Effectiveness of postexposure vaccination for the prevention of smallpox: results of a delphi analysis.

We estimated the effectiveness of postexposure smallpox vaccination in preventing or modifying disease in naive and previously vaccinated adults, using the formal Delphi technique. For persons not previously vaccinated, the median effectiveness in preventing disease with vaccination at 0-6 h, 6-24 h, and 1-3 days after exposure was estimated as 93%, 90%, and 80%, respectively, and effectiveness in modifying disease among those who develop illness was estimated as 80%, 80%, and 75%, respectively. Effectiveness was greater for those vaccinated previously. High postexposure vaccination effectiveness for preventing or modifying smallpox is consistent with the limited data available, is biologically plausible, and is similar to that seen for other viral vaccine-preventable diseases. These estimates support the Advisory Committee on Immunization Practices recommendations and provide a key parameter for mathematical models on which policy decisions may be based.