Questions and answers on influenza pandemicsArchived

At irregular intervals, an influenza A virus emerges which is different from the current human seasonal influenza viruses and can not only infect humans but can also cause disease in some of them and crucially is capable of efficient human to human transmission. The virus has to be novel enough to prevail over the seasonal A viruses, and because of its novelty there can be little specific immunity among humans, except for older people who may have met a similar virus in the past. This new virus can then spread rapidly from human to human all over the world. Because of the lack of human immunity the virus causes a variable amount of severe disease and deaths: this is an influenza pandemic. As immunity increases among humans, and the pandemic virus changes, the pandemic strain becomes part of (and may dominate) the mix of seasonal influenza A viruses, perhaps changing some of the characteristics of seasonal influenza.

Influenza pandemics vary, and in order to mitigate or even prevent some of their most concerning impacts there is a need for specific and general preparedness. All European Union countries have pandemic preparedness plans and most of them have updated these in the light of the 2009 pandemic experience and of the many evaluations which followed it.

Since 2005 ECDC, the European Commission and the WHO Regional Office for Europe have been providing joint assistance in this work to the European Member States, while the Commission was doing this before 2005.

What is influenza?

Influenza is an infectious respiratory illness caused by the influenza virus. In humans the symptoms can include all or any of: headache, fever, cough, sore throat, aching muscles and joints and generally feeling unwell. Influenza is usually more severe than a simple cold and in some people becomes very severe as complications develop. Though the illness can also be mild, and sometimes infections take place with no symptoms at all (aymptomatic infections). So there is a wide rage of illness ranging from minor symptoms through to severe pneumonia which can be life threatening. These severe illnesses may be due to the influenza virus alone or because of other infections that occur because flu has lowered the body’s defences or the influenza can precipitates a heart attack or stroke in an already vulnerable person.

What is a pandemic?

A pandemic is the rapid spread of a new human influenza around the world. Influenza pandemics happen when a new strain of a flu virus appears which can infect humans, to which most people have no immunity and which can transmit efficiently from human to human. Pandemic influenza is therefore much more severe, in terms of the number of people who become ill and the number who die, than the outbreaks of ordinary flu which happen each winter in Europe. During a pandemic many millions of Europeans become infected and ill and a proportion of these will become very ill and die.

Definition of a Pandemic

The internationally accepted definition of a pandemic as it appears in the Dictionary of Epidemiology is straightforward and well-known: 'an epidemic occurring worldwide, or over a very wide area, crossing international boundaries and usually affecting a large number of  people'.

It should be noted that this definition can apply to other infections subject to such global spread, e.g. cholera and HIV. There is no element of severity in it: while some pandemics are severe in the disease they cause in some individuals or at a population level, not all pandemics are severe.

WHO developed a more technical set of requirements for a pandemic:

The emergence of influenza A virus significant different genetically from circulating human influenza A viruses (i.e. many of the population are non-immune to the new virus) with the following three characteristics:

  • Able to infect humans,

  • Able to cause disease in humans,

  • Able to spread from human to human quite easily.

There is no reason for which an influenza pandemic has to be more or less severe than the preceding interpandemic (seasonal) influenza. Severity has never been part of the WHO definition of a pandemic,  although the pandemic virus has to be able to cause disease in at least some people. Hence once a pandemic starts it is important to assess the severity of the pandemic along a series of parameters using a Risk Based Approach such as the one developed by ECDC.

WHO pandemic phases

Phases  Description


No animal influenza virus circulating among animals has been reported to cause infection in humans.


An animal influenza virus circulating in domesticated or wild animals is known to have caused infection in humans and is therefore considered a specific potential pandemic threat.


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.


 Human-to-human transmission of an animal or human-animal influenza reassortant virus able to sustain community-level outbreaks has been verified.




The same identified virus has caused sustained community level outbreaks in two or more countries in one WHO region.

Six In addition to the criteria defined in Phase 5, the same virus has caused sustained community level outbreaks in at least one other country in another WHO region.
Post-peak Levels of pandemic influenza in most countries with adequate surveillance have dropped below peak levels.
Possible new wave Level of pandemic influenza activity in most countries with adequate surveillance rising again.
Seasonal influenza  
Post-pandemic Levels of influenza activity have returned to the levels seen for seasonal influenza in most countries with adequate surveillance.

