Table 3. Geometric Mean HI Titers against the 2009 A/H1N1 Influenza Virus in Two Groups of People with Different Seasonal Influenza Antibody Levels.

	Seroprotective for seasonal influenza prior to vaccination?
Time	Yes† (N = 38) (95% CI)	No ° (N = 20) (95% CI)
Pre (N = 58)	18 (14-24)*	12 (9-16)
day 3 (N = 58)	23 (17-31)*	14 (11-18)
day 5 (N = 58)	37 (29-48)*	25 (21-29)
day 10 (N = 58)	111 (82-150)**	53 (40-69)
day 14 (N = 58)	143 (104-198)**	70 (54-89)
day 21 (N = 58)	146 (105-203)**	70 (54-89)

^†Individuals with prevaccination seasonal influenza HI titer ≥1∶40.

° Individuals with prevaccination seasonal influenza HI titer <1∶40.

*(p<0.05) and

**(p<0.001) represent the statistical differences between the two groups, two-way Student's t-test.