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Full Version: Why does the flu bug you in winter?
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Monday, February 16, 2009
For anyone left wondering why they have been struck down with a runny nose, fever, cough and aching muscles in recent weeks it’s a discovery which will finally provide an explanation.

Scientists have discovered that the influenza virus lives longer when the air is cold and dry than when it is hot and sticky in the summer.

It could indicate that hospitals should make their wards a little more humid in the winter to combat the spread of flu on the wards — an annual problem for the NHS.

It has long been a mystery why the flu virus occurs seasonally rather than incidence being spread out evenly over the year.

One theory has been that because people are indoors more often in cold weather, this means the virus is more likely to be transmitted from person to person.

Others have argued that immunity to the virus is linked to levels of vitamin D, which is produced in the skin following exposure to sunlight. Lower levels of sun exposure in winter leads to lower immunity, it was claimed.

Now researchers in the US have found that the virus thrives in lower absolute humidity — that is when it’s cold outside and there is less moisture in the air.

They say humidity is important because it affects the transmission and survival of viral particles in airborne droplets. Cold, dry weather can dry people’s mucus, preventing the body from effectively expelling virus particles.

The cold weather also means viruses survive longer on surfaces such as doorknobs, they found. This means it is more likely to be transmitted from person to person.

However the study, in the Proceedings of the National Academy of Sciences, did not suggest any reason why absolute humidity should affect the survival of the bug.

However, they suggested that increased humidity in critical areas such as hospitals could reduce the spread of influenza.

Marc Lipsitch, an epidemiologist from Harvard University, said the study as a ‘big step forward’.

He said: “In the context of why people get more infections in the winter it is very important. This is one of the oldest observations in medicine and one that we don’t really have an explanation for — it’s a little embarrassing. We don’t have a final explanation in this study but it is a big step forward. People have wondered for a long time with why influenza is seasonal. There are a lot of good possibilities, each with a bit of evidence but this removes a lot of the ‘noise’ from relationship by looking at temperature and humidity combined.”

This winter saw the biggest flu outbreak for a decade, blamed on a combination of the coldest December for years and the arrival of Brisbane H3N2, an Australian strain of the influenza virus. It affected older people, who are more susceptible to the disease, than the younger generation.
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