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We all know influenza is more common in winter. But researchers have not known why. Virologist doctor Peter Palese has been studying the effects of heat and cold on the flu virus. He found that at higher temperatures, the flu virus didn’t spread, but at colder temperature it did. “The virus is probably more stable in cold temperature, so it hangs in the air much longer,” Dr. Palese told Ivanhoe.
People are indoors more often during the winter, they are in close contact more often, and this promotes transmission from person to person.
Cold temperatures lead to drier air, which may dehydrate mucus, preventing the body from effectively expelling virus particles.
The virus may linger longer on exposed surfaces (doorknobs, countertops, etc.) in colder temperatures.
In nations where children do not go to school in the summer, there is a more pronounced beginning to flu season, coinciding with the start of public school.
It is thought that the creche environment is perfect for the spread of illness.
Vitamin D production from Ultraviolet-B in the skin changes with the seasons and affects the immune system.
Recent research done by National Institute of Child Health and Human Development (NICHD) found that the influenza virus has a "butter-like coating". The coating melts when it enters the respiratory tract. In the winter, the coating becomes a hardened shell; therefore, it can survive in the cold weather similar to a spore. In the summer, the coating melts before the virus reaches the respiratory tract
How long influenza survives in airborne droplets seems to be influenced by the levels of humidity and UV radiation: with low humidity and a lack of sunlight in winter aiding its survival.
Another factor is that cold temperatures lead to drier air, which may dehydrate mucus, preventing the body from effectively expelling virus particles. The virus also survives longer on surfaces at colder temperatures and aerosol transmission of the virus is highest in cold environments (less than 5 °C) with low relative humidity. Indeed, the lower air humidity in winter seems to be the main cause of seasonal influenza transmission in temperate regions
However, seasonal changes in infection rates also occur in tropical regions, and in some countries these peaks of infection are seen mainly during the rainy season. Seasonal changes in contact rates from school terms, which are a major factor in other childhood diseases such as measles and pertussis, may also play a role in the flu. A combination of these small seasonal effects may be amplified by dynamical resonance with the endogenous disease cycles. H5N1 exhibits seasonality in both humans and birds.
An alternative hypothesis to explain seasonality in influenza infections is an effect of vitamin D levels on immunity to the virus.[ This idea was first proposed by Robert Edgar Hope-Simpson in 1965. He proposed that the cause of influenza epidemics during winter may be connected to seasonal fluctuations of vitamin D, which is produced in the skin under the influence of solar (or artificial) UV radiation. This could explain why influenza occurs mostly in winter and during the tropical rainy season, when people stay indoors, away from the sun, and their vitamin D levels fall.