BACKGROUND: We conducted a clinical trial in October 2009 to evaluate the immunogenicity of the AS03-adjuvanted influenza vaccine (pH1N1 vaccine) in health care workers (HCWs). By 2 weeks after vaccination, 97% had protective hemagglutinin inhibition (HI) titers (≥ 40) however, 16% were low responders (LR) and failed to maintain a protective response 90 days after vaccination. METHODS: We analyzed the humoral responses (HI, antibody-secreting cell [ASC], and serum immunoglobulin G [IgG]) in 15 LRs and 25 control HCWs. Twelve LRs were revaccinated with the pH1N1 vaccine, and 7 were subsequently vaccinated with the 2010 seasonal trivalent influenza vaccine. We conducted a long-term analysis of the humoral and CD4(+) T-helper (Th) 1 responses. RESULTS: The LRs had a slower HI antibody response than the control HCWs, with protective antibody titers not reached until 2 weeks after vaccination in the majority of the participants. The LRs also had significantly lower IgG ASCs at day 7 and HA1-specific serum IgG responses at day 21, compared with the control HCWs. Revaccination with the pH1N1 vaccine elicited rapid HI antibody, ASC, memory B cell, and multifunctional CD4(+) Th1 cell responses. CONCLUSION: This study shows that revaccination of low-responding HCWs with the pH1N1 vaccine is required for maintaining long-term protection. CLINICAL TRIALS REGISTRATION: NCT01003288.