influenza pandemic (H1N1) of 2009

influenza pandemic (H1N1) of 2009, the first major influenza outbreak in the 21st century, noted for its rapid global spread, which was facilitated by an unusually high degree of viral contagiousness. Global dissemination of the virus was further expedited by the unprecedented rates of passenger travel that characterize the modern era.

• Read more about the impact of the COVID-19 pandemic.

The pandemic virus caused a respiratory disease typical of that resulting from infection with seasonal influenza. However, despite local, national, and international efforts to contain the virus, its more contagious nature led to the infection of millions of people. The calculation of accurate global figures by entities such as the World Health Organization (WHO) was precluded by case underreporting and difficulty in obtaining samples from affected individuals, particularly in developing countries. As a result, the 622,482 cases and 18,500 deaths confirmed by laboratory analysis by WHO were considered gross underestimates. Indeed, later analyses based on statistical models that took into account countries with limited influenza-surveillance data indicated that the actual total number of deaths from the outbreak may have been as high as 284,500 to 575,400.

Symptoms and transmission

Persons infected with H1N1 experienced fever and mild respiratory symptoms, such as coughing, runny nose, and congestion. In some cases symptoms were severe and included diarrhea, chills, and vomiting, and in rare cases respiratory failure occurred. The H1N1 virus caused relatively few deaths in humans. In the United States, for example, it caused fewer deaths (between 8,870 and 18,300) than seasonal influenza, which, based on data for the years 1993–2003, causes an average of about 36,170 deaths each year. The H1N1 virus was most lethal in individuals affected by chronic disease or other underlying health conditions.

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The virus was passed from human to human primarily through inhalation of infectious particles or contact with an infected individual or a contaminated surface. These modes of transmission proved rapid and increased the potential for the virus’s global spread. The H1N1 virus of 2009 was highly contagious; between 22 and 33 percent of people who came into contact with an infected individual became infected themselves. This measure of the frequency of new cases of disease arising through contact with infected persons, which is known as the secondary attack rate, was higher for H1N1 flu than for seasonal influenza. (The typical secondary attack rate of seasonal influenza is between 5 and 15 percent.)

Early stages of the outbreak

Evidence of an influenza-like illness first appeared in February 2009 in a small town called La Gloria in Veracruz, Mex. The following month the illness emerged in Mexico City. Officials investigating the outbreak quickly traced the illness to La Gloria, where a young boy, who later became known as “patient zero,” was discovered to be infected with a previously unknown strain of influenza virus. The virus was a strain of swine influenza, and thus the outbreak adopted the name “swine flu.” Although the boy represented the first known case, researchers who continue to investigate the virus suspect that it emerged sometime in 2008. Despite substantial progress made in the characterization of the virus, its origins remain unknown.

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By the end of April more than 2,000 cases of the influenza-like illness had been reported in Mexico City and elsewhere in Mexico. Laboratory testing of a small subset of patients confirmed that a swine influenza virus was the cause of their illness. The virus that was detected was a subtype known as influenza A H1N1, though it was initially identified as a new strain of swine influenza virus because it consisted of genetic material from two different swine influenza viruses as well as genetic material from human and avian strains of influenza virus. The new H1N1 virus emerged in the United States in April 2009, in Texas, New York, California, and several other places. The virus was suspected of having been carried to those states by individuals who had been in affected areas in Mexico and then traveled from there.

On April 25, 2009, the director general of the World Health Organization (WHO), Margaret Chan, declared the outbreak a public health emergency of international concern. Within days of Chan’s announcement, the H1N1 virus reached Spain, having been carried to that country by individuals traveling by airplane from Mexico. Confirmed cases of H1N1 infection also occurred in Germany, Austria, the United Kingdom, Israel, and New Zealand. Several provinces in Canada, including Nova Scotia, Alberta, Ontario, and British Columbia, also were affected. Although most persons who fell ill recovered, there were H1N1-related deaths in Mexico and the United States. In addition, many more cases of the disease were suspected in other countries, including Australia, Chile, Colombia, and France.

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Although it was not clear whether all the cases in these other countries were caused by the H1N1 virus, several of the already confirmed cases in multiple countries demonstrated evidence of human-to-human transmission. This evidence prompted Chan and WHO on April 29 to declare a level 5 pandemic alert for the H1N1 outbreak. A level 5 pandemic alert indicated that WHO believed a swine flu pandemic was imminent and called for accelerated distribution of drugs to treatment facilities and rapid implementation of measures to control viral spread as much as possible.

At the end of April, there arose significant confusion concerning the name given to the outbreak. In some countries, “swine flu” was incorrectly believed to be caused by pigs, leading to the unnecessary slaughter of otherwise healthy pigs. This was most evident in Egypt, where a mandate for the slaughter of 400,000 pigs was issued by Minister of Health Hatem al-Gabali, though the animals showed no signs of viral infection. Egyptian pig farmers protested against the order, fueling riots that caused a great deal of concern among the country’s citizens. To relieve tensions and to avoid further confusion, WHO officially renamed the outbreak influenza A (H1N1). It later assumed the name pandemic (H1N1) 2009, or some variation thereof, being known generally as H1N1 flu. The virus itself also was given various names, including novel influenza A (H1N1) virus.