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Has there ever been a vaccination program that was so successful, so fast, that we reached nearly 100% vaccination in less than what, a year or so? Why wouldn't every other virus simply mutate in these "reservoirs" and make vaccines a failed idea?

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Not all viruses are created equal; some are much more prone to mutation than others. And indeed, vaccines for highly mutation-prone ones do "fail" in some sense -- that's why we need flu shots every year but one or two doses of the chickenpox vaccine are preventative for a lifetime: the flu is an RNA virus and also has a mechanism for swapping genes between different strains infecting an individual organism (including animals), so it changes a lot, and vaccines only last until the in-the-wild flu changes out from under them, whereas chickenpox/varicella is a DNA virus that's much more genetically stable.

> Not all viruses are created equal

Emphasis on created

* Flu, which has a particularly high mutation rate, does exactly that: in some years the flu vaccine is only 10% effective.

* Most viruses are less infectious than covid, especially delta covid.

* The covid vaccines are novel, targeting a specific protein. Natural immunity targets more than one protein, I've heard up to 28 in case of covid. This should make vaccine escape much harder, as it is significantly more difficult to mutate around 28 proteins at the same time. By 'natural immunity' I mean immunity gained by exposure to the natural pathogen, either through live infection or an inactivated pathogen vaccine.

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