influenza pandemic (H1N1) of 2009also called H1N1 flu, byname swine fluthe 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.

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 a substantial number of people. By mid-February 2010, 11 months after the outbreak was first detected in Mexico, more than 15,290 laboratory-confirmed deaths were reported by the World Health Organization (WHO). Because many countries stopped tracking individual cases in September and October 2009, WHO was unable to provide accurate global figures of laboratory-confirmed cases. Their final laboratory-confirmed case count, more than 622,482, was reported in late November 2009.

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.

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.

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.

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.

Pandemic status and response

By early June 2009 more than 25,000 cases and nearly 140 deaths from H1N1 flu had been reported worldwide, the majority of deaths having occurred in Mexico and the greatest number of cases—more than 13,000—having appeared in the United States. The continued spread of the virus across multiple regions of the world prompted WHO to announce to its member countries on June 11, 2009, that it was raising the H1N1 flu pandemic alert from level 5 to level 6. This meant that the ongoing outbreak was officially declared a pandemic.

Prior to the announcement, an upsurge in cases had occurred in Chile, Japan, Australia, and the United Kingdom. The H1N1 flu pandemic was the first influenza pandemic to be declared since the 1968 outbreak of Hong Kong flu, which caused more than 750,000 deaths. However, despite the actuation of disease-control strategies determined to prevent the further spread of H1N1 flu, by late August 2009, six months into the outbreak, a total of 209,450 cases and nearly 2,200 deaths had been reported globally.



In the United States the Centers for Disease Control and Prevention (CDC) periodically published updates on the number of H1N1 cases and deaths in that country. In late 2009 the CDC adjusted their methodology in assessing H1N1 data to account for affected individuals who had not sought medical care and who therefore had not undergone laboratory testing, which had formed the basis for earlier pandemic tracking in the country. The adjusted approach was believed to more accurately portray the actual impact of the outbreak in the United States. Based on this method, the CDC estimated that from April 2009 through mid-April 2010 between 8,870 and 18,300 deaths, between 43 million and 89 million cases, and between 195,000 and 403,000 H1N1-related hospitalizations had occurred in the United States.

On Aug. 10, 2010, Chan announced that the outbreak was no longer a level 6 pandemic. The change in status was made in response to the reduced intensity of local H1N1 outbreaks, which had come to resemble seasonal influenza, as well as to increased levels of acquired immunity against infection within communities and improved vaccination coverage worldwide.

The H1N1 virus

The influenza A H1N1 virus that caused the 2009 pandemic was suspected to have originated in pigs, although this remains a point of speculation. Because the virus was made up of genes from two strains of swine influenza virus as well as genes from human and avian influenza viruses, researchers concluded that it evolved through a process known as genetic reassortment. During reassortment, the three different types of influenza viruses—swine, human, and avian—presumably infected the same host and underwent an exchange of genetic material, thereby giving rise to the pandemic H1N1 strain. The details of how and when this occurred, however, are not clear.

Similar to all other influenza viruses, the 2009 H1N1 pandemic subtype was named for the composition of the proteins hemagglutinin (H) and neuraminidase (N) that form its viral coat. Although the pandemic virus was similar to the influenza viruses that circulate among humans seasonally, the pandemic subtype possessed unique antigens (molecules that stimulate an immune response, primarily through the production of antibodies).

Treatment and prevention

Treatment for H1N1 infection consists of administration of the antiviral drugs oseltamivir (Tamiflu) or zanamivir (Relenza). However, there is some evidence that H1N1 viruses can develop resistance to oseltamivir, which commonly is used as first-line treatment for infection. In October 2009 an intravenously administered antiviral known as peramivir, though not formally approved by the U.S. Food and Drug Administration (FDA), was given emergency-use authorization for the treatment of hospitalized H1N1 patients who had not responded to oral or inhaled antivirals or who had life-threatening illness.

The spread of the virus can be controlled through basic sanitary practices, including washing hands, wearing face masks, and disinfecting potentially contaminated surfaces. However, the most effective method of prevention for high-risk persons, including young children, women who are pregnant, and individuals with compromised immune systems, is vaccination. When the H1N1 virus emerged, there were no vaccines available that could provide immunity against infection. However, the severity of the outbreak prompted the rapid development of a novel vaccine, which was tested in clinical trials beginning in early August 2009. Because the first vaccine tested required two doses, there was immediate concern that not enough vaccine could be manufactured before a potential second wave of illness arrived in the fall in the Northern Hemisphere. The vaccine also required a three-week wait between doses, introducing the possibility that it would not have time to take effect prior to the emergence of another period of high disease activity.

By September, pilot tests of novel single-dose vaccines—developed independently by Chinese biotechnology company Sinovac and Swiss pharmaceutical manufacturer Novartis AG—indicated that sufficient protection could be provided by one injection. Sinovac received approval from the Chinese government in early September to begin mass production of the vaccine, with the goal of generating enough of the agent to vaccinate 5 percent of the Chinese population by 2010. Efforts to increase the global supply of single-dose vaccines being developed by pharmaceutical companies around the world, as well as efforts to distribute these vaccines to countries affected by the pandemic, were considered vitally important to successfully stemming the spread of illness. To aid these efforts, WHO set a goal of obtaining some 200 million donated doses for allocation to 95 low- and middle-income countries. The first stocks of donated vaccine were scheduled for delivery to those countries between November 2009 and February 2010.

In late 2009 two types of H1N1 vaccines were made available: one that could be injected and one that could be inhaled. Whereas a single dose of vaccine was presumed to be sufficient for adolescents and adults, two doses were recommended for children between the ages of 6 six months and 9 years.nine years. An example of an injectable vaccine that was used against H1N1 was Pandemrix, which was developed by GlaxoSmithKline and was approved for use in September 2009 by the European Commission. Pandemrix was used widely, with doses administered in 47 countries worldwide and with some 30 million doses used in Europe alone.

In the postpandemic period, which began on Aug. 10, 2010, vaccination remained an important means of protection against H1N1 infection for high-risk populations, which included young children, pregnant women, and persons affected by underlying respiratory diseases or chronic illness. Many people, however, were wary of vaccination, primarily because of concerns of side effects. Such concerns were reinforced in 2011 when the Pandemrix vaccine was linked to an increase in incidence of narcolepsy, a disorder characterized by sudden uncontrollable spells of sleep. In Finland, for example, the risk of narcolepsy was found to be nine times higher in Pandemrix-vaccinated persons aged 4 to 19 compared with unvaccinated persons of the same age group. A review of Finland’s cases revealed that the Pandemrix vaccine increased the risk of narcolepsy primarily in persons who carried a genetic variation predisposing them to the disorder. Similar outbreaks of vaccine-associated narcolepsy in 11 other countries were under investigation. Despite the link to narcolepsy, health officials maintained that the benefits of H1N1 vaccination outweighed the risks of possible side effects.