Objective: The aim of this study was to determine the humoral immune response to influenza A H1N1 2009 and cross reactivity against seasonal H1N1 and H3N2 strains. Methods: Baseline and convalescent sera from 88 subjects with confirmed H1N1 2009 were screened for serological responses by HAI assay. Results: Protective antibody titres to H1N1 2009 were present in 87% post-infection, but varied with age, sex and pregnancy. Some cross reactivity with seasonal influenza strains was observed. Conclusions: Females and pregnant subjects had an attenuated immune response to H1N1 2009 in comparison to the rest of the study population. Antibodies from the serum of H1N1 2009 infected subjects cross reacted with seasonal H1N1 and H3N2 influenza viruses.