Fact Sheets
Understanding Influenza
Influenza
Influenza is a highly contagious viral disease of the respiratory tract. The disease is caused by a group of closely related subtypes of virus, and different subtypes infect different animal species. It derives its public health significance from the rapidity by which epidemics evolve and spread amongst the community, and associated widespread morbidity and serious complications such as viral or bacterial pneumonia which may be fatal.
The influenza virus contains two surface proteins, haemagglutinin (H) and neuraminidase (N), which are involved in the infection of the host and production of new virus. The H protein is involved in attaching the virus to the cells it infects, while the N protein assists the virus in detaching from the cell in which it is produced. For influenza A, 16 distinct forms of H have been identified (designated H1 to H16) and 9 distinct forms of the N (designated N1 to N9). Influenza A viruses have infected many different animals, including ducks, chickens, pigs, whales, horses, and seals. However, certain subtypes of influenza A virus are specific to certain species, except for birds, which are hosts to all known subtypes of influenza A. Subtypes that have caused widespread illness in people either in the past or currently are H3N2, H2N2, H1N1, and H1N2. H1N1 and H3N2 subtypes also have caused outbreaks in pigs, and H7N7 and H3N8 viruses have caused outbreaks in horses.
Influenza A viruses normally seen in one species sometimes can cross over and cause illness in another species. For example, until 1998, only H1N1 viruses circulated widely in the U.S. pig population. However, in 1998, H3N2 viruses from humans were introduced into the pig population and caused widespread disease among pigs. Most recently, H3N8 viruses from horses have crossed over and caused outbreaks in dogs.
For the influenza B virus, only one H and one N have been identified.
Epidemics of influenza
The influenza virus has very high mutation rates and the surface antigens are prone to variation. Small mutations give rise to ‘antigenic drift' which results in the emergence of new strains of influenza A and B. Essentially this involves small changes to the viruses that are already circulating around the world. Every year, this process results in widespread epidemics, usually between late autumn and early spring. During these seasonal epidemics, attack rates usually depend upon age, reflecting whether the person has been exposed to the circulating strain before.
Pandemic Influenza
Emergence of Pandemic Influenza Viruses
There are many different subtypes of Influenza or “flu” viruses. The subtypes differ based upon certain proteins on the surface of the virus (the hemagglutinin or “H” protein and the neuraminidase or the “N” protein). Pandemic viruses emerge as a result of a process called "antigenic shift,” which causes an abrupt or sudden, major change in influenza A viruses. These changes are caused by new combinations of the HA and/or NA proteins on the surface of the virus. Changes results in a new influenza A virus subtype. The appearance of a new influenza A virus subtype is the first step toward a pandemic; however, to cause a pandemic, the new virus subtype also must have the capacity to spread easily from person to person. Once a new pandemic influenza virus emerges and spreads, it usually becomes established among people and moves around or “circulates” for many years as seasonal epidemics of influenza. The World Health Organization (WHO) have large surveillance programs to monitor and detect influenza activity around the world, including the emergence of possible pandemic strains of influenza virus. Factors that influence the Page 2 of 2 | Health Services Australia Group | The Travel Doctor-TMVC | Nat: Understanding Influenza Nov05 likelihood of a pandemic include: the emergence of a new viral subtype; the capacity for the virus to spread efficiently from person to person; and being virulent enough to cause disease. It is not possible to predict when the next pandemic will occur or how long it will last. It is over 35 years since the last pandemic.
Influenza Pandemics during the 20th Century
During the 20th century, the emergence of several new influenza A virus subtypes caused three pandemics, all of which spread around the world within a year of being detected.
- 1918-19, "Spanish flu," [A (H1N1)], caused the highest number of known influenza deaths. (However, the actual influenza virus subtype was not detected in the 1918-19 pandemic). It is estimated that between 20-40 million people may have died worldwide. Many people died within the first few days after infection, and others died of secondary complications. Nearly 60% of those who died in Australia were young, healthy adults. Influenza A (H1N1) viruses still circulate today after being introduced again into the human population in 1977.
- 1957-58, "Asian flu," [A (H2N2)], caused a milder pandemic, as the disease was milder and spread slower, and focussed on children and elderly. First identified in China in late February 1957, the Asian flu spread rapidly around the world. There was clear evidence of spread through mass gatherings.
- 1968-69, " Hong Kong flu," [A (H3N2)], caused a mild pandemic, possibly because many people had been exposed to H2N2 before and had some partial immunity. This virus was first detected in Hong Kong in early 1968. Influenza A (H3N2) viruses still circulate today.
Both the 1957-58 and 1968-69 pandemics were caused by viruses containing a combination of genes from a human influenza virus and an avian influenza virus. The 1918-19 pandemic virus appears to have an avian origin
Lessons from previous pandemics
Not knowing which influenza virus strain is going to cause the next pandemic makes planning for it very challenging. The likely impact of a pandemic depends upon characteristics of the virus such as its infectivity, attack rates in different ages (ie the proportion of the population infected for each age group) and the severity of disease it causes. A description of the three pandemics of the 20 th century demonstrates the variation in mortality, severity of illness and patterns of spread that can occur.
Consistent features of pandemics include sudden increases in morbidity and mortality, with rapid spread around the world (due to being caused by a highly contagious virus to which the population has little immunity). Pandemics usually spread to all parts of the globe within less than a year and affect more than a quarter of the total population. The ability of health and emergency systems to respond can be overwhelmed by the rapid increase in illness in the community. There is a tendency for pandemics to recur in second and sometimes third waves, which may begin simultaneously in different parts of the world.
Estimates of morbidity and mortality in Australia
If a pandemic with an attack rate of 25% (ie 25% of the population affected) were to occur again in Australia and there was no pandemic vaccine or treatment available, over a 6-8 week period it could lead to:
- 13,000-44,000 deaths
- 57,900-148,000 hospitalisations
- 2,600,000-7,500,000 outpatient visits .
The figures are estimates only and the likely outcomes associated with a pandemic will depend upon many factors such as the transmissibility and virulence of the virus, and the availability and success of health and social interventions.
In the absence of actual data on the potential pandemic virus, mathematical modelling, whereby various assumptions about the virus are included in a theoretical model to test various research questions and proposed strategies (such as school closures and quarantine measures), can provide a useful pandemic planning tool.
