Influenza Pandemics

Influenza is a highly contagious respiratory disease caused by influenza viruses. The name originated in 15th century Italy, from an epidemic of the disease attributed to the “influence of the stars.” The first pandemic, which can be defined as an epidemic occurring worldwide that crosses international boundaries, was clearly caused by the influenza virus and occurred in 1580.

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At least four pandemics of influenza occurred throughout the 19th century, while three of them occurred in the 20th century. Each one differed from the others with respect to viral subtypes, epidemiology, and disease severity. The first pandemic of the 21st century occurred between 2009 and 2010.

Three pandemics of the 20th century

Between May of 1918 and April of 1919, a highly virulent H1N1 strain of the influenza virus, also known as the Spanish flu, killed at least 50 million people worldwide. In fact, it is estimated that five times more people died from the Spanish flu as compared to those who died in World War I. The majority of deaths were a result of secondary bacterial infections, such as pneumonia, that were particularly serious in those suffering from underlying pulmonary illnesses, such as tuberculosis.

Unlike earlier pandemics and seasonal influenza outbreaks, the 1918 pandemic resulted in high mortality rates among healthy adults. In fact, the illness and mortality rates were highest among adults between 20 and 40 years of age. Epidemiologic evidence strongly suggests that the causative virus was moving from humans to swine.

The 1957-1958 pandemic has often been dubbed as the first pandemic in the era of modern virology, as it was the first time that the rapid global spread of an influenza virus was available for laboratory evaluation. Also known as the Asian flu, this pandemic resulted in two million deaths worldwide.

Although the percentage of infected people was high, the resulting disease was relatively mild compared to the Spanish flu, thus resulting in fewer deaths. Studies have revealed that the strain responsible for the Asian flu (H2N2) arose by genetic reassortment of a bird virus.

The Hong Kong 1968-1969 influenza pandemic almost passed undetected, even though it spread throughout the world in less than two years, given the already significant increase in intercontinental travel. The cause was an H3N2 strain of the influenza A virus and the death toll was significantly lower when compared to Spanish and Asian flu, making it the mildest pandemic in the 20th century.

2009 influenza pandemic

The first influenza pandemic of the 21st century was initially reported in April 2009 in Mexico and swept through the Northern Hemisphere in two waves. This pandemic raised global concerns rooted in experience from previous pandemics, especially from the notorious Spanish flu which killed 50 million people.

The virus responsible for the pandemic was a novel H1N1 strain derived from human, swine, and avian strains, hence it was informally termed “swine flu.” The cases of influenza caused by this strain have been confirmed in most of the countries throughout the world by the World Health Organization (WHO).

The swine flu pandemic of 2009 caused widespread and quite unusual outbreaks of the disease in the summer and very high levels of disease during the winter. It was also characterized by a noteworthy dominance of the pandemic virus over other seasonal influenza viruses, and also by an unusual clinical pattern where the most severe cases occurred in younger age groups.

A look back at the H1N1 outbreak

Between 150,000 and 575,000 people died from the 2009 H1N1 virus infection during the first year that the virus was in circulation. A disproportionate number of deaths occurred in Africa and Southeast Asia, where access to prevention and treatment resources was limited.

Although this influenza pandemic was hard on people younger than 65 years of age, the impact on the global population overall during the first year was less severe as compared to previous pandemics. The impact of illness was also minimized by adequate prevention, as 80 million people were vaccinated against H1N1.

On August 10, 2010, the WHO announced that the H1N1 influenza incident has moved into the post-pandemic period. Based on the data about past pandemics, the 2009 virus is expected to linger as a seasonal virus for years to come. And while the level of concern is no longer considered to be high, alertness on the part of national health authorities remains a pivotal issue.


Further Reading

Last Updated: Mar 11, 2023

Dr. Tomislav Meštrović

Written by

Dr. Tomislav Meštrović

Dr. Tomislav Meštrović is a medical doctor (MD) with a Ph.D. in biomedical and health sciences, specialist in the field of clinical microbiology, and an Assistant Professor at Croatia's youngest university - University North. In addition to his interest in clinical, research and lecturing activities, his immense passion for medical writing and scientific communication goes back to his student days. He enjoys contributing back to the community. In his spare time, Tomislav is a movie buff and an avid traveler.


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