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In page Booster dose:

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Upon its invention in the 1950s, the pertussis vaccine was whole-cell (contained the entire inactivated bacterium), and could cause fever and local reactions in people who received the vaccine. In the 1990s, people in the US started using accelular vaccines (contained small portions of the bacterium), that had lower side effects but were also less effective at triggering an immunological memory response, due to the antigen presented to the immune system being less complete.[3] This less effective, but safer vaccine, led to the development of the booster Tdap.