Age distribution for T cell reactivity to vaccinia virus in a healthy population.

The potential for bioterrorism involving smallpox has led to a debate about the durability of protective immunity against smallpox from vaccination. By assessing the T cell reactivity to vaccinia virus in a healthy population, we show that subjects who were vaccinated within the past 3 decades and who have a visible vaccination scar had remarkable T cell reactivity. However, person who were vaccinated within the past 3 decades but who do not have a scar and those who were vaccinated >4 decades ago had responses as low as those in unvaccinated subjects. Thus, we estimate that the significant T cell memory response to vaccinia virus from successful vaccination may persist for only 20-30 years. Furthermore, we found the vaccinia-specific cellular immunity could be easily assessed by determination of the frequencies of vaccinia-specific CD69 expression on T cell subsets. These data may help in the development of public health strategies to counter bioterrorism threats associated with smallpox.