Swine influenza viruses, which contain genes from human, swine, and avian influenza A viruses, have been identified in swine in the United States since 1998 (Olsen, 2002; Vincent, Ma, Lager, Janke, & Richt, 2008). From 2005 through 2009 there had been twelve cases of human infection with such viruses identified in the United States (Shinde, et al., 2009). On April 15 and April 17, 2009, the Centers for Disease Control and Prevention (CDC) identified two cases of human infection with a swine-origin influenza A virus (S-OIV) characterized by a unique combination of gene segments that had not previously been identified among human or swine influenza A viruses. This new flu virus strain was designated Swine-Origin Influenza A (H1N1) or simply H1N1.

Meanwhile, Mexico, during March and early April of this year, experienced outbreaks of respiratory illness and increased reports of patients with influenza-like illness (ILI) in several areas of the country. On April 12, the Mexican General Directorate of Epidemiology (DGE) reported an outbreak of ILI in a small community in the Mexican state of Veracruz to the Pan American Health Organization (PAHO) in accordance with International Health Regulations. On April 17, a case of atypical pneumonia in Oaxaca State prompted enhanced surveillance throughout Mexico. On April 23, several cases of severe respiratory illness laboratory confirmed as swine-origin influenza A (H1N1) virus infection were reported to the PAHO. Sequence analysis revealed that the patients were infected with the same S-OIV strain detected in two children residing in California (Ginsberg, et al., 2009).

On April 17, Mexican public health authorities responded to the increase in reports of respiratory illness by issuing a national epidemiologic alert to all influenza-monitoring units and hospitals. This alert asked hospitals to report all patients with severe respiratory illness and recommended collection of diagnostic specimens from these patients within 72 hours of illness onset.

Between March 1 and April 30, a total of 1,918 suspected cases of the new flu strain were reported, including 97 for which it could be confirmed that the new H1N1 strain was involved. As might be expected, the majority of reports of suspected cases were for hospitalized patients, reflecting the concentration of surveillance efforts within hospitals. Of 1,069 patients with suspected, probable, or confirmed cases 755 were hospitalized, and the remaining 314 were seen in outpatient settings or emergency departments. A total of 84 deaths among suspected cases have been reported.

In previous instances of human-to-human transmission of other swine-origin influenza viruses (Wells, et al., 1991; Myers, Olsen, & Gray, 2007) there have been small clusters of cases and limited generations of disease transmission -- that is, transmission from one human to another. Several findings suggest that transmission in the current epidemic in Mexico has involved person-to-person spread with multiple generations of transmission. This makes it less likely that the epidemic will be self-limiting.

The clinical spectrum of S-OIV illness is not yet well characterized in Mexico. The evidence so far suggests that H1N1 transmission is widespread and that less severe (uncomplicated) illness is common. Patients with confirmed disease have been identified in several Mexican states, and suspected cases have been identified in all states of Mexico. This suggests that H1N1 transmission is widespread in Mexico. Because, case-finding in Mexico has focused on patients seeking care in hospitals, and the selection of cases for laboratory testing has focused on patients with more severe disease, it is likely that a large number of undetected cases of illness may exist in persons seeking care in primary-care settings or not seeking care at all. As in all epidemiologic studies, conclusions based on clinical cases (and even more so on hospitalized cases) may lead to grossly exaggerated ideas of disease severity (Duncan, 1997). Therefore, we cannot yet tell whether the reports of 84 deaths possibly attributable to this new flu strain indicate that it is a severe disease threat or not. It seems likely that this strain will go pandemic, but since we have no clear evidence of either its pathogenicity or its virulence, we cannot know whether it will present us with a major threat to public health or only a threat of minor illness. Perspective may be gained by remembering that the numbers of both suspected cases and deaths being reported from Mexico are dwarfed by the numbers of cases of flu and flu mortality that occur every year in Mexico or the United States due to seasonal flu epidemics.

This article is adapted in part from material provided by the Centers for Disease Control and Prevention. Further information can be accessed online at http://www.cdc.gov/mmwr .

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Duncan, D.F. (1997). Uses and misuses of epidemiology in shaping drug policy. Journal of Primary Prevention, 17(4), 375-382. Accessed online at http://www.duncan-associates.com/UsesandMisuses.pdf

Ginsberg, M., et al. (2009) Swine Influenza A (H1N1) Infection in Two Children---Southern California, March--April 2009. MMWR 58, 400-402. Accessed online at http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5815a5.htm

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Shinde, V., et al. (2009). Triple-reassortant swine influenza A (H1) in humans in the United States, 2005–2009. New England Journal of Medicine, 2009, 361.

Vincent, A.L., Ma, W., Lager, K.M., Janke, B.H., and Richt, J.A. (2008) Swine influenza viruses: a North American perspective. Advances in Virus Research, 72, 127-154.