Creating immunity
=================

* John Hoey

Regarding our editorial,1 Susan Fletcher's conclusion that “herd immunity through vaccination is a flawed concept” is based on 2 undeniably true facts: first, infectious disease rates have fallen over time as living conditions have improved; second, that immunity after exposure to infectious disease (by vaccination or infection) also declines over time. But it is illogical to conclude that because living conditions are important, immunity is not. Both factors have contributed enormously to the observed decreases in death and illness resulting from infectious diseases.

Science and common sense concur that if there is less infectious disease in the community (because almost everyone is vaccinated), then there is much less chance that those not immunized or only partly immunized will come into contact with the infectious agent and become ill. Furthermore, if very large proportions of the “herd” are effectively immunized, diseases that survive only in humans can be eliminated. Polio and measles in Canada and many other countries are good examples (not to mention smallpox). Vaccination and herd immunity undoubtedly play important causal roles in the declining rates of infectious diseases, but they are not the only factors. 

I agree with Fletcher that we need better reporting systems for adverse reactions to vaccines. The public health sector in this country and elsewhere also need to find better ways to share existing knowledge about the serious health effects of infectious diseases and about the risks and benefits of vaccination.1

**John Hoey** *CMAJ*

## Reference

1.  1. A patchwork policy: vaccination in Canada [editorial]. CMAJ 2003;168(5):533.
    
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