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The immune system is not an all or nothing defense. For some diseases we develope life-long immunity (chicken pox), but for others we develop a transient rise in antibody levels that dwindle over time. Without a "reminder" to the immune system, the level of circulating antibodies fall below the effective range. That's the reason you need a tetanus shot every ten years, but never need to redo your polio immunity IF you got the attenuated Sabine vaccine. The Salk vaccine made with dead virus extracts did not confer lifetime immunity.

Each disease is different. Hell, each strain of the flu is somewhat different. Even as a health officer, I don't normally take the flu shot myself. I did this year along with all five of my girls after I read the early reports in October. I have also rat-holed Tamiflu and amantadine.

Stay safe, stay warm, and stay away from Democrats....

That's the reason you need a tetanus shot every ten years, but never need to redo your polio immunity IF you got the attenuated Sabine vaccine.

But aren't both examples above related to the frequency of change in the immunizing antigens (not all are well preserved in killed vaccines). Even when killed is not as good as attenuated, isn't protection for more than several months?