Influenza viruses cause annual epidemics of respiratory infection globally. Pregnant women are thought to be at increased risk for morbidity and mortality from respiratory infections, including seasonal influenza, because of changes in anatomy and the immune and cardiovascular systems that accompany pregnancy. Data from high-income countries indicate that pregnant women are at increased risk of hospitalization because of influenza compared to non-pregnant women of childbearing age and the general population. Risks of influenza-associated hospitalization during pregnancy also appear to increase by trimester, with women in their third trimesters at greatest risk.

Although studies show that pregnancy confers an increased risk for influenza-associated hospitalizations, few data are available about the incidence of influenza and influenza-associated complications during pregnancy. Influenza virus infection is hypothesized to increase the risk of preterm birth and fetal growth restriction through the ensuing inflammatory response and/or by triggering immune dysregulation that may activate pathways precipitating early labor and alter placental transfer to the fetus of nutrients or cytokines that influence metabolism. However, studies on the effect of influenza during pregnancy on perinatal outcomes have mixed results with some studies finding an association between influenza and stillbirth, spontaneous abortion, preterm birth and fetal growth restriction while others find no association. There are no studies from low- or middle-income countries of the effect of laboratory-confirmed seasonal influenza during pregnancy on perinatal outcomes, despite the fact that pregnancies in these countries account for > 90% of births each year. Data from low- or middle-income countries on the incidence of influenza virus infection during pregnancy are only available from the placebo or control arms of a few randomized controlled trials evaluating influenza vaccines during pregnancy.

Influenza vaccines are the most effective method of influenza prevention. Studies document that influenza vaccines are safe and effective at preventing influenza among pregnant women. Despite the recommendation by the Strategic Advisory Group of Experts (SAGE) on Immunization in 2012 to prioritize pregnant women for expansion of influenza vaccination programs, influenza vaccines remain underutilized or are not used at all among pregnant women in many countries. Data about the incidence and impact of influenza virus infection during pregnancy could inform decisions about the potential value of influenza vaccines for pregnant women, particularly in low- and middle-income countries that will likely weight the introduction of influenza vaccines against other potential public health prevention programs. If findings demonstrate the impact of influenza virus infection during pregnancy on birth and perinatal outcomes, including increased infant morbidity and mortality, this would likely provide policy makers, healthcare personnel, and pregnant women with compelling evidence about the value of influenza vaccination.

The Pregnancy and Influenza Multinational Epidemiologic (PRIME) study is a prospective, longitudinal cohort study of pregnant women in three middle-income countries designed to address three primary objectives: 1) to evaluate the effect of laboratory-confirmed influenza virus infection during pregnancy on pregnancy and perinatal outcomes; 2) to estimate the incidences of all-cause acute respiratory illness, febrile acute respiratory illness, and laboratory-confirmed influenza virus infection during pregnancy; and 3) to examine the clinical spectrum of illness associated with influenza viruses, including duration and severity of illness.

By: Arriola CS, Suntarattiwong P, Patel A, Dawood FS, Soto G, Das P et al.

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