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Why did H1N1 activity increase in the United States during autumn 2009?

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Caring for flu victims — influenza pandemic (H1N1) of 2009 Doctors at the Mexico City Navy Hospital wearing protective gear as they tend to patients complaining of H1N1 flu-like symptoms.

The increase in H1N1 activity in the United States during the autumn of 2009 was an expected development, as autumn traditionally marks the onset of the seasonal influenza season in the Northern Hemisphere.1

By mid-September 2009, disease activity rose dramatically, and by late October, 48 states had reported widespread influenza-like illness. This surge occurred while the country faced significant challenges in vaccine distribution; although the U.S. Department of Health and Human Services had planned for the availability of 120 million vaccine doses by mid-October, only about 11 million doses had been delivered by that time. These production delays left a large percentage of the population susceptible to infection, prompting President Barack Obama to declare the outbreak a national emergency on October 24 to facilitate the use of federal resources for response efforts.1