Flu season is an annually recurring time period characterized by the prevalence of an outbreak of influenza (flu). The season occurs during the cold half of the year in each hemisphere. It takes approximately two days to show symptoms. Influenza activity can sometimes be predicted and even tracked geographically. While the beginning of major flu activity in each season varies by location, in any specific location these minor epidemics usually take about three weeks to reach its pinnacle, and another three weeks to significantly diminish.
Annually, about 3 to 5 million cases of severe illness and 290,000 to 650,000 deaths from seasonal flu occur worldwide.
Cause
Three virus families, Influenza virus A, B, and C are the main infective agents that cause influenza. During periods of cooler temperature, influenza cases increase roughly tenfold or more. Despite the 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 sub type. The associated sub type changes each year, due to 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. 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. A seasonal decline in the amount of ultraviolet radiation 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. Cold temperatures lead to drier air, which may dehydrate mucous membranes, preventing the body from effectively defending against respiratory virus infections. Viruses are preserved in colder temperatures due to slower decomposition, so they linger longer on exposed surfaces (doorknobs, countertops, etc.). 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 day care 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.
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.
Research done by the National Institute of Child Health and Human Development (NICHD) in 2008 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.
Timing
In the United States, the flu season is considered October through May. It typically reaches an apex in February, with a seasonal baseline varying between 6.1% and 7.7% of all deaths. In Australia, the flu season is considered May to October. It usually peaks in August. For other southern hemisphere countries such as Argentina, Chile, South Africa, and Paraguay also tend to start around June. Brazil has a complex seasonality component for its flu season, due to part of its being in a tropical climate, but its further south latitudes have their flu peaks in June–July, during the southern hemisphere winters.
Flu seasons also exist in the tropics and subtropics, with variability from region to region. In Hong Kong, which has a humid subtropical climate, the flu season runs from December to March, in the winter and early spring.
Flu vaccinations
Flu vaccinations are used to diminish the effects of the flu season and can lower an individual's risk of getting the flu by about half. Since the Northern and Southern Hemisphere have winter at different times of the year, there are actually two flu seasons each year. Therefore, the World Health Organization (assisted by the National Influenza Centers) recommends two vaccine formulations every year; one for the Northern, and one for the Southern Hemisphere.