JENNERIAN VACCINATION. 459 



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 individuals 

 who have contracted the disease. While vaccination is 

 undoubtedly efficacious in protecting against smallpox, 

 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 epidemics 

 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 revaccination 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 investi- 

 gated in this country by a Royal Commission, whose 

 general conclusions are as follows. Vaccination diminishes 

 the liability to attack by smallpox, and when the latter 

 does occur, the disease is milder and less fatal. Protection 

 against attack is greatest during nine or ten years after 

 vaccination. It is still efficacious for a further period of 



