Comparison of the Long-Term Immunogenicity of Two Pandemic Influenza A/H1N1 2009 Vaccines, the MF59-Adjuvanted and

Since the first reports of theA/H1N1 virus inApril 2009, the pandemic influenza virus spread globally and circulated for a long time. The primarymethod for the control of influenza is vaccination, but levels of influenza vaccine-induced antibody are known to decline rapidly during a 6-month period. In adults aged 18 to 64 years, we compared the long-term immunogenicity of two of the influenza A/H1N1 2009monovalent vaccines, 3.75-gMF59-adjuvanted vaccine and 15-g unadjuvanted vaccine. The serumhemagglutinin inhibition (HI) titerswere determined prevaccination and at 1, 6, and 10months after vaccination.One hundred six (88.3%) of the 120 subjectsweremonitored for the entire 10-month period after receiving the influenzaA/H1N1 2009monovalent vaccine. Therewere 60 patientswho received the unadjuvanted vaccine and 46 patientswho received theMF59-adjuvanted vaccine. The seroprotection rates, seroconversion rates, and the geometricmean titer (GMT) folds fulfilled the criteria of the EuropeanMedicinesAgency (EMA) for in-fluenzaA/California/7/2009 (H1N1) at 1month after vaccination irrespective of the vaccine composition. Although theGMTs at 1 month postvaccinationwere somewhat higher in the unadjuvanted vaccine recipients than in theMF59-adjuvanted vaccine recipients, the differencewas not significant (P 0.29). The seroprotection rates at 6 and 10months postvaccinationwere preserved above 70% but only in theMF59-adjuvanted vaccine recipients. In conclusion, low-doseMF59-adjuvanted influenza vaccine, evenwith 3.75g hemagglutinin antigen,might induce excellent long-term immunity that is comparable to the conventional dose of unadjuvanted vac-cine amonghealthy adults aged 18 to 64 years. The pandemic influenza A/H1N1 virus, first reported in April2009, spread globally and circulated for a year. Although it i