Mechanism for seasonal nature of influenza

The exact mechanism behind the seasonal nature of influenza outbreaks is unclear. Some proposed explanations are:
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.
Vitamin D production from Ultraviolet-B in the skin changes with the seasons and affects the immune system.
Research in guinea pigs has shown that the aerosol transmission of the virus is enhanced when the air is cold and dry. The dependence on aridity appears to be due to degradation of the virus particles in moist air, while the dependence on cold appears to be due to infected hosts shedding the virus for a longer period of time. The researchers did not find that the cold impaired the immune response of the guinea pigs to the virus.
A recent research done by National Institute of Child Health and Human Development (NICHD) on influenza virus identified the virus as having "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.