Pandemic Overview

Updated: 15 June 2015

What is a pandemic?

A pandemic is a world-wide disease outbreak. Several human flu pandemics occur each century, affecting millions of people. The last influenza pandemic was in 2009-2010 and experts agree it is inevitable another will occur.

How do pandemics occur?

Human influenza pandemics can occur when a "new" virus emerges that has the following properties:
  1. Humans have little or no immunity to it, 
  2. The virus can cause significant illness or death, 
  3. The virus can be spread easily from person to person. 
If these three criteria are met, the virus can then spread globally. The severity of the pandemic depends on the qualities of the virus. Some cause more severe illness and are more infectious than others. 

Influenza pandemics of the 20th and 21st Century

Three pandemics occurred during the 20th century with the "Spanish Flu" being by far the most severe.

1918: Spanish Flu occurred immediately after World War 1 and lasted about 15 months. Even though this was the era before commercial airline travel, the disease spread across the world within 2 months. It was a global catastrophe. Approximately 30 percent of the world's population was infected and about 50 million people died - many within 48 hours of becoming ill. It was caused by an H1N1 strain of influenza A. The virus may have originated in Northern France or the USA. One of the current seasonal influenza strains is a “descendant” of this virus. 

Several “waves” of infection occurred. The first wave of illness struck Europe, Asia, Africa and the USA. WHO described this first wave as “not especially deadly.” The second wave caused greater numbers of infection and saw more deaths. 

Spanish flu was especially able to kill young adults between the ages of 15 and 35, as well as the very young, the elderly and the infirm. No antibiotics or vaccinations were available during this pandemic. Quarantine efforts did not prevent the disease from spreading globally, except possibly to delay its introduction into Australia. Due to its isolation, Australia remained unaffected until 1919. By the time the virus reached Australia, the flu strain was not as virulent (serious) but may have remained active for a longer period. 

1957: Asian Flu The virus responsible was first identified in the Far East. It was an influenza A/H2N2 strain. (This strain no longer circulates in humans). Low immunity rates in people less than 65 years of age allowed scientists to predict a pandemic and a vaccine became available in August 1957. The pandemic occurred in waves and most deaths occurred between September 1957 and March 1958. The elderly had the highest rates of death. 

1968: Hong Kong Flu In early 1968, an influenza pandemic virus was first detected in Hong Kong, and was an A/H3N2 strain. The pandemic lasted from September 1968 until March 1969. Deaths from this virus peaked in December 1968 and January 1969, with those over the age of 65 most likely to die. A form of this virus still circulates as seasonal flu. 

2009-2010: “Swine Flu” (later termed pandemic (H1N1) 2009) The first pandemic of the 21st century spanned April 2009 through August 2010. Despite being markedly more severe in pregnant women than most flu infection, this virus predominantly caused illness similar to ‘regular’ flu and was considered mild even as it infected people worldwide in out-of-season outbreaks. A form of this virus still circulates as seasonal flu.

Which virus will cause the next pandemic?

Bird flu A strains H5N1 and H7N9 have met the first two of the criteria above. At this time, they do not spread easily from person to person. However if they develop this ability, they could cause a pandemic. No one can predict which virus strain will cause the next pandemic, how rapidly the disease will spread, or how severe it will be. It is uncertain which population groups will be at the highest risk of disease and death. 

What we do know

Flu pandemics spread in waves. Quarantines and other public health interventions may slow but are unlikely to prevent global spread. A vaccine is unlikely to be available in the first months of the pandemic, and it is uncertain if there will be an effective antiviral against a new pandemic virus. Other "non-pharmaceutical" interventions will be needed to try and reduce the virus’s spread and minimise the impact to individuals and organisations.