Table 2  Numbers (percentages) of subjects who seroconverted two and six months after vaccination (intention to treat analysis) Month Antigen (µg) No of subjects 2 6 5 25 13 (52) 15 (60) 10 25 18 (72) 19 (76) 20 25 17 (68) 16 (64) 40 25 21 (84) 20 (80) Total 100 69 (69) 70 (70) Odds ratio for doubling in dose of vaccine (95% confidence interval) 1.56 (1.05 to 2.33) 1.26 (0.85 to 1.86) P value 0.03 0.24