Background Influenza vaccination is recommended for patients with inflammatory bowel disease (IBD). The 2009 H1N1 influenza vaccine produced seroprotection rates of >85% in the general population but there are no data on the immunogenicity of the vaccine in patients with IBD.

Methods An observational prospective open-label study was conducted to examine the immunogenicity of the 2009 H1N1 influenza vaccine in 108 patients with IBD. Patient details, medications and disease activity were recorded. Pre- and post-vaccination haemagglutinin inhibition titres and geometric mean titres were measured. A functional assay of T lymphocyte activity was measured at vaccination in a subset of patients as an alternative measure of immunosuppression. Subjects were followed for 6 months post-vaccination.

Results Of 108 patients enrolled, 105 completed the study. The post-vaccination seroprotection rate was 50%. Immunosuppressed subjects had a lower rate of seroprotection than non-immunosuppressed subjects (44% vs 64%, p=0.06). The proportion with seroprotection was significantly lower in subjects on combination immunosuppression than in those receiving no immunosuppression (36% vs 64%, p=0.02). Patients receiving combined immunosuppression had a significantly lower fold increase in geometric mean titres than those on monotherapy immunosuppression (3.5 vs 11.5, p=0.03). An assay of T lymphocyte activity was performed in a subgroup of 48 subjects. Those with intermediate activity had lower seroprotection than those with high activity (28% vs 61%, p=0.02). The vaccine was well tolerated.

Conclusions Patients with IBD vaccinated with the 2009 H1N1 influenza vaccine had a low rate of seroprotection, particularly among those who were immunosuppressed. Although there is a need for studies of the clinical benefit of vaccines in this population, patients with IBD need to be aware of this reduced immunogenicity.