Adapted from "Pandemic influenza preparedness and response: a WHO guidance document (2009)"

Historical pandemics

(date and common name)

Considered Area 
of Emergence

Influenza A 
Virus Type

Reproductive number

Estimated Case 
Fatality Rate

Estimated attributable excess mortality worldwide

Age groups most affected  (simulated attack rates)

1918 - 1919 
Spanish Influenza





20 – 50 million

Young adults

1957 - 1958 
Asian Flu

Southern China




1-4 million

Children most affected

1968 - 1969 
Hong Kong Flu

Southern China




1-4 million

Across all age groups

Mexican Flu       

Mexico/Southern United States


1.1-1.4 (95% CI

Up to 0.03%

152-576 thousand

Young people (5 to 30 years of age)

Other influenza viruses (mostly H1N1 types) have emerged in the 20th Century. They have not caused pandemics but they have required inclusion in the seasonal vaccines. In 1977 one H1N1 virus emerged, again from China and spread world wide mostly affecting children and adolescents.

What happens during a pandemic – how many people are affected?

This is impossible to predict. Certainly a higher proportion of the population become infected with the new influenza virus than with seasonal influenza. Often experts estimate that about a quarter of the population (25%) become infected. A proportion of the people infected with the virus become very ill, and some of them die. These proportions can be quite small but multiply them by 25% of the population and the numbers become very large. A pandemic can come in a single wave or as two or wave separated by few months or a year. Sometimes the second wave is more severe than the first. Each wave can last up to four or five months rising and then falling in intensity like seasonal influenza in the winter. These waves do not happen at exactly the same time or to the same extent in every part of a country. In Europe there is often a tendency for the waves to progress from West to East, but often starting in a pandemic with the countries that have the most contact with the countries first affected in another part of the world.

What happens during a pandemic – how do health services and hospitals cope?

One earlier planning estimate of the additional stress that a European health service caring for 100 000 people might experience over the first wave of a pandemic is as follows:

  • Infected ill people = 25 000
  • Additional medical consultations (primary care) 5 000
  • Additional hospitalisations = 150
  • Additional deaths = 100

These data can be scaled up for larger populations so for a Million Population multiply by ten.

The same estimates came up with the following effect on the workforce of the first wave of a pandemic 25% overall would be affected and off work at some point. At the peak 4% of the work-force would be off because they would be ill at week 14 which would be double normal rate. However, perhaps the same proportion might be off because they were caring for someone else who is in the family.

This is only one estimate undertaken for planning and there are others estimates that can be made which will look worse or better. That is why you can read different guesses at the number of people who will die early in a pandemic. But in a real pandemic the pattern will always be more complicated.

With these kind of numbers health services come under stress during a pandemic. Primary care can be stressed additionally by people who are anxious and seeking reassurance or if it is decided to make antivirals available to everybody who becomes unwell. This can be made even worse by staff who will be off sick themselves or caring for others. That is why planning is so important and fortunately health services in many countries do such planning all the time, planning what they will do with different crises, major accidents, bombs going off, etc. However a pandemic is especially difficult because it runs on and affects all the health services in every part of the country. But pandemics have been planned for in every European country to greater or lesser extents. For example hospitals have often planned to stop routine surgery and primary care services will ‘triage’ sending only those to hospital who most need hospital care and can benefit most.

What happen during a pandemic – are other services affected beyond the health services?

That depends on how bad the pandemic is, but they can be. Certainly one as bad as 1918-19 would affect services. However again that is why planning for a pandemic is not just in the health services. Many other sectors like the power companies, communication and food and fuel distributors also have to consider how they will maintain services with up to 8% of their staff off sick at the peak of the first wave of a pandemic.

Are all pandemics the same?

No. Pandemics do not come in a standard size. Of the three pandemics that took place in the 20th Century, those of 1957 and 1968-70 were roughly as severe as each other in terms of the numbers of additional deaths attributed to them but they still differed considerably in their other characteristics (for example the most affected groups). The 1918-19 pandemic was far worse than the other two while the first pandemic of the 21st Century, in 2009, was considerably milder.

When will the next pandemic take place, and which virus will cause it?

We simply do not know and there is no way of knowing. An influenza pandemic could start this winter, it could start next summer or it might not happen for more than five or ten years from now. Influenza viruses are inherently unpredictable. Scientists have expressed special concern about the A/H5N1 bird flu virus, currently circulating in poultry mostly in East and southeast Asia. However, this is just one of several ways a pandemic could start - as we saw in 2009 when a swine (pig) influenza virus resulted in an A(H1N1) pandemic starting in the Americas. Note there are over 70 types of animal influenza viruses, H7N7, H5N9, H7N4, etc., most of which are not going to end up causing a pandemic. This is work underway now to determine which of the animal viruses are more likely to undergo pandemic change and which should be prepared for. But this is certainly not a matter of making predictions.

How do influenza pandemics start?

