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Background: Influenza is a leading cause of morbidity and mortality in subjects with chronic diseases, who may also exhibit reduced immunogenicity to conventional influenza vaccines. MF59-adjuvanted influenza vaccine may enhance their immune response. Methods: We compared immunogenicity and safety of MF59-Adjuvanted Trivalent Influenza Vaccine (ATIV; Fluad®, Novartis Vaccines) and non-adjuvanted subunit (TIV; Agrippal®, Novartis Vaccines) in adults with at least one moderate to severe chronic condition. In this phase III, randomised, controlled, observer-blind study all subjects (18- 60 years of age) received one dose of ATIV (N=180) or TIV (N=179) vaccine during 2006/07 NH influenza season. Immunogenicity was tested using Hemagglutination Inhibition (HI) assay against vaccine and mismatched strains. Subjects were followed for safety for six months. Results: ATIV elicited significantly higher HI geometric mean titres (GMTs; P < .01) and mean-fold increases in titres (GMRs; P < .01) against all vaccine strains, compared with TIV. Seroprotection rates (HI ≥ 40) were 67–93% and 49–78% for ATIV and TIV groups, respectively (P < .01). ATIV also induced significantly higher GMTs against three mismatched strains (P < .05), and significantly greater GMRs against mismatched A strains (P < .05). Both influenza vaccines were well tolerated and safe, although ATIV elicited more solicited local and systemic (both 49%) reactions than TIV (both 28%). Most reactions (> 97%) were mild to moderate and all resolved spontaneously. Conclusion: ATIV is well tolerated, safe and confers higher and broader immunogenicity, when compared with a TIV, in adults with underlying chronic diseases.