An influenza pandemic is a worldwide outbreak of influenza. It is a rare but recurrent event. An influenza pandemic occurs when a new influenza A virus emerges from a specific place, that is antigenically different to the circulating influenza strain(s), that is able to spread easily from person to person and against which most of the population have little or no immunity.
The two types of influenza viruses capable of causing significant human illness are termed influenza A and B. Both these virus types are able to cause annual winter epidemics of varying size and severity, but only influenza A viruses have the ability to cause a pandemic. Influenza A and B viruses are continually altering through a process of random mutation (termed antigenic drift), every few years this can result in a significant epidemic. Influenza A can change substantially (antigenic shift), leading to a new subtype with the potential to cause a pandemic. This can be as a result of reassortment of genes between different influenza viruses, or through direct transmission of an avian virus to humans.
Three worldwide influenza pandemics occurred in the 20th century: the 1918/1919 'Spanish flu', the 1957/1958 'Asian flu' and the 1968/1969 'Hong Kong flu'. Each differed from the others with respect to the causative influenza A virus subtype, epidemiology and disease severity. More recently the world experienced the first influenza pandemic of the 21st century: the H1N1 2009 'swine flu' pandemic.
This pandemic is considered one of the deadliest disease events in human history.
The "Spanish flu" or 1918 influenza pandemic we now know from molecular-archaeological studies was caused by direct transmission of an H1N1 avian influenza virus to humans. This virus caused an unusually severe and deadly illness, towards and soon after the end of World War I.
The origin of the pandemic remains unknown. It was called 'Spanish flu' because the pandemic received greater attention in Spain than in the rest of Europe (affected by World War I). Both China and North America are named as possible starting points of the pandemic. However, first outbreaks were reported in American military establishments in March 1918. There is also some limited evidence of pre-pandemic outbreaks in military encampments in France from 1916 onwards.
From the USA, the pandemic moved to Europe in April/May 1918. While World War I was not the cause of the pandemic, the close proximity of military quarters, and the mass deployment and movement of troops may have accelerated its initial spread throughout Europe. It later moved to North Africa and by June 1918 the pandemic had reached India, China, New Zealand and the Philippines. Many countries, including the UK, experienced second and third waves in 1918-1919 of a more virulent form of infection.
The social effects were intense due to the fast spread of the pandemic and its high case fatality rate (2.0 - 2.5%). Every effort was made to control the transmission. These included isolation, personal hygiene, use of disinfectants and the prevention of public gatherings. Many institutions, including schools, were closed. In some places quarantine was enforced with different levels of success.
Although no figures exist in many parts of the world, the pandemic is estimated to have infected 50% of the world's population; approximately 25% suffered a clinical infection (illness with symptoms) and the total excess mortality was between 40 to 50 million. There are reports of people waking up well in the morning but dying by nightfall - so rapid was the disease process. The attack rate and mortality rates were highest among healthy adults (20-40 years old). In the USA more than 600,000 people died; in England the figure reached 200,000. Entire villages perished in Alaska and Southern Africa. An estimated 17 million died in India, about 5% of India's population.
The 'Asian flu' or 1957 influenza pandemic was caused by genetic reassortment between the existing H1N1 virus and an avian H2N2 influenza virus. The pandemic started in China in February 1957 and spread worldwide that same year, lasting until 1958. Influenza disease surveillance is more developed now than in the 1950's. Nevertheless, three weeks after the initial outbreak, influenza A H2N2 virus was identified as the cause of the pandemic.
Within six months, the pandemic spanned the entire globe. Infection spread to India, Australia, and Indonesia by May; to Pakistan, Europe, North America and the Middle East by June; to South Africa, South America, New Zealand and the pacific Islands by July; and to Central, West and East Africa, Eastern Europe and the Caribbean by August. In Europe the epidemic coincided with the September return to school. Cases were concentrated in school-aged children and those crowded together, but in the UK the impact on mortality was in the elderly.
By the end of 1957, the worst seemed to be over. However, a second wave of infection was observed early in 1958, which broke out in numerous regions including Europe (but not in UK), North America, the former USSR and Japan. This wave caused high rates of illness and increased fatalities. Quarantine measures were generally found to be ineffective, at best merely postponing the transmission by weeks.
The two waves together affected some 40-50% of people, of which 25-30% experienced clinical disease. The mortality rate was estimated at approximately 1 in 4000. Thus, the total death toll probably exceeded 1 million people.
As in 1957, the 'Hong Kong flu' or 1968 influenza pandemic, resulted from genetic reassortment and arose in Southeast Asia. However, this outbreak did not begin in Hong Kong but in China in July 1968, spreading to Hong Kong that same month, from where it spread rapidly across the whole world. Half a million cases were reported in Hong Kong in just two weeks. The virus was rapidly identified as a novel influenza A subtype, H3N2, and in August 1968 WHO warned about the emergence of a possible pandemic. Further spread occurred rapidly throughout most of South-East Asia, although a significant outbreak did not occur until January 1969 in Japan.
The Hong Kong influenza reached the US in September 1968, via US Marines returning from service in Vietnam. By December the illness was widespread and morbidity and mortality was as high as in the 1957-1958 pandemic. In Europe the disease was diagnosed from September 1968 onwards; symptoms were mild and excess deaths negligible. In the United Kingdom the epidemic began in December, and demands on medical services were not excessive. However, the number of fatalities due to influenza sharply increased in Europe one year later, during the 1969-1970 season. Finally, the virus reached South America and South Africa in mid-1969.
Vaccine manufacture began within two months of the virus being isolated. However, only 20 million doses were ready by the time the epidemic peaked in the United States.
Estimates of victims of the 1969 pandemic show a range of 1-3 million fatalities, of which over 30,000 were from the United Kingdom.
The H1N1 2009 'swine' influenza pandemic virus emerged in humans following multiple reassortment events between swine viruses, with the resultant pandemic virus having a novel mix of genes previously not reported in swine or human influenza viruses. It is believed to have emerged in Mexico in mid-February 2009 when an outbreak of influenza-like illness occurred in the Mexican town of La Gloria, Veracruz. By early April, high numbers of cases prompted the Mexican authorities to notify the World Health Organisation (WHO). In mid-April the new virus was identified in California as an H1N1 virus of swine origin. On April 27 WHO raised the pandemic alert from 3 to 4 signalling human to human transmission and then again on 29 April from 4 to 5 indicating that more than two countries in a WHO Region were affected. By 21 May, 41 countries had reported 11,034 cases including 85 deaths.
In the UK the first imported cases were detected on April 27 and a wave of activity gradually took off peaking in mid/late July. Transmission was interrupted by the school summer holidays but a second wave of activity occurred in the autumn that peaked in mid-October 2009. Rates of infection were highest in children and young adults and lowest among the elderly. The latter may have had some cross protective immunity from exposure in the 1950s to related influenza viruses.
In terms of morbidity and mortality, the 2009 pandemic is now considered mild. However, severe disease was seen in persons with underlying co-morbidities, the elderly and pregnant women. The pandemic vaccine was not available until October 2009 and so had little impact on the progression of the pandemic. On August 10 2011, the WHO declared the pandemic over and the world moved into the post pandemic phase. The H1N1 (2009) virus, however, remains one of the seasonal influenza viruses in circulation globally and is likely to remain so for the foreseeable future. More information on the 2009 pandemic is available from the pandemic archive pages.