We do not know for sure. Theoretically there are at least three ways this could happen:

  • One scenario is that a person or an animal become infected at once with two types of influenza - one human and one an animal or bird strain. They then exchange genetic material and produce a new reassortant virus that is new to man.
  • A second possibility is of a strain of animal influenza changing and becoming able to infect humans and transmit among them.
  • A third theoretical scenario is of an existing human influenza mutating into a new virus type.

Why has there been so much concern about the bird flu outbreaks involving A(H5N1)?

Public health officials are concerned by the unprecedented worldwide outbreaks in poultry with A(H5N1) viruses for at least three reasons. Firstly this strain is persisting among domestic poultry in a number of countries where poultry and human are in close contact, therefore there is a constant risk or humans being infected and the virus adapting to them. Secondly this virus has already shown the ability to infect and adapt to a number of quite different bird species, which is unusual. Thirdly in the few humans that have been infected by these viruses, they cause severe disease with a high mortality, much higher than it is usually seen when animal influenzas infect humans - although to date these viruses have not been at all efficient in their human to human transmission. Therefore there is some risk that through mutation or recombination the H5N1 becomes the next pandemic virus, and if it did it might be an especially severe pandemic.

So are you saying A/H5N1 will inevitably become a pandemic virus?

No! But the longer it is around, the more potential there is. What is therefore especially worrying is that H5N1 is now deeply embedded in poultry in a number of countries despite many attempts to eradicate it there. Fortunately to date the high biosecurity levels in the EU and EFTA countries have stopped it becoming embedded in the poultry in our Member States, though it is known to be carried by certain wild birds like ducks and waterfowl.

Can a pandemic be contained once it has started?

Probably not. Some experts believe that if a pandemic is found just after it has started, if it is still in a single locality, if it is in a rural area without too many communications, if the right actions are taken quickly and if the plans work well, then an influenza pandemic might be contained and averted. That is a lot of ‘ifs’. The more likely scenario is as in 2009 that by the time it is realised a pandemic has started, the virus is far to widely distributed to be contained. However there are many things that can be done to mitigate or reduce the damage and impact of a pandemic.

Can the risk of a pandemic occurring in the first place be reduced?

We think it can. The first priority, and the major line of defence, is to reduce opportunities for human exposure to the largest reservoir of the virus: infected poultry and other infected animals. This is achieved through the rapid detection of poultry outbreaks and the emergency introduction of control measures, including the destruction all infected or exposed poultry stock, and the proper disposal of carcasses. All available evidence points to an increased risk of transmission to humans when outbreaks of highly pathogenic avian H5N1 influenza are widespread in poultry. As the number of human infections grows, the risk increases that a new virus subtype could emerge, resulting in an influenza pandemic. This link between widespread infection in poultry and increased risk of human infection has been demonstrated in some Asian countries, where small numbers of humans continue to be infected each year from A(H5N1) caught from their domestic poultry. WHO has stressed the urgency of the situation and the need (if possible) for rapid action in the animal and agricultural sectors. Examples are the culling in 1997 of Hong Kong’s entire bird population – an estimated 1.5 million chickens and other birds –  in 3 days; in 2003, the culling of nearly 30 million birds (out of a total bird population of 100 million) in the Netherlands within a week when they were infected with another bird influenza virus A(H7N7). Rapid action in both of these situations potentially averted an influenza pandemic in humans. However as a reality check it has to be remembered that there are many parts of the world where the surveillance of influenza in domestic poultry and animals and the people who work or live closely with them is weak or non-existent. In those countries, mass culling cannot be done.

Could a pandemic start in Europe?

Yes – it certainly could. Though in a sense it does not matter where a pandemic starts (it will come to affect your country eventually, as in 2009) there are advantages to pandemics coming later to your country as it gives a chance to see what a pandemic is like first in another country and give some time for final preparation.

Is everyone equally at risk in a pandemic?

No – for a number of reasons. Firstly some people can be immune because of some similarity between the pandemic virus and older viruses to which people have been exposed. Secondly there will always be people who are more likely to have severe disease if they are infected – usually people with underlying diseases, pregnant women and older people – people in so called risk groups. However, the precise risk groups vary from one pandemic to another.

Are presently available seasonal influenza vaccines useful in an influenza pandemic?

Probably not. They were not in the 2009 pandemic. However they may be given in a pandemic at the usual time of year, shortly before a pandemic vaccine becomes available because it is unknown whether or not seasonal viruses will also be circulating.

Are any drugs available for prevention (prophylaxis) and treatment?

Yes. The main class of drugs available are the neuraminidase inhibitors (oseltamivir and zanimivir) and some countries have national stockpiles of these drugs as a back up to the drugs available through the usual sources. These drugs were generally beneficial in the 2009 pandemic, especially if given early in the illness. They are particularly beneficial for those at higher risk of developing complications and in the period between the pandemic starting and new specific vaccines becoming available.