The proportion of persons 18-60 years of age with a ≥4-fold increase in HAI titer who received 1 dose (15 μg) of monovalent pandemic (H1N1) 2009 nonadjuvant vaccine was 96.2%, and the proportion with an increased HAI titer ≥40 was 97.1%, results similar to those of a recent study (12) . However, the geometric mean titer in healthy vaccinated persons was 237.8, a 34.5-fold increase over the prevaccination titer, which was greater than that for patients naturally infected with pandemic (H1N1) 2009 virus (12) . This fi nding may have resulted from impaired adaptive immune responses against pandemic (H1N1) 2009 virus in the initial phase, which included decreased numbers of CD4 and B lymphocytes and an increase in T regulatory cells (4) .