Influenza comes to Oregon and the rest of the northern hemisphere every winter. Some years the peak of illness is memorable and typically occurs between December and March. Although influenza is often referred to as “the flu”, it is a specific viral infection that is responsible for a substantial number of hospitalization and deaths each winter- it is not just the common cold. Every year is different but across the US an estimated 30,000 excess deaths are caused by influenza each year; in Oregon the number of extra deaths is probably about to 300 each year. The continuous genetic changes in the virus, called “antigenic drift”, mean that people can get sick from the same bug year after year; this year’s immunity doesn’t prevent next year’s illness.
Historians estimate that over the last 3 centuries there have been 10 influenza “pandemics”. Influenza pandemics occur when the virus undergoes such dramatic changes that virtually no one has any resistance to infection; when this occurs the number of people infected is much higher than during ordinary winters. In addition, during some pandemics, the severity of illness is higher. For example, in 1918-1919, not only was the number of ill people high but the death rate was 50 times higher than usual. Furthermore, there was a shift in the age group most severely affected from the elderly to those from 20-40 years of age. In contrast, the more recent pandemics of 1957 and 1968 caused far less dramatic increases in the death rate (only about two times the norm).
Although considered inevitable, the timing and severity of the next pandemic cannot be predicted. The outbreak of the H5N1 influenza strain in wild and domestic birds, which began in Asia in late 2003, is being carefully watched as a potential precursor to a pandemic since approximately 100 humans have been infected. Although the number of human infections is quite small, about half have died demonstrating the severity of disease when it occurs. The conditions to trigger a pandemic are not yet present since, at this point, the H5N1 virus is not efficiently transmitted from bird to human and rarely spreads between people.