What are the major reasons for concern over the possibility of a pandemic?
Computer modeling in Thailand suggests the possibility, with immediate early aggressive coordinated international intervention, of stopping a potential pandemic strain.
This would be analogous to quenching a small fire with forest-fire potential. Once human-to-human transmission reaches a major metropolitan area, however, the past history of influenza pandemics indicates that spread throughout the entire world can be expected to occur in only a matter of weeks.
Most health authorities doubt that it will be able to be stopped, once a pandemic is underway, only mitigated, at best. A pandemic virus will be spread rapidly by the coughing or sneezing by infected persons. Unlike SARS, individuals shed the virus for about two days before symptoms appear.
Asymptomatic air travelers will be very difficult to screen because spread may occur without fever. Attack rates during a pandemic, by definition, reach 25-35% of the total population.
Even with a mild pandemic virus, such as occurred with the 1957 "Asian flu" global death rates were in the neighborhood of two to 7.4 million deaths.
The 1918 pandemic caused over 40 million deaths with a mortality rate of 2.5%. Presently known mortality rates among humans confirmed to be infected with H5N1 are extremely high by historic standards, in excess of 50%.
It is true that this number does not take into account a denominator made larger by those with less severe infections who never present to health authorities.
It is also hoped that in becoming more easily transmissible, a pandemic virus will lose a great deal of its lethality. A pandemic is expected to cause large surges of people seeking or requiring hospitalization, temporarily overwhelming health services.
High rates of worker absenteeism can interrupt other essential services such as law enforcement, transportation, and communications. These social and economic disruptions may be temporary if the rates of illness peak rapidly, however, today's closely interrelated systems of trade and commerce may amplify the disruptions.
It is expected that a second wave of global spread would occur within a year of the first during which those who were ill and survived the first wave could once again become sick during the second. Three such waves occurred during the 1918 pandemic.
Understanding Avian Influenza