each immunity and vaccine last for certain year in your body. 20 year is max length. if it is expired you need to renew it by getting a shot or be wary to avoid that disease. so to answer your question your doctor is right.
20 year immunity cycles. i keep getting sinusitis every 6-8 months for the last 2 years. prior to that i was getting this every 2-4 years. i asked my physician why is it that i am getting sinusitis at a much greater frequency. he told me that immunity can hold for 20 years cycles in which time immunity can be lost and one has to reacquire immunity. this is the first time i heard of this and can not find anything to support my doctors claims. is it possible he was misinformed? is immunity not lifelong?