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Why did the World Health Organization declare H1N1 a pandemic on June 11, 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 World Health Organization (WHO) declared the H1N1 influenza outbreak a pandemic on June 11, 2009, by raising its alert level from 5 to 6.1

This decision was prompted by the virus's continued spread across multiple regions of the world. By early June, more than 25,000 cases and nearly 140 deaths had been reported globally, with significant upsurges occurring in countries such as Chile, Japan, Australia, and the United Kingdom.1

Under the WHO's pandemic preparedness plan, a level 6 declaration signifies that an outbreak is characterized by globally widespread and sustained disease transmission among humans. This move officially recognized that the virus had moved beyond localized outbreaks to establish a pandemic state, marking the first such declaration since the 1968 Hong Kong flu.21