The second way is to see whether or not there is a typical skin reaction following vaccination with a traditional smallpox vaccine, given about three months after vaccination with the MVA vaccine. The typical reaction in an unvaccinated person to smallpox vaccine is formation of a blister or "pox" which occurs at the site of vaccination. In a person with immunity to smallpox the skin reaction is much less, and typically consists of a little swelling at the site of vaccination.