2009 swine flu outbreak in Mexico

In March and April 2009, an outbreak of a new strain of influenza commonly referred to as "swine flu" infected many people in Mexico and other parts of the world, causing illness ranging from mild to severe. Initial reports suggested that the outbreak had started in February due to farming practices at a pig farm half-owned by Smithfield Foods. Smithfield Foods stated that it had found no clinical signs or symptoms of the presence of swine influenza in the company's swine herd, or among its employees at its joint ventures in Mexico, that it routinely administers influenza virus vaccination to their swine herds and that it conducts monthly testing for the presence of swine influenza. The new strain was identified as a combination of several different strains of Influenzavirus A, subtype H1N1, including separate strains of this subtype circulating in humans and in pigs. The World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC) have expressed serious concerns that the new strain, which transmits between humans and has had a relatively high mortality rate in the possible and confirmed Mexican cases, has the potential to become an influenza pandemic. It is reported that, because the virus is already widespread, containment will be impossible.

As of April 27, 2009, all schools nationwide remained closed until at least May 6. Health Minister Córdova noted that the number of new cases had declined during the three days: from 141 on Saturday to 119 on Sunday and 110 on Monday. It is unclear why there were more deaths in Mexico than in other areas, as there were multiple potential variables, such as a stronger strain of the virus or more exposure to it. No definite conclusion had been reached, however the CDC reported that swine flu viruses in the US and Mexico matched.

The first infected was registered in San Diego, California on April 2, but it was missclassed as an H2N3 case. It was not until April 13, with the first death in Oaxaca, that further research was carried out, officially detecting the new type of virus on April 16. The pandemic outbreak was first detected in the Federal District, where surveillance began picking up a surge in cases of influenza-like illness starting on March 18. The surge was assumed by authorities to be "late-season flu" (which usually coincides with a mild Influenzavirus B peak) until April 21, when a CDC alert concerning two isolated cases of a novel swine flu was reported in the media (see 2009 swine flu outbreak in the United States). Both cases were in US counties on the Mexico – United States border, adjacent to one state in Mexico where the influenza surge had been detected. This was the missing link that connected the surge and the new strain, and established the high suspicion of an outbreak. One study estimated that by the end of April, at least 113,000 to 375,000 people in Mexico had been infected. Then, in early May, influenza activity began to decline, likely due to the shutdown of most public places in response to the outbreak. However, H1N1 remains present in Mexico, and as of December, was classified by WHO as widespread.

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