous research has reported in utero protection from COVID-19 vaccines during the months after they became available in 2021. This study provides new perspective on vaccine effectiveness through mid-2022, once the omicron variant spread. It also expands on previous findings to include data on protection up to 6 months of age.

The study found that vaccine protection waned over the first 6 months of the baby’s life in both time periods. In the delta period, likelihood of testing positive for COVID-19 was 84% lower in the babies of vaccinated mothers at 2 months, 62% at 4 months and 46% at 6 months. In the omicron period, they found protection at 21% at 2 months, 14% at 4 months, and 13% at 6 months.

Over the entire study period, incidence of hospitalization for COVID-19 was significantly lower for children of vaccinated mothers than those of unvaccinated mothers. Overall, the risk was low: during their first 6 months, 0.03% (10 babies of the 30,311 studied) were hospitalized. Among the 10 babies hospitalized, 1 baby was born from a mother who was vaccinated and 9 from unvaccinated mothers. The authors were unable to determine specific vaccine effectiveness against infant hospitalization for COVID-19 because of the small number of babies who were hospitalized.

Nicola Klein, MD, PhD

“Even though the effectiveness of mRNA vaccines was less during the omicron period, the vaccines still provided some protection for infants against both infection with the SARS-CoV-2 virus and hospitalization,” said senior author Nicola Klein, MD, PhD, director of the Kaiser Permanente Vaccine Study Center. “Maternal vaccination is the best way to protect infants under 6 months of age who are not yet old enough to be vaccinated.”

The analysis also examined whether infants testing positive differed by the pregnancy trimester in which mothers were vaccinated. The authors found mothers who had 1 dose of COVID-19 vaccine during their third trimester had babies with significantly reduced risk of testing positive for the virus during the omicron period; results for the delta period were less precise and much lower.

The study did find greater protection with more than 1 inoculation, the authors said, suggesting that while a single shot is valuable, babies will receive even greater protection if their mothers receive the complete COVID-19 vaccine series.

The study was funded by the National Institute of Allergy and Infectious Disease.

Additional co-authors were G. Thomas Ray, MBA, Bruce Fireman, MA, Evan Layefsky, BA, Kristin Goddard, MPH, Edwin Lewis, MPH, and Pat Ross, BA, of the Vaccine Study Center; Saad Omer, PhD, of Yale University; and Mara Greenberg, MD, of The Permanente Medical Group.

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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. It seeks to understand the determinants of illness and well-being, and to improve the quality and cost-effectiveness of health care. Currently, DOR’s 600-plus staff is working on more than 450 epidemiological and health services research projects. For more information, visit or follow us @KPDOR.

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