JENNERIAN VACCINATION. 499 



practised all over the world, as it is at the present 

 day. 



The evidence in favour of vaccination is very strong. 

 There is no doubt that inoculation with lymph properly 

 taken from a case of cowpox, can be maintained with very 

 little variation in strength for a long time by passage from 

 calf to calf, and such calves are now the favourite source 

 of the lymph used for human vaccination. When lymph 

 derived from them is used for the latter purpose, immunity 

 against smallpox is conferred on the vaccinated individual. 

 It has been objected that some of the lymph which has 

 been used has been derived from calves inoculated, not 

 with cowpox, but with human smallpox. It is possible 

 that this may have occurred in some of the strains of lymph 

 in use shortly after the publication of Jenner's discovery, 

 but there is no doubt that most of the strains at present in 

 use have been derived originally from cowpox. The most 

 striking evidence in favour of vaccination is derived from 

 its effects among the staffs of smallpox hospitals, for here, 

 in numerous instances, it is only the unvaccinated in- 

 dividuals who have contracted the disease. While vaccina- 

 tion is undoubtedly efficacious in protecting against small- 

 pox, Jenner was wrong in supposing that a vaccination in 

 infancy afforded protection for more than a certain number 

 of years thereafter. It has been noted in smallpox epi- 

 demics which have occurred since the introduction of 

 vaccination, that whereas young unprotected subjects 

 readily contract the disease, those vaccinated as infants 

 escape more or less till after the i3th to the i5th years. 

 It has become, therefore, more and more evident that re- 

 vaccination is necessary if immunity is to continue, and 

 where this is done in any population, smallpox becomes a 

 rare disease, as has happened in the German army, where 

 the mortality is practically nil. The whole question of the 

 efficacy of vaccination has recently been investigated in 

 this country by a Royal Commission, whose general con- 

 clusions are as follows. Vaccination diminishes the 

 liability to attack by smallpox, and when the latter does 



