Gender effects on humoral immune responses to smallpox vaccine

There are no data currently available on gender and racial variation in smallpox vaccine immune responses. We recruited 1076 healthy adults 18–40 years old who received one dose of the US-licensed smallpox vaccine (Dryvax®). Vaccinia neutralizing antibody titers in each subject's serum were determined using a high throughput neutralization assay based on a recombinant, β-gal expressing vaccinia virus. Results are reported as the serum dilution inhibiting 50% of virus activity (ID50). The median ID50 for all subjects was 132.2 (inter-quartile range (IQR) = 78.8, 205.6). While no significant differences were observed with race and ethnicity, females had significantly higher neutralizing antibody titers than males (158.5 [93.2, 255.8] vs. 124.1 [75.2, 185.9]; p < 0.0001). As expected, time since vaccination was also associated with variations in neutralizing antibody titers in our subjects. These data indicate that neutralizing antibody titers following primary smallpox vaccination vary by gender.