Lessons from the Past: Methodological Issues Arising from Comparison of the Disease Burden of the Influenza A (H1N1) Pandemic 2009–10 and Seasonal Influenza 2010–2019 in the United States

BACKGROUND: Annual influenza outbreaks constitute a major public health concern, both in the United States and worldwide. Comparisons of the health burdens of outbreaks might lead to the identification of specific at-risk populations, for whom public health resources should be marshaled appropriately and equitably. METHODS: We examined the disease burden of the 2009–10 influenza A (H1N1) pandemic relating to illnesses, medical visits, hospitalizations, and mortality, compared to influenza seasons 2010 to 2019, in the United States, as compiled by the Centers for Disease Control. RESULTS: With regard to seasonal influenza, rates of illnesses and medical visits were highest in infants aged 0–4 years, followed by adults aged 50–64 years. Rates of hospitalizations and deaths evinced a starkly different pattern, both dominated by elderly adults aged 65 and over. Youths aged 0 to 17 years were especially adversely affected by the H1N1 pandemic relative to hospitalizations and mortality compared to seasonal influenza; but curiously the opposite pattern was observed in elderly adults (aged 65 and older). CONCLUSIONS: Determination of a baseline influenza mortality profile in the United States over the 2010–19 decade is not straightforward. The disease burden of the 2009–10 influenza A pandemic among the elderly was strikingly unlike that observed in the subsequent influenza seasons 2010 to 2019: the past did not predict the future.