22 Influenza 2006

In less severe cases, most patients experienced typical influenza with a 3- to 5-day fever followed by complete recovery (Kilbourne 2006). In contrast to subsequent pandemics, most deaths during the 1918 pandemic were among young and healthy persons aged 15 to 35 years old, and 99 % of deaths occurred in people younger than 65 years.

The recovery of the genomic RNA of the 1918 virus from archived formalin-fixed lung autopsy material and from frozen, unfixed lung tissue from an influenza victim who was buried in permafrost in November 1918 (Taubenberger 1997) has enabled the complete coding of the sequences of all eight viral RNA segments of the 1918 H1N1 virus (Taubenberger 2005). According to this investigation, the 1918 virus was not a reassortant virus (like those of the 1957 and 1968 pandemics), but more likely an entirely avian-like virus that adapted to humans.

1957

The 1957 pandemic was caused by H2N2, a clinically milder virus than the one responsible for the 1918 pandemic. Outbreaks were frequently explosive, but the death toll was much lower. Mortality showed a more characteristic pattern, similar to that seen in seasonal epidemics, with most excess deaths confined to infants and the elderly (WHO 2005b). Patients with chronic underlying disease and pregnant women were particularly at risk of developing pulmonary complications (Louria 1957). The global excess mortality of the 1957 pandemic has been estimated at 1–2 million deaths.

1968

The 1968 pandemic, was also a mild pandemic. The mortality impact was not even particularly severe compared to the severe epidemic in 1967–1968 (the last H2N2 epidemic), as well as two severe H3N2 epidemics in 1975–1976, and in 1980–1981 (Simonsen 2004). The death toll has been estimated to have been around 1 million, and in the United States, nearly 50 percent of all influenza-related deaths occurred in the younger population under 65 years of age. Sero-archaeological studies showed that most individuals aged 77 years or older, had H3 antibodies before they were exposed to the new pandemic virus (Dowdle 1999) and that pre-existing antiH3 antibodies might have protected the elderly (> 77 years old) during the 1968 H3N2 pandemic.

Since 1968, there has been only one episode – in 1976 – when the start of a new pandemic was falsely anticipated (Dowdle 1997, Gaydos 2006, Kilbourne 2006).

Current Situation

Major pandemics have occurred throughout history at an average of every 30 years and there is a general consensus that there will be another influenza pandemic. It is impossible to predict which influenza strain will be the next pandemic virus. One possible candidate is the avian H5N1 strain which has become endemic in wild waterfowl and in domestic poultry in many parts of Southeast Asia, and is recently spreading across Asia into Europe and Africa. Recent research has shown that just ten amino acid changes in the polymerase proteins differentiate the 1918 influenza virus sequences from that of avian viruses, and that a number of the same changes