A pandemic is the spread of a human illness on a global scale which rapidly affects all parts of the world and more than 25% of the global population. A pandemic occurs when a new virus appears against which the human immune system has no defence.
What is a pandemic?
Which evolution must an influenza virus undergo to trigger a pandemic?
Most of animal influenza viruses do not normally infect human beings. However, on occasion, certain animal viruses do infect humans. An influenza pandemic occurs when an animal influenza virus to which most humans have no immunity acquires the ability to cause sustained chains of human-to-human transmission. Such a virus has the potential to spread worldwide, causing a pandemic.
The development of an influenza pandemic can be seen as the result of the transformation of an animal influenza virus into a human influenza virus. Pandemic influenza viruses may arise through:
- genetic reassortment: genes from animal and human influenza viruses mix together to create a human-animal influenza reassortant virus.
- genetic mutation: genes in an animal influenza virus change allowing the virus to infect humans and transmit easily among them.
Have there been influenza pandemics in the past?
There were three influenza pandemics in the 20th century: in 1918, 1957 and 1968.
- The 1918/19 pandemic resulted in more than 40 million deaths in less than a year.
- The 1957 pandemic cost the lives of more than two million people, and was due to a less virulent virus than the one in 1918.
- As for 1968, there were a million deaths, the illness having spread more slowly than in the earlier pandemics.
How many phases of pandemic does the World Health Organisation distinguish?
The World Health Organisation distinguishes 6 phases:
- Phase I: In nature, influenza viruses circulate continuously among animals, especially birds. Even though such viruses might theoretically develop into pandemic viruses, no viruses circulating among animals have been reported to cause infections in humans.
- Phase II: An animal influenza virus circulating among domesticated or wild animals is known to have caused infection in humans, and is therefore considered a potential pandemic threat.
- Phase III: An animal or human-animal influenza reassortant virus has caused sporadic cases or small clusters of disease in people, but has not resulted in human-to-human transmission sufficient to sustain community-level outbreaks. Limited human-to-human transmission may occur under some circumstances, for example, when there is close contact between an infected person and an unprotected caregiver. However, limited transmission under such restricted circumstances does not indicate that the virus has gained the level of transmissibility among humans necessary to cause a pandemic.
- Phase IV: It is characterized by verified human-to-human transmission of an animal or human-animal influenza reassortant virus able to cause "community-level outbreaks". The ability to cause sustained disease outbreaks in a community marks a significant upwards shift in the risk for a pandemic. Any country that suspects or has verified such an event should urgently consult with WHO so that the situation can be jointly assessed and a decision made by the affected country if implementation of a rapid pandemic containment operation is warranted. Phase 4 indicates a significant increase in risk of a pandemic but does not necessarily mean that a pandemic is a forgone conclusion.
- Phase V: It is characterized by human-to-human spread of the virus into at least two countries in one WHO region. While most countries will not be affected at this stage, the declaration of phase 5 is a strong signal that a pandemic is imminent and that the time to finalize the organisation, communication, and implementation of the planned mitigation measures is short.
- Phase VI, the pandemic phase, is characterized by community level outbreaks in at least one other country in a different WHO region in addition to the criteria defined in phase 5. Designation of this phase will indicate that a global pandemic is under way.
- During the post-peak period, pandemic disease levels in most countries with adequate surveillance will have dropped below peak observed levels. The post-peak period signifies that pandemic activity appears to be decreasing; however, it is uncertain if additional waves will occur and countries will need to be prepared for a second wave. Previous pandemics have been characterized by waves of activity spread over months. Once the level of disease activity drops, a critical communications task will be to balance this information with the possibility of another wave. Pandemic waves can be separated by months and an immediate "at-ease" signal may be premature.
- In the post-pandemic period, influenza disease activity will have returned to levels normally seen for seasonal influenza. It is expected that the pandemic virus will behave as a seasonal influenza A virus. At this stage, it is important to maintain surveillance and update pandemic preparedness and response plans accordingly. An intensive phase of recovery and evaluation may be required.
After the appearance of the A(H1N1) influenza virus in March 2009, the World Health Organisation declared pandemic alert level 6 on 11 June 2009. It announced the move into the post-pandemic period on 10 August 2010.
How long does it take for the virus responsible for an influenza pandemic to spread around the entire world?
Influenza pandemics which occurred in the 20th century spread round the world within six to nine months.
Taking account of the intensity of international traffic and exchanges in the 21st century, the virus responsible for an influenza pandemic spreads around the entire planet in less than three months.
How many deaths will there be in the event of an influenza pandemic?
It is impossible to predict with much accuracy how many deaths result from an influenza pandemic. On basis of the three influenza pandemics which occurred in the 20th century, the World Health Organisation estimates that in the case of a new influenza pandemic mortality would be in the order of 2 to 7.4 million people around the world.
However, all estimates of the number of deaths are purely speculative, as death rates are largely determined by four factors:
- the number of infected people,
- the virulence of the virus,
- the vulnerability of affected populations
- as well as the effectiveness of preventive measures.
How effective would antiviral medicines be in the initial phase of an influenza pandemic?
The administration of antiviral medicines to ill people as well as to members of their family, close contacts and carers could be useful at the very beginning of an influenza pandemic.
- On the one hand, antiviral medicines would increase the survival chances of infected people.
- On the other hand, they could prevent the virus spreading to people in the entourage of the ill person and thus improving its capacity for human-to-human transmission.
Is there a vaccine against the virus responsible for an influenza pandemic?
It is only possible to perfect a vaccine from the time when the virus responsible for an influenza pandemic has appeared and when it has been identified.
How long does it take to produce a vaccine against the virus responsible for an influenza pandemic?
Between the time when the virus responsible for an influenza pandemic has been isolated and identified and the time when the vaccine can be produced on a large scale it takes at least six months.
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