|MadSci Network: Immunology|
The short answer is no. It is difficult to be sure if you still have immunity to something you were immunized with 20 years ago. The strength and length of memory depends on the nature of the antigen that you received, either as an immunization or as the natural disease. However, based on experience, it appears that immunity to most childhood diseases is life long, either because of the persistence of memory cells, or because you are no longer susceptible for some non-immunologic reason. Of the immunizations you have probably received, the only one routinely re-administered is tetanus, an inactivated toxin from the bacterium Clostridium tetani that apparently doesn't elicit good memory cell responses. Smallpox vaccination is no longer practiced since the disease has been eradicated from the world due to a combined effort of the World Health Organization and local governments. The virus only exists (hopefully) in two high-containment research facilities, and there has been a vigorous debate about whether or not these stocks should be destroyed.
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