Sometimes a new type of influeza virus appears that causes an illness that is more serious than is usually the case for flu. This happened, for example, in 1918, when a flu pandemic killed between 20 million and 100 million people. Recently, there have been concerns about the new type of bird (avian) flu. At present the virus responsible does not pass easily from birds to humans, and it does not seem to pass from one human to another. However, the fear is that the virus might change and that human-to-human infection could then be possible. Should all this happen, the changed virus would be a major threat to human health.
With current technology, it would take several months to produce enough vaccine against such a new virus for even a small proportion of the world's population. By that time, it would probably be too late; the virus would already have spread to most parts of the world.
Health authorities must therefore consider all the methods that might control the spread of the virus. With the increase in international travel that has taken place, the virus could spread more quickly than in previous pandemics. Restrictions on international travel might be considered necessary, particularly travel by air. However, it is important to estimate how useful restrictions on air travel might be in controlling the spread of a flu virus. Travel restrictions are usually unpopular and could themselves be harmful. If they are not effective, resources could be wasted on enforcing them.
Researchers of the Centre for Infections, Health Protection Agency, UK used the techniques of mathematical modelling. In other words, complex calculations were done using information that is already av ailable about how flu viruses spread, particularly information recorded during a worldwide flu outbreak in 1968?969. Using this information, virtual experiments were carried out by simulating worldwide outbreaks on a computer. The researchers looked at how the virus might spread from one city to another and how travel restrictions might reduce the rate of spread. Their calculations allowed for such factors as the time of the year, the number of air passengers who might travel between the cities, and the fact that some people are more resistant to infection than others.
From the use of their mathematical model, the researchers concluded that restrictions on air travel would achieve very little. This is probably because, compared with some other viruses, the flu virus is transmitted from one person to another very quickly and affects many people. Once a major outbreak was under way, banning flights from affected cities would be effective at significantly delaying worldwide spread only if almost all travel between cities could be stopped almost as soon as an outbreak was detected in each city. It would be more effective to take other measures that would control the spread of the virus locally. These measures could include use of vaccines and antiviral drugs if they were available and effective against the virus.