Sara van den Berg

27 2 CMV on influenza vaccination response. Higher pre-vaccination antibody titers (pre-titers) indeed were shown to result in lower post-vaccination antibody titers to subsequent vaccination [7, 35]. Furthermore, one could expect a larger effect of immunosenescence on de novo immune responses [36, 37]. A seasonal influenza vaccination is therefore a suboptimal study setting to investigate the effect of latent CMV infection on influenza vaccine antibody response. We hypothesize that the effect of latent CMV infection on the antibody response to influenza vaccination can best be studied when a novel influenza virus strain is introduced into a naïve population. In this study, we had the unique opportunity to investigate the effect of latent CMV infection on the antibody response during the pandemic season of 2009 to the novel H1N1pdm vaccine strain in a large study population and at multiple time points after vaccination. This allowed a sophisticated study design to test the effect of latent CMV infection on a de novo influenza vaccine response by minimizing pre-existing immunity due to previous exposure by vaccination or natural infection. As a control, the influence of latent CMV infection on the memory antibody response to the vaccination in the subsequent year was also investigated, which included both the same H1N1pdm vaccine strain and an H3N2 vaccine strain. MATERIAL AND METHODS Study population and design The current study is embedded in a trial that evaluated the immune responses to pandemic and seasonal influenza vaccination that was conducted in 2009-2011 (the Pandemic influenza vaccination trial, Netherlands Trial Register NTR2070). This study was carried out in accordance with the recommendations of Good Clinical Practice with written informed consent from all subjects. All subjects gave written informed consent in accordance with the Declaration of Helsinki. The protocol was approved by the Central Committee on Research Involving Human Subjects of the Netherlands. Healthy individuals, between 18 and 52 years of age, were recruited among health care workers in the Utrecht area in the Netherlands. Individuals over 52 years of age were not included because of potential pre-existing immunity due to exposure to the influenza/A/H1N1 strain that circulated until 1957 [38]. Serum samples and questionnaires were used from the vaccine group of the Pandemic influenza vaccination cohort. Vaccines In the pandemic season, individuals received two doses of the monovalent MF59-adjuvanted influenza vaccine containing influenza A/California/7/2009(H1N1pdm09) with a three-week interval (Focetria, Novartis, Italy). Blood samples were collected before vaccination (T1), three weeks after vaccination at which also the second pandemic vaccine dose was given (T2), six weeks after the first vaccination (T3), 26 weeks after the first vaccination (T4) and if participants continued with the study during the 2010-2011 season, also 52 weeks after the first vaccination (T5) ( Figure 1 ). Self-reported vaccine history (2006-2009) was extracted from the questionnaires. If study subjects received seasonal trivalent vaccination in 2009-2010