Influenza virus infection during pregnancy can increase the risk of hospitalization and illness when compared to non-pregnant women. Year-round vaccination during pregnancy may decrease the associated morbidities in mothers and their infants, and may have better overall efficacy than seasonal vaccinations.
Each year many people worldwide are infected with the influenza virus, or as it is more infamously known as, “the flu”. This viral infection is often accompanied with feelings of malaise, body aches, fever, chills, cough, nasal congestion, and headache, to name a few. Though these symptoms can negatively impact anyone, the most vulnerable individuals in the society are often the most severely affected.
According to a study recently published in The Lancet, pregnant women who have been infected with the flu are at increased risk of illness and hospital admission compared to non-pregnant women. This has caused the World Health Organization (WHO) to recommend yearly immunization of pregnant women in order to allow for the transfer of protective maternal antibodies to the fetus. In this study, the researchers set out to investigate the efficacy of immunizing pregnant women living in Southern Nepal with the influenza vaccine. They wanted to see if providing year-round influenza vaccination to women who were pregnant resulted in fewer incidences of the flu in the infant and/or mother in the 180 days following delivery.
The researchers chose to target pregnant Nepalese women because Nepal has been considered as a region that is broadly representative of South Asia. In Nepal, where the influenza virus circulates throughout the year, Steinhoff and colleagues studied 3693 pregnant women from April 25, 2011, to September 9, 2013 in a randomized, placebo-controlled trial with two successive yearly cohorts. In both cohorts, the individuals who were randomly assigned to the placebo group received sterile saline in place of the trivalent, inactivated influenza vaccine, Vaxigrip (Sanofi-Aventis Pharma, Mumbai, India), that was given to the experimental group in both cohorts. Cohort 1 enrolled 2090 women (1041 were given the placebo and 1049 the vaccine), and cohort 2 enrolled 1603 women (805 randomly assigned to the placebo group and 798 the vaccine group). A total of 3646 live born infants were included in the study, and 1010 of them were infants of mothers who were the recipients of the trivalent vaccination. In order to be eligible to participate in the study, women needed to be married, between 15-40 years of age, at 17-34 weeks of gestation at the time of enrolment, and not previously vaccinated with any influenza vaccine that season. Only babies who were born more than two weeks after the mother had been immunized were included in the analysis.
In order to assess the morbidity of participants, each week starting from the day of enrolment to 180 days post partum, trained staff would visit the women at their homes and collect data regarding signs and symptoms experienced by them and their infants. This information reflected any influenza-like illness, including maternal self-reported fever, sore throat, myalgia, persistent cough, and nasal congestion in the preceding week. For infants, the signs and symptoms explored were high fever, cough, wheeze, difficult or rapid breathing, and any ear discharge. Additional information was solicited in the form of a midnasal swab from any woman who reported having fever plus an additional sign (persistent cough, sore throat, nasal congestion, or myalgia) on any of the past seven days. Midnasal swabs were also taken from infants whose mothers reported them as having any one of fever, cough, wheeze, difficulty breathing, or ear discharge on any of the past seven days.
This particular study on influenza vaccination in pregnant women had three primary outcomes. Primarily, Steinhoff and colleagues were interested in determining the incidence of laboratory-confirmed influenza illness (via positive polymerase chain reaction (PCR) nasal swabs) in the infant from birth to 180 days. Secondly, they assessed the incidence of influenza-like illness (i.e. fever AND either cough or sore throat on at least 1 day) in the mothers during pregnancy and to 180 days postpartum. Over 36 months of study, 254 influenza cases were documented in 24 of the months; 182 episodes were due to influenza type A, and 129 were due to influenza type B. The researchers determined that vaccination of pregnant women with the inactivated trivalent influenza vaccine significantly reduced laboratory-confirmed influenza infection in their infants. They also discovered that vaccination would decrease the incidence of maternal influenza-like illness, and the proportion of babies with low birth weight (<2500g). It is also important to note that there were no serious adverse events were associated with receiving the vaccination. These findings suggest that giving the influenza vaccination throughout the year may have greater global advantages than seasonal vaccinations, especially in climates where the influenza virus is present for many months.
The strength of the study was in the design. The placebo-controlled randomized distribution allows for the greatest comparison between those who were immunized and those who were not. A limitation of the study was the lack of first trimester ultrasounds to verify the actual gestational ages of each pregnancy. Another limitation was that fever was a required criterion for diagnosis of maternal influenza-like illness. This may have resulted in a lower amount of maternal midnasal swab specimens collected, and fewer cases of proven influenza illness in the mothers when compared to their infants.
Written By: Kimberly Spencer B.Sc. (Hons)