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Susceptibility to an influenza virus strain is dependent on the immune status of the human host, since our bodies become immune only to the strains that we have been exposed to previously (either naturally or by vaccination).

The lack of previous exposure to this subtype resulted in the rapid spread and increased number of infections, especially in the younger population. Respiratory infections occurred, even among those individuals who had received the seasonal flu vaccine because that vaccine was not formulated to protect against the influenza A 2009 H1N1 subtype.