Pre- and postvaccine SBA titers by meningococcal serogroup. Pre- and postvaccination serogroup SBA titers were measured. SBA titers were measured using serial dilutions of patient serum in the presence of human complement to determine the minimum antibody titer capable of killing N meningitidis in vitro. Vaccine response rates were measured a median of 54 days after vaccination. The dashed line indicates a titer of 1:8. Subjects with a prevaccination titer ≥1:8 for a given serogroup are indicated with red dots. Median and IQR bars are shown.