Table 3  Numbers (percentages) of subjects who were seroprotected at two and six months after vaccination (intention to treat analysis) Month Antigen (µg) No of subjects 2 6 5 25 5 (20) 1 (4) 10 25 3 (12) 6 (24) 20 25 8 (32) 10 (40) 40 25 11 (44) 9 (36) Total 100 27 (27) 26 (26) Odds ratio for a doubling in dose of vaccine (95% confidence interval) 1.63 (1.07 to 2.49) 1.87 (1.20 to 2.92) P value 0.02 0.006