FIG. 3.

Durable antiviral antibody responses are induced by both smallpox infection and vaccination. (a) Quantitation of antiviral IgG responses by ELISA. In these experiments, ELISA plates were coated with VAC lysate and the titers of virus-specific serum IgG were determined and expressed as ELISA units (EU). Samples that scored <100 EU are considered seronegative, and samples from naive, unvaccinated subjects (n = 10) score below this cutoff value (designated by the dashed line). (b) Quantitation of orthopoxvirus-specific neutralizing antibody responses. In these studies, 2-fold dilutions of serum were incubated with VAC, and plaque reduction neutralization tests were performed. The NT₅₀ is defined as the reciprocal of the serum dilution required to reduce infectious virus (i.e., VAC plaques) by 50%. Symbols and group definitions are as described in the legend to Fig. 1.