Why did confirmed H1N1 case and death totals underestimate the pandemic's true impact?

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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 initial reliance on laboratory-confirmed cases and deaths significantly underestimated the true impact of the 2009 H1N1 pandemic because this method failed to account for the large number of affected individuals who never sought medical care.1

Because these individuals did not visit healthcare providers, they were never subjected to the laboratory testing that formed the basis of early tracking efforts. Recognizing this limitation, the Centers for Disease Control and Prevention (CDC) adjusted its methodology in late 2009 to better reflect the actual scale of the outbreak. By shifting from strictly laboratory-confirmed counts to statistical estimations, the CDC revealed a much higher burden of disease, estimating that between 43 million and 89 million cases and up to 18,300 deaths had occurred in the United States alone between April 2009 and mid-April 2010.1