Only 13% of 11,277 pregnant women received the influenza vaccine prior to the 2017 or 2018 influenza seasons.
Influenza vaccination can be underused among pregnant women, despite evidence it can protect both the mother and infant from the flu.
A team, led by Fatimah S. Dawood, MD, Influenza Division, Centers for Disease Control and Prevention, quantified the risk of antenatal influenza and examined its association with perinatal outcomes.
In the prospective cohort, the investigators examined 11,277 pregnant women in India, Peru, and Thailand prior to the 2017 and 2018 influenzas seasons, of which 1474 (13%) received influenza vaccines. Each participant was at least 18 years old with expected delivery dates 8 weeks or more following the start of the influenza season. The median age of the patient population was 26 years old and the gestational age was 19 weeks.
A total of 310 patients had influenza (3%), 270 of which had influenza A (87%) and 40 of which had influenza B (13%).
The researchers contacted the women twice a week until the end of the pregnancy to identify illnesses with symptoms of myalgia, cough, runny nose or nasal congestion, sore throat, or difficulty breathing. They also collected mid-turbinate nasal swabs from symptomatic women for influenzas real-time RT-PCR testing.
The investigators assessed the link between antenatal influenza and preterm birth, late pregnancy loss (at least 13 weeks’ gestation), small for gestation age (SGA), and birthweight of term singleton infants using Cox proportional hazards models or generalized linear models to adjust for potential confounders.
The investigators weighted influenza incidences by the population of women of childbearing age in each study country. These weighted incidences were 88.7 per 10,000 pregnant woman-months (95% CI, 68.6-114.8) during the 2017 season and 69.6 per 10,000 pregnant woman-months (95% CI, 53.8-90.2) during the 2018 season.
Antenatal influenza was not linked to preterm birth (aHR, 1.4; 95% CI, 0.9-2.0; P = 0.096) or having a SGA infant (aRR, 1.0; 95% CI, 0.8-1.3; P = 0.97).
However, antenatal influenza was associated with late pregnancy loss (aHR, 10.7; 95% CI, 4.3-27.0; P = 0.0001) and reduction in mean birthweight of term, singleton infants (-55.3 g; 95% CI, -109.3 to -1.4; P = 0.0445).
“Women had a 0.7–0.9% risk of influenza per month of pregnancy during the influenza season, and antenatal influenza was associated with increased risk for some adverse pregnancy outcomes,” the authors wrote. “These findings support the added value of antenatal influenza vaccination to improve perinatal outcomes.”
Recently, researchers found a plant-based influenza vaccine demonstrated both safety and efficacy.
In terms of safety profile, 1.1% of those who received the vaccine experienced an adverse an adverse event—versus 1.0% in the placebo group.
Furthermore, 0.1% in both the vaccine and placebo groups had severe treatment-related treatment-emergent adverse events.
The primary outcome for this study was relative vaccine efficacy to prevent laboratory-confirmed influenza-like illness caused by any influenza strain.
Thus, it achieved its primary non-inferiority endpoint with a relative vaccine efficacy of 8.8% (95% CI, -16.7 to 28.7).
The study, “Incidence of influenza during pregnancy and association with pregnancy and perinatal outcomes in three middle-income countries: a multisite prospective longitudinal cohort study,” was published online in The Lancet Infectious Diseases.