The vaccine was recommended to pregnant women in their second or third trimester and to groups at high risk of severe influenza.6It was mandatory to report pandemic vaccination to the national immunisation register (SYSVAK).4,5Approximately 54% of pregnant Norwegian women were vaccinated.4 In contrast, among 46000 pregnancies in Norway during the pandemic, only 516 cases of laboratoryconfirmed influenza infection were registered. than women without these symptoms. Women who reported being very ill or illness duration of >5 days had higher HItiters than women reporting less severe illness or illness of shorter duration, respectively. == Conclusions == Antibody RH1 waning was comparable in vaccinated women and women with ILI. More severe ILI or longer duration of illness was associated with higher HItiters. Most unvaccinated pregnant women with ILI had low HItiters, probably due to moderate illness and HItiter waning between exposure and sampling. Keywords:antibodies, influenza, pandemic H1N1pdm09, pregnancy, vaccination, waning == 1. INTRODUCTION == During the 2009 influenza A(H1N1) pandemic, pregnant women Rabbit Polyclonal to Cytochrome P450 27A1 were at increased risk of hospitalization and death due to severe influenza contamination.1,2,3Following the first reports on this association, a populationbased cohort of pregnant women (The Norwegian Influenza Cohort Study, NorFlu) was established in Norway, to study the potential effects of maternal pandemic influenza and vaccination on the women and their children. In Norway, the main pandemic period occurred between October 1, 2009, to December 31, 2009, peaking in early November.4,5A vaccination campaign with the AS03adjuvanted A(H1N1)pdm09 vaccine (Pandemrix) started 19th October. The vaccine was recommended to pregnant women in their second or third trimester and to groups at high risk of severe influenza.6It was mandatory to report pandemic vaccination to the national immunisation register (SYSVAK).4,5Approximately 54% of pregnant Norwegian women were vaccinated.4 In contrast, among 46 000 pregnancies in Norway during the pandemic, only 516 cases of laboratoryconfirmed influenza infection were registered. Due to limited laboratory capacity during the pandemic, testing of patients with severe illness was prioritized. Furthermore, only 8.9% of women who gave birth in 2009 2009 or 2010 were diagnosed with influenza by a physician.4However, national influenza surveillance data indicated a clinical attack rate of approximately 30% in the Norwegian population.5 Data on laboratoryconfirmed influenza were limited also for women in the NorFlu pregnancy cohort, but antibodies against the A(H1N1)pdm09 virus, measured as hemagglutination inhibition (HI)titers, were measured in maternal blood samples taken RH1 at delivery, 39 months after the pandemic period. As the 2009 2009 A(H1N1)pdm09 computer virus was an antigenically novel computer virus7and the frequency of preexisting RH1 antibodies to the computer virus was low in the Norwegian populace prior to the pandemic,8high HItiters might serve as a proxy for contamination in unvaccinated individuals. Antibodies induced by influenza contamination or vaccination are known to wane over time,9,10,11,12,13and HItiters have been suggested to decline faster after vaccination with pandemic vaccines than after contamination.14,15,16Studying antibody waning is usually important for understanding the longevity of the maternal antibodies. A small study from the 2009 2009 pandemic found no significant difference in HItiter waning between vaccinated and infected pregnant women, although the HItiters declined at a slightly slower rate in the infected women.17As the immune system is altered to tolerate the fetus, pregnancy may influence the immune response to pandemic influenza infection and vaccination.18However, studies on influenza vaccination of pregnant women mostly indicate that RH1 their immune responses are comparable to those of nonpregnant healthy individuals.19 Due to the long time interval between the pandemic exposure and blood sampling at delivery, we aimed to estimate HItiter waning in the unvaccinated pregnant women with ILI during the 2009 pandemic, and compare with the estimated waning in the vaccinated women. In addition, as HItiters have been reported to be positively associated with RH1 influenza symptoms and severity.