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The immune system has a very long memory after live viral infections, say researchers who examined repeated blood samples from 45 volunteers working in a primate research centre in Oregon. The samples spanned up to 26 years of follow-up and showed that antibodies generated in response to natural viral infections have half lives ranging from an estimated 50 years for varicella zoster virus to more than 11000 years for Epstein-Barr virus. Antibodies against mumps, measles, and rubella were also remarkably stable, implying lifelong immunity (estimated half lives 542 years, 3014 years, and 114 years).
Humoral immunity against tetanus and diphtheria, induced by vaccination with protein antigens, decayed much faster than that triggered by a natural viral infection. In vaccinated volunteers, tetanus specific antibodies had an estimated half life of just 11 years, and those directed against diphtheria had a half life of 19 years.