For all you seasonal flu vaccination types:
Quote:
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Flu vaccines confer immunity for one to two years. Actually getting sick with the flu can result in immunity from that particular strain that lasts 20 years or more, CDC tests show. That's why older adults may have a certain degree of protection against H1N1 — because many of them were exposed to a close cousin of this strain that circulated before 1957.
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http://www.usatoday.com/news/health/...children_N.htm