Why flu viruses are prevalent in cold environments

The following scientific facts about the disproportionate spread of flu viruses across seasons (summer and winter) can be used to explain the possible global distribution of the Coronavirus disease (COVID-19). It does not, however, mean complete absence of the virus in some countries but rather the likelihood of reduced cases in some hotter countries all things being equal (i.e., having some control measures in place).

MODEL Three flu virus families, Influenzavirus A, B, and C are the main infective agents that cause influenza. During periods of cooler temperature, influenza cases increase roughly tenfold (10×) or more. Despite higher incidence of manifestations of the flu during the season, the viruses are actually transmitted throughout populations all year round.

Each annual flu season is normally associated with a major influenza virus subtype. The associated subtype changes each year, due to the development of immunological resistance to a previous year's strain (through exposure and vaccinations), and mutational changes in previously dormant viruses strains. The exact mechanism behind the seasonal nature of influenza outbreaks is unknown. However, some home proposed explanations are:

1. People are indoors more often during the winter, they are in close contact more often, and this promotes transmission from person to person.

2. A seasonal decline in the amount of ultraviolet radiation during winter may reduce the likelihood of the virus being damaged or killed by direct radiation damage or indirect effects (i. e. ozone concentration) increasing the probability of infection.

3. Cold temperatures lead to drier air, which may dehydrate mucous membranes, preventing the body from effectively defending against respiratory virus infections. Air conditioners at low temperatures create same drivers cold environment similar to winter.

4. Viruses are preserved in colder temperatures due to slower decomposition (lipid bilayer of virus membranes is preserved in cold), so they linger longer on exposed surfaces (doorknobs, countertops, etc.).

5. 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 daycare environment is perfect for the spread of illness. Keeping kids at home and not playing outside may worsen flu infection in households.

6. Vitamin D production from Ultraviolet-B in the skin changes with the seasons (high in sunny days) and affects the immune system.