Sunday, December 25, 2011

Biosurveillance for Influenza

Influenza is the most widespread naturally occurring infectious disease in recent human history that has caused countless deaths worldwide. There were three influenza pandemics in the 20th century – the “Spanish” flu of 1918-19 (“the mother of all pandemics”), the “Asian” flu of 1957-58, and the “Hong Kong” flu of 1968-69. The 1918 flu, caused by a strain of H1N1, was by far the most deadly. Between 50 and 100 million people died totally as a result of the Spanish flu, possibly more than during the entire course of The Black Death. It makes the Spanish flu the deadliest natural disaster in human history. For comparison, death toll of WWI is estimated between 10 and 20 million, and WWII – between 62 and 78 million. The 1957 pandemic was due to a new H2N2 strain of influenza virus and killed two million people, while the 1968 pandemic resulted from an H3N2 strain and killed one million. In addition, there were 3 so-called flu pandemic scares (unrealized pandemics). The first pandemic of 21st century was the Swine flu (April 2009 – July 2010). Fortunately, this H1N1 Swine flu pandemic was mild. It is convenient to show these results in the form of a table:

Pandemic            Years      Strain          Death Toll

Spanish flu      1918 - 1919    H1N1     50 - 100 million
Asian flu        1957 - 1958    H2N2            2 million
Hong Kong flu    1968 - 1969    H3N2            1 million
Swine flu        2009 - 2010    H1N1           25,174 (!)     

The names of influenza strains: H1N1, H2N2, H3N2, etc., are related to the two proteins that play a very important role in the process of influenza transmission: hemagglutinin  (where we get the H) and neuraminidase (from which the N comes from). It is known that there exist 16 different H-proteins: H1 – H16, and 9 different N-proteins: N1 – N9, overall 16 x 9 = 144 combinations. Although the most recent pandemic was rather mild, the uncertainty of “what is next?” is really alarming, Actually, pandemics happen every few decades. They occur when a new subtype of influenza A arises that has either never circulated in the human population or has not circulated for a very long time (so that most people do not have immunity against the virus), and it can spread easily through the human population.

Summarizing one can conclude that influenza is the number one permanent threat to the human population health, which deserves special attention and a particular approach. That is why in all developed countries there exist continuously operating influenza-specific reporting systems (such as FluView (CDC), EuroFlu (the WHO European Region), GoogleFlu (worldwide), FluWatch (Canada) etc. And that is why every day at 10 a.m. during flu season, a report on the number of cases of flu-like illness in the United Kingdom is placed on the desk of the British Prime Minister.

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