The smallpox vaccine Vaccinia was successfully used to eradicate smallpox, but 
although very effective, it was a very reactogenic vaccine and responsible for 
the deaths of one or two people per million vaccinated. Modified Vaccinia virus 
Ankara (MVA) is a replication-deficient and attenuated derivative, also used in 
the smallpox eradication campaign and now being developed as a recombinant viral 
vector to produce vaccines against infectious diseases and cancer. Many clinical 
trials of these new vaccines have been conducted, and the findings of these 
trials are reviewed here. The safety of MVA is now well documented, 
immunogenicity is influenced by the dose and vaccination regimen, and 
information on the efficacy of MVA-vectored vaccines is now beginning to 
accumulate.