216 INFECTION AND IMMUNITY 



was made compulsory the number of deaths fell to 

 80 per million. In Sweden the mortality in the eight- 

 eenth century exceeded 2000 per million of the in- 

 habitants. From 1801 to 1815, when vaccination was 

 largely practised but was not compulsory, the death- 

 rate fell to 631 per million. Since this preventive 

 measure has been made compulsory the rate has con- 

 tinued to fall until it is not more than 8 per million. 



The immunity resulting from vaccination is not 

 absolute and it is well known that very susceptible 

 persons may suffer a second or even a third attack of 

 smallpox. But attacks in vaccinated persons are com- 

 paratively rare and comparatively mild. 



This is shown by statistics collected by the London 

 Board of Health, which show that the mortality 

 among unvaccinated persons was 20.85 ^> while among 

 the vaccinated it was only 3.40$. The gradual loss 

 of immunity after vaccination is established by the 

 well-known fact that revaccination after a longer or 

 shorter interval is successful in a considerable pro- 

 portion of the cases. This is therefore to be recom- 

 mended whenever smallpox is prevalent. 



As a result of neglect of vaccination an epidemic 

 occurred in Sheffield, England, some years ago. 

 Statistics collected during this epidemic (1887-88) 

 show that among children less than ten years of age 

 the vaccinated were attacked in the proportion of 



