Why did the CDC change its method for estimating H1N1 cases and deaths in the United States?

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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.

In late 2009, the Centers for Disease Control and Prevention (CDC) updated its methodology for tracking the H1N1 pandemic to better reflect the true impact of the outbreak in the United States.1

Previously, the CDC relied on laboratory-confirmed cases to track the virus. However, this approach failed to account for the many individuals who contracted the virus but did not seek medical care and therefore never underwent laboratory testing. By adjusting its assessment method to include these uncounted cases, the CDC was able to produce estimates that more accurately portrayed the actual scale of the pandemic, including total infections, hospitalizations, and deaths.1