Effect of Dose, Safety, Tolerability of a New Smallpox Vaccine in Adults Without Previous Smallpox Vaccination

• Number of Participants Reporting Adverse Events of Severe Intensity Post-vaccination With ACAM2000 or Dryvax® Smallpox Vaccine [ Time Frame: Days 0 to 30 post-vaccination ] [ Designated as safety issue: No ]
  The severity of each reported adverse event was classified by the investigator according to the following definitions. None - no symptom; Mild - awareness of sign or symptoms, but easily tolerated; Moderate - discomfort enough to cause interference with usual activity; and Severe - incapacitating with inability to work or perform usual activity.
  
  
• Neutralizing Antibody Response Titers Post-vaccination With ACAM2000 or Dryvax® Smallpox Vaccine. [ Time Frame: Day 30 post-vaccination ] [ Designated as safety issue: No ]
  
• Participants With ≥ 4-fold Increase in Plaque-Reduction Neutralization Test (PRNT50) Titers Post-vaccination With ACAM2000 or Dryvax® Smallpox Vaccine. [ Time Frame: Day 30 post-vaccination ] [ Designated as safety issue: No ]
  

The objective of this study is to determine the minimum dose of ACAM2000 that is calculated to produce a major cutaneous reaction in at least 90% of a population of healthy adults 18-29 years of age and naïve to smallpox vaccine. Specifically, the objectives of this study are to:

1. Compare the safety and tolerability of four dose levels of ACAM2000 and a standard dose of Dryvax® in healthy adults 18-29 years of age and naïve to smallpox vaccine. Safety and tolerability will be determined by examination of the local cutaneous reaction, adverse events, physical examinations, vital signs, structured interviews, and laboratory analysis.

2. Determine the immunogenicity of four dose levels of ACAM2000 and a standard dose of Dryvax® in healthy adults 18-29 years of age by comparing:

   1. the proportion of subjects at each dose level who develop a major cutaneous reaction;
   2. the proportion of subjects in each treatment group who develop neutralizing antibodies, including the fold-increase in antibody titer between Baseline and Day 30 sera; and the geometric mean vaccinia neutralizing antibody titer on Day 30.
3. Determine the minimum dose of ACAM2000 that is calculated to produce a major cutaneous reaction in at least 90% of a population of healthy adults 18-29 years of age and naïve to smallpox vaccine.