History of pandemics

Influenza Pandemics of the Twentieth Century

An influenza pandemic is a worldwide outbreak of influenza. It is a rare but recurrent event. An influenza pandemic occurs when a new influenza virus emerges, that is able to spread easily from person to person and against which people have little or no immunity.

Influenza viruses are grouped into three types, designated A, B, and C. Influenza A and B viruses are of concern for human health.

The virus is able to cause both annual winter epidemics of varying size and severity, and occasional more severe pandemics. Influenza A and B viruses alter gradually through a process of random mutation ("antigenic drift"), every few years this will result in a significant epidemic. Influenza A may also change abruptly ("antigenic shift") leading to a new subtype and causing a pandemic. Only influenza A has the ability to cause a pandemic.

Three worldwide influenza pandemics occurred in the last century: in 1918/1919 " Spanish flu", in 1957/1958 " Asian flu" and in 1968/1969 " Hong Kong flu". Each differed from the others with respect to the causative virus, epidemiology and disease severity.

Spanish Influenza

The "Spanish flu" or 1918 influenza pandemic was caused by an unusually severe and deadly strain of the H1N1 subtype of Influenza A, 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 from there the pandemic reached India, China, New Zealand and the Philippines by June 1918. Many countries, including the UK, experienced second (1918-1919) and third waves (1919-1920) 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.

This pandemic is considered one of the deadliest disease events in human history. 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.

Asian influenza

The "Asian flu" was a pandemic outbreak of avian influenza that started in China in February 1957 and spread worldwide that same year, lasting until 1958. Within 6 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.

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.

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.

Hong Kong Influenza

As in 1957, the 1968 influenza pandemic arose in Southeast Asia. This virus is known as the "Hong Kong Influenza". However, this outbreak began in China in July 1968, spreading to Hong Kong that same month, from where it spread rapidly to 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. However, a significant epidemic did not occur in Japan until January 1969.

The Hong Kong influenza reached the USA 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.

Last reviewed: 13 August 2008