REPORT OF THE ROYAL VACCINATION COMMISSION. 675 



small-pox, on the lack of sanitary conditions, was a feature of the history 

 of small-pox during the eighteenth century. 



Upon the whole, then, we think that the marked decline of small-pox 

 mortality in the first quarter of the present century affords substantial 

 evidence in favour of the protective influence of vaccination. 



Age Incidence of Small-pox. 



A study of the age incidence of small-pox mortality is very instructive. 

 In connexion with this point it is necessary to bear in mind that experi- 

 ence has led to the conclusion that, whatever be the protective effect of 

 vaccination, it is not absolutely permanent ; the most convinced advocates 

 of the practice admit that after the lapse of nine or ten years from the 

 date of the operation its protective effect against an attack of small-pox 

 rapidly diminishes, and that it is only during this period that its power in 

 that respect is very great ; though it is maintained that, so far as regards 

 its power to modify the character of the disease and render it less fatal, 

 its effect remains in full force for a longer period, and never altogether 

 ceases. The experience upon which this view is founded is derived almost 

 exclusively from the case of infantile vaccination. It has been supposed 

 by some that the transitory character of the protection results from 

 changes connected with the growth from infancy to adult years. Whether 

 this be so or not, we have no means of determining. 



No doubt, when Jenner drew the attention of the public to the value 

 of vaccination, he believed that a single successful inoculation of vaccine 

 matter secured absolute immunity for the future from an attack of small- 

 pox. It is certain that in this he was mistaken. It may well be doubted 

 whether the anticipation was a reasonable one. No such immunity is 

 secured by an attack of small-pox, though there are few who would 

 maintain the proposition that it is without protective influence against 

 another attack. A priori there would seem to be no sound ground for 

 expecting that vaccinia would afford more potent protection than small- 

 pox itself. The extent of the protection afforded (assuming that there is 

 some protective influence) could only be determined by experience. It soon 

 became apparent that Jenner had, in the first instance, overrated the 

 effect of vaccination. That he should thus have overestimated it 

 is not to be wondered at, when the tendency to be unduly sanguine, 

 which besets the discoverer of any new prophylactic, and, indeed, every 

 discoverer, is borne in mind. 



We think, taking it all together, that the evidence bearing upon 

 the question whether the vaccinated are less liable to be attacked by 

 small-pox than the unvaccinated, points to two conclusions : first, that 

 there is, taking all ages together, less liability to attack among the 

 vaccinated than among the unvaccinated ; and next, that the advantage in 

 this respect enjoyed by vaccinated children under ten years of age is 

 greatly in excess of that enjoyed at a more advanced period of life. 



In considering whether vaccination has been the principal cause of the 

 decline, we must inquire whether the other causes suggested by those who 

 deny the efficacy of vaccination will satisfactorily account for it. 



