Vaccination against influenza in pregnant women – safety and effectiveness

Abstract

Influenza is a major cause of morbidity and mortality worldwide. During seasonal influenza epidemics and pandemics, pregnancy places otherwise healthy women at an increased risk of complications from influenza. The factors believed to increase the susceptibility of complicated influenza infection during pregnancy are linked to the physiologic changes, including immunologic changes (attenuation of the cell-mediated immune responses, selective suppression of T-helper 1 cell mediated immunity while the adaptive humoral immunity remains unimpaired), increased cardiac output and oxygen consumption and tidal volume. Pregnant women have similar incidence of seasonal influenza as the general population, however, because of the physiological changes, they are at an increased risk of complications (including secondary pneumonia, acute respiratory insufficiency, increased risk of stillbirth, premature deliveries) and death. Immunization of pregnant women against influenza is currently recommended in many countries. Vaccination against influenza with trivalent inactivated vaccine (TIV) has been proven to be safe and effective. Lack of harmful effect of TIV on pregnant women and newborns has been demonstrated in several studies: no increased risk of spontaneous abortions, preterm birth, low birth weight, congenital malformations, cesarean section have been reported. Vaccination against influenza has been proven to be effective in reducing rates and severity of the disease in vaccinated mothers and their children. Several studies revealed a decreased risk of influenza-like illnesses among mothers who were vaccinated during pregnancy, but also a decreased risk of laboratory confirmed cases of influenza and hospitalizations due to influenza and its complications among newborns and infants born to vaccinated mothers. Currently available inactivated influenza vaccines are not licensed for use in infants younger than 6 months. Protection of young infants against the infection in early life thus requires a cocooning strategy to reduce the number of vulnerable individuals among care givers and contacts. Neonates and infants may be also protected against influenza directly by antibodies of maternal origin that cross the placenta or are transferred via breast milk. The duration of passively acquired antibodies depends on the initial blood concentration and is probably less than 6 months. Vaccine coverage among pregnant women remains low. Possible explanations include lack of education by health care workers, the feeling among the general public that influenza is not a serious problem, and the failure of prenatal care providers to offer the vaccine. Overall, the most important factor for a woman to decide to be immunized during pregnancy was to have a clear recommendation from the health care provider. Reasons evoked by obstetricians for not providing influenza vaccines included lack sufficient data on safety and efficacy, concerns about the medical legal risks of vaccination during pregnancy and the perception that pregnant women would not want to be vaccinated. Educational intervention targeting health care workers in charge of pregnant women should be primary implemented to provide higher influenza vaccine coverage and to protect pregnant women and young infants from influenza related morbidity.