Study shows mRNA COVID-19 vaccination in pregnancy can prevent hospitalization among infants

By Dr. Tomislav Meštrović, MD, Ph.D.Reviewed by Emily Henderson, B.Sc.Feb 16 2022

A report published in the CDC’s Morbidity and Mortality Weekly Report (MMWR) shows how primary two-dose vaccination series against coronavirus disease 2019 (COVID-19) during pregnancy is linked to a reduced risk of COVID-19–associated hospitalization among infants six months of age or younger.

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Study: Effectiveness of Maternal Vaccination with mRNA COVID-19 Vaccine During Pregnancy Against COVID-19–Associated Hospitalization in Infants Aged <6 Months — 17 States, July 2021–January 2022. Image Credit: milkumistock / Shutterstock.com

Infection with the acute respiratory syndrome coronavirus 2 (SARS-CoV-2) during pregnancy is associated with severe COVID-19 and death, with pregnant women more frequently experiencing preterm birth, stillbirth, and a myriad of other factors pregnancy complications. Consequently, vaccination is highly recommended in pregnancy to prevent the development of COVID-19, including severe forms of the disease and death.

Our experience with other vaccine-preventable diseases implies that maternal immunization can indeed protect infants via passive transplacental antibody transfer, particularly during the first six months of life. As previous studies have shown the possibility of transplacental transfer or SARS-CoV-2–specific antibodies, a logical question was to assess its protective role.

Previous studies have shown that infants are at risk for life-threatening complications from COVID-19, including acute respiratory failure. Moreover, multisystem inflammatory syndrome in children (MIS-C) can complicate their clinical presentation even further; hence, the protection offered by maternal antibodies (for example, after vaccination) may prevent these dire scenarios.

However, the optimal timing of vaccinating future mothers for the adequate transfer of antibodies is currently not clear, while the direct effect of maternal COVID-19 vaccination in preventing severe clinical presentation in infants has not been described thus far.

A recent study by the Overcoming COVID-19 Investigators, led by Dr. Natasha B. Halasa from the Vanderbilt University Medical Center and Dr. Samantha M. Olson from the CDC COVID-19 Emergency Response Team, aimed to answer some of these open and pertinent questions.

A comparative, case-control approach

By employing a case-control study design, vaccine performance in preventing serious outcomes in infants has been assessed by comparing the odds of completing a two-dose primary mRNA COVID-19 vaccination series during pregnancy among mothers of infants with the disease and a control group (i.e., those with negative SARS-CoV-2 test results).

Infants included in the study were aged six months or less and were admitted outside of their birthing ward to one of twenty pediatric hospitals in the period between July 1, 2021 and January 17, 2022. Of note, the SARS-CoV-2 Delta variant was pervasive in the United States, before the predominance of SARS-CoV-2 Omicron variant of concern after mid-December.

By pursuing parent or guardian interviews, the researchers and study personnel have obtained baseline demographic data, clinical information, as well as SARS-CoV-2 testing history during hospitalization or after discharge. Furthermore, the review of infants’ electronic medical records has also been pursued.

Maternal vaccine prevents infant hospitalizations

This real-world evaluation showed that the maternal completion of a two-dose primary mRNA COVID-19 vaccination series during pregnancy is linked to a reduced risk for COVID-19 hospitalization among infants aged six months or less.

More specifically, the effectiveness in preventing infant hospitalizations was 61% overall for mothers who received their vaccination course during pregnancy. The protection was also more robust when the vaccine was administered in the later stages of the pregnancy.

Even though booster doses are endorsed for pregnant women as well, vaccine effectiveness of maternal booster doses received during pregnancy could not be assessed in this study due to the rather small sample size, resulting in underestimation.

Endorsing COVID-19 mRNA vaccine in pregnancy

In addition to the small sample size, the report is not without other limitations, such as no assessment of prior infection with SARS-CoV-2 in pregnancy, self-reported data for several participants, as well as residual confounding. Still, the findings reveal a significant trend that needs to be considered when developing public health recommendations.

“With infants not currently age-eligible for vaccination and infant hospitalization rates remaining at the highest levels of the pandemic, this study suggests that maternal COVID-19 vaccination during pregnancy might protect infants aged <6 months from COVID-19–related hospitalization”, say study authors in this paper.

Further studies should appraise the optimal timing of vaccination before pregnancy compared with during pregnancy. But, for now, US Centers for Disease Control and Prevention (CDC) recommends that women who are pregnant, might become pregnant, or who are trying to in the future should definitely stay up to date with COVID-19 vaccination.