COVID-19 vaccination during pregnancy protects babies from SARS-CoV-2 infection in their first 4 months of life, according to a study yesterday in JAMA Internal Medicine.
In the cohort study, which included all infants born in Norway late last year and early this year, COVID-19 incidence was lower in babies born to women who received either the Moderna or Pfizer-BioNTech vaccine during the second or third trimester of pregnancy.
Protection, however, was more than twice as high during the period dominated by the Delta variant as during the Omicron period.
Protection for infants 33% during Omicron
The researchers—primarily from the Norwegian Institute of Public Health—note that studies have shown that babies born to women vaccinated during pregnancy are protected against diseases like pertussis (whooping cough) and flu. So they aimed to assess whether the phenomenon held true for COVID vaccines.
They note that, of 21,643 Norwegian babies born from September 2021 through February 2022, 9,739 of them (45.0%) were born to women who received a second or third dose of an mRNA COVID-19 vaccine during pregnancy.
The investigators analyzed data on polymerase chain reaction (PCR) test results for mothers and their babies, all of which are registered in the Norwegian Surveillance System for Communicable Diseases. They included data on the first SARS-CoV-2–positive PCR test for babies at least 1 day after birth up to 4 months of age, as well as information on women who had a positive test 14 or more days before giving birth. In Norway, free PCR tests are widely available.
The incidence rate of a positive COVID-19 test for infants 0 to 4 months old overall was 5.8 per 10,000 follow-up days. For babies born to vaccinated women during the Delta period, it was 1.2 per 10,000 follow-up days, versus 3.0 for babies born to unvaccinated mothers, for a vaccine effectiveness of 71% (adjusted hazard ratio [aHR], 0.29; 95% confidence interval [CI], 0.19 to 0.46).
In the Omicron period, however, the incidence rate was 7.0 per 10,000 follow-up days in the mother-vaccinated group, compared with 10.9 per 10,000 follow-up days in the unvaccinated group. That translates to a vaccine effectiveness in the newborns of 33% (aHR, 0.67; 95% CI, 0.57 to 0.79).
Compared with vaccinated mothers, unvaccinated mothers were younger and had more children and less education, and a lower proportion of them were born in Scandinavia. The percentage of infants born to vaccinated mothers increased during the study period. Most newborns with a positive COVID-19 test during the fall of 2021 were born to unvaccinated mothers, but an infant's age at the time of a positive test was similar between the groups.
The rate of infant hospitalization for COVID-19 as the main diagnosis was 0.07% in both groups.
Study strengths, limitations
The authors say the strengths of the study include the breadth and depth of the Norwegian population data, which is possible because of mandatory reporting in the country. They write, "We believe our study results are generalizable to other pregnant populations. This assumption is strengthened by the fact that the findings align with the results from the US study examining maternal COVID-19 vaccination and risk of infant hospitalization for COVID-19."
Limitations include lack of testing for variants, the possible antibody protection provided by breastfeeding, potential healthy vaccinee bias, varied follow-times, and a lack of information on the infants' disease symptoms. They did not characterize any of the limitations as major, however.
The authors conclude, "The findings of this study provide early evidence to suggest that infants benefit from passive protection from SARS-CoV-2 infection following maternal COVID-19 vaccination during pregnancy."
