Antibody titers were 16 000 AU/mL for 50% protection, 31 000 AU/mL for 70% protection, 43 000 AU/mL for 80% protection, and 63 000 AU/mL for 90% protection for total participants; 27 000 AU/mL for 50% protection and not applicable at the other protection levels for infection-naive participants; and 17 000 AU/mL for 50% protection, 35 000 AU/mL for 70% protection, 50 000 AU/mL for 80% protection, and 76 000 AU/mL for 90% protection for previously infected participants.