Why Plan For Influenza Pandemic?
What Is an "Influenza Pandemic"?
Influenza pandemics are characterised by the spread of a novel type of influenza virus to many parts of the world, causing unusually high morbidity (illness) and mortality for perhaps two to three years. Most people do not have immunity to the virus and therefore are susceptible to influenza infection. A pandemic can overwhelm the resources of a society due to the exceptional number of those affected.
A pandemic may occur as a result of the emergence of a new viral sub-type with the capacity to spread efficiently from human to human.
What Does an Influenza Pandemic Look Like?
Past pandemics over the centuries have swept quickly through populations, and left considerable damage in their wake. Recovery was impeded by the tendency of pandemics to recur in second and third waves. Age groups and geographical areas not affected initially may prove vulnerable during subsequent waves.
For example, in the 1918-1919 "Spanish flu" pandemic,15 there were three waves. For whatever reason, the virus in the first wave, in June - July, caused illness that appeared to be indistinguishable from seasonal influenza. In November, a far more virulent illness appeared. (The first wave provided some protection from the second - those who became ill in the first wave were less likely to get sick in the second wave.) The third wave, in 1919, was much smaller and less intensive than the previous two.
By contrast, the 1957-1958 "Asian flu" pandemic was essentially one long wave, lasting about 3 months, with a very high total attack rate (possibly 70% - 80% of the New Zealand population) and no significant following waves. The mortality rate was very low.
It is not possible to predict pandemic wave activity or other features before a pandemic. It is probably safe to say that if there is a very large wave with a very high total attack rate (as in 1957) there won't be another of any size resulting from the same virus (or a slightly mutated form) because a high proportion of the population will have developed natural immunity. However a 20% wave wouldn't preclude another larger one at a later stage (as in 1918).
Current national planning aims to keep influenza out of New Zealand or substantially delay its entry, and if it arrives, control clusters within New Zealand until a vaccination campaign could be run. Vaccination will protect the general population against pandemic influenza. However, given the time lapse (several months at least) between virus recognition and production of a vaccine, planning must take into account the possibility that the pandemic may reach New Zealand, and that there may be more than one "wave" of illnesses.
MoH has also prepared a number of possible scenarios to assist with planning for pandemic influenza - these are in Appendix 2. In addition, the international weekly journal of science, Nature, illustrates how a pandemic might play out with a future scenario in the form of a blog (Avian Flu Special: The Flu Pandemic: Were We Ready? ).
How Likely Is an Influenza Pandemic?
The World Health Organisation (WHO) and MoH advise that it is certain there will be an influenza pandemic at some time in the future, but no-one can say when.
On average, influenza pandemics occur three times every century, but with no recognisable pattern in timing. In the last century, pandemics occurred in 1918-1919 (the "Spanish flu"), 1957-1958 (the "Asian flu"), and 1968-1969 (the "Hong Kong flu").
The WHO considers the risk of avian influenza morphing into the next pandemic to be very high. The H5N1 virus has recently expanded its geographical area: originally it had affected several east and southeast Asian countries,16 but recently has spread to Russia and Kazakhstan. Even if the risk from avian influenza goes away, another influenza virus can be expected to come along months or years later.
The WHO is advising Governments worldwide to take precautionary measures and develop pandemic influenza response plans.
What Is Avian Influenza ("Bird Flu")?
Bird flu or avian influenza is a contagious viral infection that can affect all species of birds. Migratory waterfowl (ducks and geese) are a natural reservoir for avian influenza virus overseas, and may carry the viruses without becoming ill. Fortunately, New Zealand is not on the regular migratory pathways of any waterfowl and only very occasionally do waterfowl reach our shores, generally originating from southern Australia.
Bird flu outbreaks among chickens and other birds occur from time to time around the world due to a variety of strains of avian influenza virus. The current outbreak of highly pathogenic avian influenza (HPAI) due to the H5N1 strain is of concern because of the size of the outbreaks, the number of countries becoming affected and the fact that humans have become infected.
The H5N1 virus is highly infectious among birds and in a number of species can be rapidly fatal. Because of their living conditions domestic poultry flocks are particularly vulnerable to the rapid spread of the disease. The disease is not normally spread to humans but some cases have been reported. Most cases to date appear to have resulted from close direct contact with infected birds. There is no suggestion yet that the virus is easily spreadable from person to person.
Clinical experts are, however, concerned at the potential for H5N1 to adapt to humans and thereby acquire the ability to spread readily from human to human. If this happens there could be a worldwide influenza pandemic. The WHO considers the current risk to be high, and is advising Governments worldwide to take precautionary measures and develop pandemic influenza response plans.
Why Should New Zealanders be Concerned?
Beginning in late July 2005, official reports to the World Animal Health Organisation from government authorities indicate that the H5N1 virus has expanded its geographical range. Both Russia and Kazakhstan reported outbreaks of avian influenza in poultry in late July, and confirmed H5N1 as the causative agent in early August. Deaths in migratory birds, infected with the virus, have also been reported. Outbreaks in both countries have been attributed to contact between domestic birds and wild waterfowl via shared water sources.
These are the first outbreaks of HPAI recorded in the two countries. Both countries were previously considered free of the virus.
Experience in south-east Asia (Viet Nam, Thailand, Cambodia, and Indonesia) indicates that human cases of infection is rare but that there is a high mortality rate (there have been 112 laboratory-confirmed human cases of avian influenza December 2003, of which 57 were fatal). Most, but not all, human cases have been linked to direct exposure to dead or diseased poultry, notably during slaughtering, de-feathering, and food preparation.
Influenza viruses are highly unstable. This means that over time, viruses change and may develop the ability to readily infect humans. Also, when animal influenza viruses are circulating at the same time as human viruses there is potential for the two to "meet" and create a new influenza virus to which humans would have little, if any, protective immunity, and which can spread easily from person to person.
H5N1 is showing signs of changing and the expanding geographical presence of the virus creates expanded opportunities for human exposure. The emergence of an HPAI strain that is readily transmitted among humans would mark the start of a pandemic.
Where Can We Find International Information Updates?
The Avian Influenza provides updates on the global occurrence of avian influenza, risks to humans, vaccine and anti-viral developments. It also provides useful background information about the nature and characteristics of avian influenza and past pandemics.
For an international perspective and updates on infection in birds, see Update on Avian Influenza in Animals (Type H5) .
The New Zealand Ministry of Health's Pandemic Influenza also provides much relevant information.
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