Results of this testing were also validated using external quality with samples from the Vitamin D External Quality Assessment Scheme (DEQAS) [40]. Data were collected from baseline questionnaires and medical record review by study 8-Bromo-cAMP coordinators. analyses examined the association between 25(OH)D levels [categorized as 20 ng/ml (deficiency) vs. 20 ng/ml] with the primary outcome of seroconversion. Secondary outcomes included seroprotection; a 4-fold increase in titers; and geometric mean titers post-vaccination. Analyses were repeated using 25(OH)D levels as a continuous variable. Results: A total of 128 adults [64 HIV-infected (median CD4 count 580 cells/mm3) and 64 HIV-uninfected] were included. Seroconversion at day 28 post-vaccination was achieved in fewer HIV-infected participants compared with HIV-uninfected participants (56% vs. 74%, p=0.03). Vitamin D deficiency was more prevalent among HIV-infected persons vs. HIV-uninfected persons (25% vs. 17%), although not significantly different (p=0.39). There were no associations found between lower 25(OH)D levels and poorer antibody responses at day 28 or 6 months for any of the study outcomes among either HIV-infected or HIV-uninfected adults. Conclusion: Vitamin D deficiency was common among both HIV-infected and HIV-uninfected adults, but lower levels did not predict antibody responses after H1N1 (2009) influenza vaccination. Low 25(OH)D levels do not explain poorer post-vaccination responses among HIV-infected persons. Background Influenza remains a leading cause of seasonal epidemic disease resulting in excess morbidity and mortality. Vaccination remains the main preventive strategy against influenza and is currently recommended 8-Bromo-cAMP for persons 6 months [1]. Protection against influenza after vaccination varies widely by the match with circulating strains as well as host characteristics. Immunosuppression, including HIV infection, has been associated with reduced vaccine effectiveness [2-9]. This is of particular concern since HIV-infected persons are at higher risk for influenza-related complications [10-13]. Methods to improve vaccine responsiveness among HIV-infected persons have been studied including the use of higher influenza vaccine doses (e.g., Fluzone High-Dose)[14] and use of adjuvants (e.g., AS03, MF59) 8-Bromo-cAMP [15-17]. However, neither of these potential strategies are currently recommended by vaccine guidelines [1,18]. Vitamin D may affect both the innate and adaptive immune responses, and may have an immunomodulating role in improving immune responses to vaccines mediated through its actions on antigen-presenting cells including the dendritic cells [19-25]. While a study among prostate cancer 8-Bromo-cAMP patients found evidence of an association between low baseline 25-hydroxyvitamin D [25(OH)D] levels and poorer influenza vaccine responses [26], studies in healthy persons found no 8-Bromo-cAMP associations [27,28]. Hence further data are needed especially among immunocompromised hosts [27]. Among HIV-infected patients, only three published studies have examined this potential relationship. A study (n=91) showed no relationship between baseline 25(OH)D levels and vaccine antibody responses at day 21 post-vaccination with the 2010/2011 trivalent influenza vaccine [29], while a second study [n=90] found similar 25(OH)D levels (both groups: 20 ng/ml) among responders and non-responders after 2009 H1N1 influenza vaccination at day 21 post-vaccination [30]. A third study found that use of Rabbit Polyclonal to STK10 vitamin D supplementation at the time of influenza vaccination had no effect on post-vaccination antibody levels during the 2008C2009 season, but this study did not evaluate actual 25(OH)D levels [31]. Since only one of these studies had a HIV-negative group [29], each involved differing influenza seasons, and none evaluated long-term post-vaccination responses (responses were measured at 3C8 weeks) [29-31], further research is needed. Given that HIV-infected persons have reduced immune responses after influenza vaccination [2-9] and a high prevalence of vitamin D deficiencies [32,33], we sought to determine if low 25(OH)D levels help explain the poorer post-vaccination immune responses in this population compared with HIV-uninfected adults. Hence, the purpose of this study was to evaluate vitamin 25(OH)D levels among HIV-infected and HIV-uninfected adults and its potential relationship with influenza vaccine immunogenicity. Methods Study Design A prospective cohort study was conducted to compare the immunogenicity of the monovalent 2009 influenza A (H1N1) vaccine (strain A/California/7/2009/H1N1; Novartis Vaccines and Diagnostics) among HIV-infected and HIV-uninfected adults during the 2009C2010 influenza season (Clinical Trials #”type”:”clinical-trial”,”attrs”:”text”:”NCT00996970″,”term_id”:”NCT00996970″NCT00996970)[9]. Vaccination was performed October 29CDecember 2, 2009, and participants were voluntarily enrolled at the Naval Medical Center San Diego, San Diego, California; Naval Medical Center, Portsmouth, Virginia; and Walter Reed.