People immunized years ago have lost most if not all protection from smallpox.
Length of Protection
Smallpox vaccination provides high level immunity for 3 to 5 years and decreasing immunity thereafter. If a person is vaccinated again later, immunity lasts even longer. Historically, the vaccine has been effective in preventing smallpox infection in 95% of those vaccinated. In addition, the vaccine was proven to prevent or substantially lessen infection when given within a few days of exposure. It is important to note, however, that at the time when the smallpox vaccine was used to eradicate the disease, testing was not as advanced or precise as it is today, so there may still be things to learn about the vaccine and its effectiveness and length of protection.
http://www.bt.cdc.gov/agent/smallpox/vaccination/facts.aspStudies have shown that even after a single vaccination, 75yrs ago, most people still have some neutralizing antibodies which will give some level of protection. Not enough immune response for complete protection, but enough to greatly reduce the disease progression. Of course, since routine smallpox vaccination stopped in the mid-late '70s, anyone born since then would not have been routinely vaccinated.
http://www.sciencentral.com/articles/view.php3?language=english&type=article&article_id=218392037