Flu epidemics and pandemics

The word "pandemic" has a different formal definition when used by the World Health Organization, and when used in the popular press. Essentially, the restricted WHO definition is that a disease is pandemic when there is confirmed person to person transmission in at least two countries in one region and in a country in another region. Thus WHO may deem a pandemic exists even if there is a relatively narrow transmission of a relatively mild disease in only a few countries. By this definition there is a flu pandemic every year!

The more common usage of the term differentiates a situation in which there is a disease which affects a large number of people worldwide with a large number of people affected by very serious disease.

Incidentally, I wonder whether the effect of the demographic transition has been worked into our understanding of epidemiology. In flu, for example, the virus population drifts among closely related viruses during the year and from year to year, with an occasional introduction of a new strain that is significantly different than the existing strains. Immunizations and past exposure to the disease gives some protection as the existing resistance reduces the virulence of new infections.

There are a limited number of strains of flu that have caused epidemics in the human population. A "new" strain that is introduced often will not be entirely new, but rather a strain which was common at some time in the past, which disappeared when there was sufficient herd immunity, but which become epidemic again when the herd immunity has sufficiently decreased.

Where birthrates are high (and lives short) it takes a relatively short time for a large number of children never exposed to a viral strain to enter the population. When the birthrate is low, a large portion of the population has been present for decades and the level of herd immunity decays less rapidly. Thus the time between major epidemics and pandemics may increase as the demographic transition takes place.

So the reduction of birth rates that has occurred in the past half century may perhaps counteract in part the increase in human population and the increased urbanization that increases contact rates, and thus is likely to increase the the spread of infectious diseases.