Influenza infection during pregnancy was associated with an increased risk of late pregnancy loss and reduced birthweight in singleton term infants, highlighting the value of antenatal influenza vaccination to improve perinatal outcomes, according to study results published in the Lancet Infectious Diseases.

This prospective study included pregnant women (median age, 26 years; median gestational age, 19 weeks) from India, Peru, and Thailand during the 2017 and 2018 influenza seasons to assess the effect of antenatal influenza on pregnancy and perinatal outcomes. The primary outcomes were preterm birth and birthweight of term singleton infants. Secondary outcomes were late pregnancy loss, defined as gestational age of at least 13 weeks, and small for gestational age.

Of the 11,277 pregnant women, 1474 (13%) received an influenza vaccine during the current influenza season. The weighted incidence of influenza was 88.7 per 10,000 pregnant woman-months (95% CI, 68.6-114.8) during the 2017 influenza season and 69.6 per 10,000 pregnant woman-months (95% CI, 53.8-90.2) during the 2018 influenza season.  The highest incidence of influenza was during the first trimester (149.9 per 10,000 pregnant woman-months; 95% CI, 96.7-232.3).

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Influenza during pregnancy was not associated with an increased risk of preterm birth (adjusted hazard ratio [aHR], 1.4; 95% CI, 0.9-2.0; P =.096) or small for gestational age (adjusted relative risk, 1.0; 95% CI, 0.8-1.3; P =.97). However, having influenza during pregnancy was significantly associated with an increased risk of late pregnancy loss (aHR, 10.7; 95% CI, 4.3-27.0; P <.0001) and a reduction in mean birthweight (–55.3 g; 95% CI, –109.3 to –1.4; P =.0445). When the analyses was stratified by trimester, there was a significant reduction in mean birthweight for only the second trimester exposures (β coefficient, –113.9; 95% CI, –207.9 to –19.9; P =.018).

Limitations of this study included the absence of placental histopathology to assess for alternative causes of pregnancy loss. Researchers also noted that results were not generalizable to women without access to antenatal care or populations with high HIV infection rates.

Findings highlighted the “potential added value of influenza vaccination during pregnancy to improve perinatal outcomes in addition to the established benefits of protecting mothers and their infants,” the researchers concluded.


Dawood FS, Kittikraisak W, Patel A, et al. Incidence of influenza during pregnancy and association with pregnancy and perinatal outcomes in three middle-income countries: a multisite prospective longitudinal cohort study. Lancet Infect Dis. Published online October 29, 2020. doi:10.1016/S1473-3099(20)30592-2