What is a pandemic?

A pandemic is an outbreak of a new infectious disease, which causes serious illness and spreads widely from person to person across more than one geographical region. HIV/Aids can thus be described as a pandemic illness — it emerged to infect many people relatively recently. Cancer cannot: it is not by and large caused by infection, and neither is it new.

How does a pandemic differ from an epidemic?

An epidemic occurs whenever the number of cases of a particular disease exceeds the number that would normally be expected. Epidemics of infectious disease can escalate into pandemics, but do not always do so.

Is swine flu going to cause a pandemic?

Probably. Professor Roy Anderson, a leading epidemiologist who is also Rector of Imperial College, London, said yesterday that he considers a pandemic is already under way — the question that remains is how serious it will be. Many scientists were saying the same thing privately even before the World Health Organisation raised the threat level to phase 5 on Wednesday, indicating that it considers a pandemic to be imminent.

How bad will it be?

We don’t know. The early indications are that the virus spreads easily, but is not especially lethal — at least outside Mexico. In the US, there have been 109 confirmed cases and only one fatality and that was a Mexican toddler who had travelled to the US after becoming infected. However, there have been too few infections confirmed to date for scientists to be confident about its virulence.

What will determine its course?

There are two crucial factors: the transmissibility of the virus, and its virulence. The first will determine how quickly flu will spread and how many people will become infected. The latter will determine how many people who are infected fall seriously ill or die.

How is virulence measured?

The key measure is the case fatality rate — the proportion of infected people who die. For the Spanish flu of 1918-19, this was about 2.5 per cent, while for the 1968 pandemic it was below 0.5 per cent. This explains the huge difference in the death tolls.

It has so far been impossible to establish the case fatality rate for swine flu with much reliability. To calculate it, you need to know the total number of infections as well as the total number of deaths, and reliable data on the Mexican outbreak is not available.

Even a low case fatality rate, of 0.1 per cent, would still be dangerous if the virus spreads very widely. If 40 per cent of the UK population became infected, as the worst-case scenarios suggest, there could be as many as 240,000 deaths.

How is transmissibility measured?

The measure here is the reproduction number, sometimes known as R. This is the average number of people that each person with the virus will infect. If R is greater than one, then the virus is spreading. If it is below one, then it is starting to peter out. The reproduction number can be affected by many factors: the biology of the virus, the level of immunity in the population, containment measures such as isolation, school closures and travel restrictions, and the use of antiviral treatments and vaccines.

Again, the reproduction number for swine flu remains unknown, though it is clearly above one. As epidemiologists establish its value over the coming days and weeks, they will develop a much better idea of the likely course of the pandemic.

Some scientists say they’re hoping for a “mild pandemic”. Isn’t this a contradiction in terms?

No. All pandemics are serious, but they do not necessarily have to feature very high levels of mortality. A disease can fulfil all the criteria of a pandemic without killing very large numbers of people — that is what is meant here by “mild”.

The 1968-9 flu pandemic is an example of a past event that is generally considered to be mild. It killed an estimated 1 million people worldwide — many more than the usual annual death toll for seasonal flu of 250,000 to 500,000, but many fewer than the 50 million who are thought to have died in the Spanish Flu pandemic of 1918-19.

How does swine flu compare with avian flu as a pandemic threat?

The H1N1 swine flu virus is much more readily transmissible than the H5N1 avian virus — the latter has not yet developed the ability to pass readily from person to person. However, H5N1 is much more virulent. WHO figures show it has killed 257 of the 421 people infected so far — a case fatality rate of 61 per cent. If it were to become more transmissible, the resulting pandemic would almost certainly be a lot worse.

Have studies of the virus offered any clues to virulence?

Yes. Early indications are encouraging. Preliminary analysis suggests that it infects the upper part of the airway, which is generally a hallmark of less virulent viruses. It also appears to lack a protein that is involved in triggering extreme immune system responses known as cytokine storms, which are often the cause of death with highly pathogenic flu viruses such as the 1918 strain.

Source: timesonline.co.uk