How Vaccinating People Can Also Protect Others

Abstract

Edward Jenner demonstrated direct protection against smallpox in a human challenge study in a single subject conducted a century before the pioneering work of Pasteur and Koch laid the foundations of our current understanding of the microbial causes of infection. His paper «On the origin of the vaccine inoculation» published in 1801 concludes with the words: «..and it now becomes too manifest to admit of controversy, that the annihilation of the Small Pox, the most dreadful scourge of the human species, must be the final result of this practice». So Jenner accurately predicted the eradication of smallpox some 175 years later using the technique he had discovered. There are no words with which adequately to do justice to his remarkable foresight. However, Jenner must have taken his observation in James Phipps, the boy he vaccinated with material from a cowpox lesion and then repeatedly challenged with material from smallpox lesions and multiplied it in his head by the number of people living on the planet. Even he could not have known then, what we know now, namely, that his vaccine and nearly all the others developed and widely used since, can do much more than protect recipients against target infections. Setting aside the possibility of non-specific effects, which are beyond the scope of this chapter (see ► Chap. 1), vaccines can break the train of transmission of their target infections between humans, and so vaccinating just some people can be enough to protect everyone. In the cases of smallpox and more recently polio virus type 2, mass vaccination has led to eradication and thus protection for everyone who will ever live. No other advance in medicine comes anywhere close to this extraordinary power of vaccines.