Discussion
Immunization with inactivated influenza vaccine is the most effective method to prevent influenza infection and influenza-related complications, particularly in pregnant women. The CDC's Advisory Committee on Immunization Practices has recommended that all women who are pregnant or will be pregnant during influenza season should be vaccinated.[7] This recommendation was based on studies showing the increased risk of hospitalization for respiratory illness in pregnant women, as well as the demonstrated efficacy and safety of vaccination during pregnancy.
Regarding the use of vaccine against 2009 H1N1 influenza virus, the CDC's Advisory Committee on Immunization Practices identified pregnant women as one of the five initial target groups for vaccination efforts.[8] A single dose of monovalent vaccine against 2009 H1N1 influenza virus showed a favorable immune response in pregnant women.[9,10] The efficiency of vaccination during pregnancy was not influenced by maternal immunological status or by the gestational stage when the vaccine was received.[9] The safety of vaccination against 2009 H1N1 influenza virus during pregnancy has been reported in a recent study.[11] There was no increase in spontaneous abortion or major malformations among the fetuses of women who were vaccinated at any time during pregnancy or ≤4 weeks prior to conception.
Maternal influenza vaccination might provide protection for not only the vaccinated women, but also the infants born to these mothers, via transplacental transfer of anti-influenza antibodies.[12,13] Seasonal influenza vaccine reduced the risk of influenza illness and hospitalization for an influenza-like illness among infants up to 6 months of age. Increased blood anti-influenza antibody titers were confirmed in infants through 2–3 months of age.
Data from stored serum samples from people who received vaccination with recent (2005–2009) seasonal influenza vaccines suggest that seasonal vaccines are unlikely to provide protection against 2009 H1N1 influenza virus.[14] Before vaccination, a cross-reactive antibody to the 2009 H1N1 influenza virus was detected in 6–9% of people aged 18–64 years and 33% of those aged >60 years. In our study,[9] the pre-vaccination antibody positivity rate among 124 pregnant women demonstrated a similar rate (7.2%). For the novel influenza A (H1N1) virus, few women of childbearing age had pre-existing antibodies and the initial vaccine formulation available was monovalent, in contrast to the trivalent seasonal vaccines. Therefore, the emergence of the 2009 pandemic influenza was a rare opportunity to evaluate the immunogenicity of the influenza vaccine without the influence of past vaccinations.
This study examines the persistence of maternal antibodies and the transplacental transfer of antibodies following vaccination among pregnant women. Unfortunately, the number of participants was rather small. The baseline antibody titers prior to vaccination were not measured and maternal blood samples were collected only once, at delivery. Each subject had a different period between vaccination and blood draw, depending on the gestational stage when the vaccine was administered. The mean maternal antibody titers in the infected and vaccinated groups were at least 1:40, but these antibody titers tended to decrease over time after infection and vaccination. Most of the women who received the vaccination more than 100 days before delivery did not have an effective antibody titer. The number of days taken for the antibody titer to fall below 1:40 was approximately 150 days in vaccinated women and 225 days in infected women. In terms of humoral immunity, vaccination would have an approximately 150-day period of effectiveness at preventing influenza infection. Considering the linear relationship between antibody titers in the maternal and umbilical cord blood, antibody transfer to the fetus would also decrease over time after vaccination. Our study findings demonstrated a similar tendency: antibody titers in the maternal and umbilical cord blood clearly fell more than 20 weeks after vaccination.
These data are helpful to evaluate whether maternal vaccination confers sufficient passive immunity to the newborn. As a result of inactivated influenza vaccine not being indicated for infants younger than 6 months of age, antenatal immunization might be a useful strategy.
Expert Rev Vaccines. 2012;11(8):903-905. © 2012 Expert Reviews Ltd.
