Study on Pregnant Women Points to Importance of Timely Flu Vaccination


Researchers examined the effects of severe acute respiratory infections on pregnancy and reported some concerning findings.

pregnancy vaccination

A recent study found that poor timing between season influenza transmission and national annual vaccination campaigns led to an increase in premature births, lower birthweight and an increase in babies delivered by cesarean section.1,2 The study underscores the importance for timely influenza vaccinations amid the continuing COVID-19 pandemic.

Globally, respiratory infections are a leading cause of disease and death, but its adverse effects on pregnant women remain understudied and are especially pronounced in low- and middle-income countries, according to researchers.1 Study authors cited findings from several previous studies, which found that influenza during pregnancy is associated with increased hospitalization, late pregnancy loss and low birthweight, among other effects.

Investigators from the University of Virginia School of Medicine and the Federal University of Ceará in Brazil collaborated to analyze the effect of SARI on pregnancy by identifying peak influenza activity among various regions in Brazil and reviewing SARI, influenza, and influenza vaccinations in the state between 2013 and 2018. In the semiarid state of Ceará, peak seasonal influenza activity normally begins in mid-May, but influenza circulation may start as soon as mid-March, according to the study. Importantly, despite regional differences among influenza seasons – for example, Fortaleza, the state capital, has a seasonal peak 2-3 months earlier than Ceará – Brazil utilizes the same national vaccination schedule.

“Because vaccine-acquired immunity against influenza usually develops 2 weeks after immunization, we hypothesized that pregnant women and their fetuses in the semiarid region might not be adequately protected against influenza,” study authors wrote.

Between 2013 and 2018, of 3297 recorded SARI cases, 145 (4.4%) occurred in pregnant women. The research team reported that 30 to 40 weeks after the peak of the influenza season, birthweights decreased by 40 g and prematurity rates increased from 10.7% to 15.5%. The study identified 61 children born to mothers with SARI during pregnancy, who weighed 10% less at birth and were more likely to be premature than 122 newborn controls. In addition, mothers who had SARI during pregnancy had significantly less formal education than mothers who did not have SARI.

“Our study aligns with earlier reports showing the importance of prevention and adjustment of influenza vaccine campaign schedules to avoid complications of influenza,” authors wrote. “Further work is urgently needed to model and study the optimal timing, potential impact, logistics, economics, and implementation of such a diversified national influenza strategy,” they concluded.


  1. Filho JQ, Junior FS, Lima TBR, et al. Perinatal outcomes of asynchronous influenza vaccination, Ceará, Brazil, 2013-2018. Centers for Disease Control and Prevention (CDC). September 2021. Accessed October 13, 2021.
  2. Troubling birth findings show importance of timely flu vaccination. News Release. ScienceDaily; October 13, 2021. Accessed October 13, 2021.

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