A pandemic occurs when a new influenza virus emerges and starts spreading as easily as normal influenza – by coughing and sneezing. Because the virus is new, the human immune system will have no pre-existing immunity. This makes it likely that people who contract pandemic influenza will experience more serious disease than that caused by normal influenza.
In contrast to epidemics, pandemics are rare events that occur every 10 to 50 years. They have been documented since the 16th century, and in the last 400 years, at least 31 pandemics have been recorded. During the twentieth century, three influenza pandemics occurred. Their mortality impact ranged from devastating to moderate or mild. The 1918 pandemic was caused by a H1N1 virus of apparently avian origin, whereas the subsequent pandemic strains - H2N2 in 1957 and H3N2 in 1968 - were reassortant viruses containing genes from avian viruses.
THE STORY OF INFLUENZA: 1918 AND BEYOND
An influenza pandemic is a rare but recurrent event. Three pandemics occurred in the previous century: “Spanish influenza” in 1918, “Asian influenza” in 1957, and “Hong Kong influenza” in 1968. The 1918 pandemic killed an estimated 40–50 million people worldwide. That pandemic, which was exceptional, is considered one of the deadliest disease events in human history. Subsequent pandemics were much milder, with an estimated 2 million deaths in 1957 and 1 million deaths in 1968.
1918 Spanish influenza
The first influenza pandemic of the 20th century spread more or less simultaneously in 3 distinct waves during a 12-month period in 1918-1919, across Europe, Asia, and North America. It was the worst pandemic in history, killing more people than World War I, and it is generally assumed that at least 50 million people died. The first wave, which started during the spring of 1918, was highly contagious but not particularly deadly. Only the second wave, beginning in September, spread the deadly form of the pandemic.
The virus of 1918 (H1N1) was extremely virulent and caused many deaths through secondary bacterial pneumonia. The primary viral pneumonia could kill previously healthy young individuals within 2 days. The clinical course of severe cases was so unfamiliar that investigators doubted it was influenza. Symptoms in 1918 were so unusual that, initially, it was misdiagnosed as dengue fever, cholera, or typhoid.
The 1918 virus was not a reassortant virus (like those of the 1957 and 1968 pandemics), but more likely an entirely avian-like virus that adapted to humans.
1957 Asian influenza
The 1957 pandemic was caused by H2N2, a clinically milder virus than the one responsible for the 1918 pandemic. Outbreaks were frequently explosive, but the death toll was much lower. Mortality showed a more characteristic pattern, similar to that seen in seasonal epidemics, with most excess deaths confined to infants and the elderly. The global excess mortality of the 1957 pandemic has been estimated at 1-2 million deaths.
1968 Hong Kong influenza
The 1968 pandemic, caused by H3N2, was also a mild pandemic. The death toll has been estimated to have been around 1 million, and in the United States, nearly 50 percent of all influenza-related deaths occurred in the younger population under 65 years of age.
As unpredictable as influenza pandemics are, as unpredictable is the virus itself. We know nothing about the pathogenic potential of the next pandemic strain. The next pandemic may be relatively benign, as it was in 1968 and 1957, or truly malignant, as was the 1918 episode. We don't know if the next pandemic will be caused by the current avian influenza virus, H5N1, or by another influenza strain. We ignore how the next pandemic will evolve over time, how rapidly it will spread around the world, and in how many waves. We don't know which age groups are at the highest risk of severe outcomes. We have no idea whether the next pandemic will kill 2, 20, or 200 million people.
Not surprisingly, healthcare professionals are becoming sensitised to the risk of a new pandemic. The ongoing outbreak of H5N1 influenza among birds, with occasional transmission to human beings is of major concern because of intriguing parallels between the H5N1 virus and the 1918 influenza strain. Such H5N1 avian influenza also known as "bird flu".
Health experts have been monitoring a new and extremely severe influenza virus – the H5N1 strain – for almost eight years. The H5N1 strain first infected humans in Hong Kong in 1997, causing 18 cases, including six deaths. Since mid-2003, this virus has caused the largest and most severe outbreaks in poultry on record. In December 2003, infections in people exposed to sick birds were identified.
Since then, over 260 human cases have been laboratory confirmed for ten countries worldwide, including the four seriously hit Asian countries (Indonesia, Viet Nam, Thailand, and China), and more than half of these people have died due to H5N1 avian influenza. Most cases have occurred in previously healthy children and young adults.
As there have been increasing update of avian influenza (bird flu) cases reported in more countries such as Japan and Hong Kong (see report here), cumulative number of confirmed human cases of avian influenza A/(H5N1) reported to WHO as at 15 January 2007 since 2003 showing a total of 267 cases (only laboratory-confirmed cases are reported) with 161 deaths including Vietnam, China, Indonesia, Azerbaijan, Cambodia, Djibouti, Egypt, Iraq, Thailand and Turkey. (see WHO report here)
See the following reports for further details and updates:
- Influenza pandemic
- Ten things you need to know about pandemic influenza
- Influenza Report 2006
- Flu Fears
- WHO's Avian Influenza Outbreak
- CDC's Avian Influenza (Bird Flu)