Seasonal Flu is a worldwide killer. Affecting between 3 million to 5 million people each year, severe infections requiring hospitalisation can occur especially in the young, the old, and those with a pre-existing medical condition. In fact up to 500,000 deaths are attributed to seasonal flu each year around the globe. The advent of a flu vaccine has been effective in the reduction in the number of those affected by influenza. Each year a prediction of the likely flu viruses that will be prevalent in winter is made, and this forecast is used to structure the cocktail of influenzas contained in the year’s flu jab. Clearly, there can be a mismatch between the forecasted vaccination and the flu viruses that actually cause disease by the time winter comes. Usually the effectiveness of the vaccine has been at least 50%, but last year the immunisation was said to be only 34% effective. Moreover, this year there is some concerning preliminary evidence that only 10% of those receiving the flu inoculation will receive protection from this season’s flu.
The data comes from Australia. As their winter ‘precedes’ that of the Northern Hemisphere, there is always interest to see if the flu vaccine has been effective in reducing cases of influenza. This year there has been a large increase in influenza notifications down under. Even during the H1N1 flu pandemic of 2009, the number of Australian flu cases was ‘only’ 59,022: this year’s case number was around 200,000. The better news, however, is that the number of fatalities has not risen to this extent, meaning that although there are more influenza cases, the virus is less severe than flu viruses in previous years.
The main culprit responsible for this season’s epidemic is the H3N2 virus. This year’s vaccine did actually include an H3N2 component, but the flu that emerged during the Australian winter was a variant of this, and was still able to cause disease in those immunised. As flu viruses can mutate at a rapid rate, one of the difficulties of producing a vaccine in advance is that the virus may have changed in the meantime – this is called antigenic shift.
As the Northern Hemisphere enters its winter, many expect that like Australia the prevalent flu will be H3N2. As those in the North have been immunised with a vaccine that is similar to the Australian version, very limited protection against influenza is feared. We will know for sure all too soon.
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