The 2009 Influenza Pandemic: An Overview

On April 29, 2009, in response to the global spread of a new strain of influenza, the World Health Organization WHO raised its influenza flu pandemic alert level to Phase 5, one level below declaring that a global influenza pandemic was underway. On June 11, as the virus continued to spread on several continents, WHO declared the outbreak to be an influenza pandemic Phase 6. WHOs pandemic declaration is based on the geographic spread of the virus, not on a worsening of the severity of the illnesses it causes. Officials now believe the outbreak of the new flu strain began in Mexico in March 2009, or perhaps earlier. The novel H1N1 swine flu was first identified in California in late April. Health officials quickly linked the new virus to many of the illnesses in Mexico. Since then, cases have been reported around the world. As of June 11, 2009, almost 29,000 cases were reported in 74 countries, on all continents but Antarctica. Most of the reported cases are in Mexico, the United States, and Canada. However, increasing numbers of cases are now reported in Argentina, Chile, Australia, and other countries in the Southern Hemisphere, as their winter approaches and flu transmission becomes more efficient. Health officials note that reported cases likely represent only a fraction of actual infections. For example, a U.S. official commented in May that there may actually have been upwards of 100,000 cases thus far in the United States.