2009 Swine Flu Pandemic: The H1N1 Influenza Outbreak Worldwide

The 2009 swine flu pandemic was a worldwide outbreak of a new strain of influenza A, known as H1N1, that was first identified in Mexico and the United States in April 2009. The World Health Organization (WHO) declared it a pandemic on 11 June 2009, the first influenza pandemic of the 21st century. Although milder than many experts had feared, it killed an estimated 150,000 to 575,000 people in its first year and led to one of the largest vaccination campaigns in history.

Background: influenza pandemics

Influenza viruses change constantly, causing seasonal epidemics every winter. Occasionally, a strikingly new virus emerges to which most people have little immunity, and it can spread around the world as a pandemic. The 20th century saw three: the Spanish flu of 1918, the Asian flu of 1957 and the Hong Kong flu of 1968.

In the early 2000s, health authorities feared a new pandemic, especially from the H5N1 bird flu virus spreading in Asia. The SARS outbreak of 2002–2003 had also shown how quickly a new respiratory disease could travel by air. Many governments therefore prepared pandemic plans and stockpiled antiviral drugs such as oseltamivir (Tamiflu).

Emergence in Mexico and the United States

In March and April 2009, Mexico saw an unusual rise in severe respiratory illness, including in the town of La Gloria in Veracruz state and in Mexico City. In mid-April, the US Centers for Disease Control and Prevention (CDC) identified a new H1N1 virus in two children in California, and the same virus was soon confirmed in Mexican patients.

The new virus was a mixture of genes from swine, avian and human influenza viruses. Most of its genes came from viruses that had circulated in pigs, which is why it was quickly nicknamed “swine flu”. Later genetic studies suggested it had circulated undetected in pigs for years before passing to humans, probably in Mexico, though the exact origin remains uncertain.

On 23–24 April 2009 Mexico closed schools in Mexico City, and later suspended many public events and non-essential activities. On 25 April the WHO declared a Public Health Emergency of International Concern, the first under the revised International Health Regulations.

Global spread and the pandemic declaration

The virus spread rapidly by air travel. By late April cases had appeared in Canada, Spain, the United Kingdom, New Zealand and other countries.

Governments reacted in very different ways. Some screened arriving passengers for fever, and a few countries quarantined travellers from Mexico or restricted flights, measures the WHO did not recommend. Several countries banned imports of pork from Mexico and the United States, although there was no evidence that eating pork could spread the disease. Egypt ordered the slaughter of its pigs, a decision widely criticised by health experts. Mexico’s tourism and economy suffered heavily in the early weeks. The WHO raised its pandemic alert level to phase 4 on 27 April and phase 5 on 29 April.

On 11 June 2009, WHO Director-General Margaret Chan declared phase 6, a full pandemic, because the virus was spreading in communities in several regions of the world. She stressed that most cases were mild. By the end of the pandemic, laboratory-confirmed cases had been reported in more than 200 countries and territories.

The Southern Hemisphere, entering its winter, saw a large wave between June and August 2009. In the Northern Hemisphere, a second, larger wave began in the autumn of 2009, peaking in October and November in North America and much of Europe.

Who was affected

Unlike seasonal flu, which mostly kills elderly people, the 2009 H1N1 virus hit children and young and middle-aged adults hardest. Many older people had some protection, probably because they had been exposed to related H1N1 viruses earlier in life.

For most people the illness was similar to ordinary flu, with fever, cough, sore throat and aches, and they recovered within a week. A minority, however, developed severe viral pneumonia, and intensive care units in some cities came under heavy pressure during the peaks. Pregnant women, people with chronic diseases and people with severe obesity faced higher risks of serious illness. Indigenous populations in several countries, including Canada, Australia and New Zealand, were also affected disproportionately.

How many died

About 18,500 laboratory-confirmed deaths were reported to the WHO by August 2010. Experts agreed this was a large undercount, because most people who died were never tested.

A study led by CDC scientists and published in 2012 estimated that between 151,700 and 575,400 people died worldwide from the virus during the first 12 months of its circulation, with a large share in Africa and South-East Asia. Because victims were relatively young, the years of life lost were higher than in a typical flu season.

Vaccines and treatment

Manufacturers began producing a pandemic vaccine within weeks of the virus’s identification. Vaccination began in China, Australia, the United States and Europe from September and October 2009, but supplies arrived after the autumn wave had already begun in many countries. Governments prioritised groups such as pregnant women, health workers, young children and people with chronic conditions.

Hundreds of millions of doses were produced. Countries also used antiviral drugs widely, though their effectiveness in reducing deaths later became a matter of scientific debate. Studies in some European countries later linked one vaccine, Pandemrix, to a small increase in cases of the sleep disorder narcolepsy, particularly among children and adolescents.

Controversy over the response

Because the pandemic proved milder than feared, the response drew criticism. The Parliamentary Assembly of the Council of Europe and some journalists questioned whether the WHO had exaggerated the threat and whether experts advising it had conflicts of interest with drug manufacturers. Several countries were left with large surpluses of unused vaccine.

An independent review committee reported to the WHO in 2011. It found no evidence of improper commercial influence, but criticised the WHO’s communication, its definition of a pandemic based on spread rather than severity, and the lack of transparency about its advisers. It concluded that the world was not well prepared for a severe pandemic.

Others defended the WHO, arguing that in the first weeks no one could know how severe the new virus would be, and that preparing for the worst was the responsible choice. Many public health experts concluded that the main lesson was the need for faster vaccine production and more flexible plans that could be scaled to the actual severity of a pandemic.

On 10 August 2010, Margaret Chan announced that the world had moved into the post-pandemic period. The H1N1 virus continued to circulate as one of the seasonal flu strains.

Timeline

• 2009, March–April: Unusual respiratory illness is reported in Mexico.
• 2009, mid-April: The CDC identifies the new H1N1 virus in two Californian children.
• 2009, 25 April: The WHO declares a Public Health Emergency of International Concern.
• 2009, 29 April: The WHO raises the pandemic alert to phase 5.
• 2009, 11 June: The WHO declares a pandemic (phase 6).
• 2009, October–November: The second wave peaks in the Northern Hemisphere as vaccination begins.
• 2010, 10 August: The WHO announces the end of the pandemic.

Why it matters

The 2009 swine flu pandemic was the first real test of the world’s modern pandemic plans. It showed how fast a new virus could circle the globe and how hard it was to make vaccines in time, and the debates over its handling shaped preparations for later emergencies, including the COVID-19 pandemic.