Tuesday, April 28, 2009

More on influenza A

The numbers that are cropping up in the paper about people infected with influenza A (that may be swine flu) are seemingly quite alarming. But they really do have to be put into context.

Every year, somewhere between 380,000 and 760,000 New Zealanders become infected with one of the many strains of influenza. Many of those cases are varieties of influenza A (strains that originated in animals) - like the Spanish flu-strain, California, New Caledonia, or Brisbane strains.

There is an influenza surveillance summary released by ESR (who are based in Kenepuru) each month. This surveillance gives an indication of the number of cases and distribution of influenza in the community, and identifies the main strains of the virus that are doing the rounds. It's a sample of cases, and not the complete statitisics for the country, and the numbers obviously don't include those who didn't bother going to the doctor. But anyway, in June 2008, 141 cases of influenza were confirmed by testing - 114 of those were strains of influenza A. July 2008 recorded 295 cases of influenza, 155 of which were influenza A strains. Other months show comparable proportions.

There are seasonal surges in people fronting up to their doctors with influenza-like illness, and there are also surges when something triggers concern, such as the threat of a pandemic. Fear lowers the threshhold for going to the doctor. Many kiwis will suffer through what they think is just a cold, but is probably a mild case of influenza, without visiting a doctor. Bring up the threat of a pandemic and people head to the doctor at the drop of a sneezed-in tissue. With the expected proportions of influenza A cases in mind, when you increase the the number of tests performed, you'll increase the number of confirmed cases. Some of the people being tested have said that it just feels like they have a cold, and are effectively only being tested because of circumstances.

Until the cases of influenza A are confirmed as being the actual swine flu, I'm not really going to worry. We're not outside the bounds of statistical normalcy.

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