Flu season is the portion of the year in which there is a regular outbreak in flu cases. It occurs during the cold half of the year in each hemisphere. Flu 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 3 weeks to peak and another 3 weeks to significantly diminish.^[1] Individual cases of the flu however, usually only last a few days. In some countries such as Japan and China, infected persons sometimes wear a surgical mask out of respect for others.

Cost

The annual flu (also called "seasonal flu" or "human flu") in the U.S. "results in approximately 36,000 deaths and more than 200,000 hospitalizations each year. In addition to this human toll, influenza is annually responsible for a total cost of over $10 billion in the U.S." ^[2]

Cause

The garden variety flu that comes around every year is caused by Influenzavirus A, Influenzavirus B, or Influenzavirus C and are also known as human flu virus strains which is to say it has made genetic changes to adapt to its human hosts. It passes from human to human all year round and never goes away completely. When it is cold, infection from "human flu" increases roughly tenfold or more. Different strains of flu virus circulate in different years as it is constantly mutating. The influenza vaccine for the 2005 - 2006 flu season contains proteins from the coat of two subtypes of species A and from species B. Species B and C don't have subtypes.

It has long been unclear why outbreaks of the flu occur seasonally rather than uniformly throughout the year. Proposed explanations included:

1. Because people are indoors more often during the winter, they are in close contact more often, and this promotes transmission from person to person.
2. Cold temperatures lead to drier air, which may dehydrate mucus, preventing the body from effectively expelling virus particles.
3. The virus may linger longer on exposed surfaces (doorknobs, countertops, etc.) in colder temperatures.
4. Increased travel and visitation in the northern hemisphere due to the holiday season.^[3]

Research in guinea pigs has shown that the aerosol transmission of the virus is enhanced when the air is cold and dry.^[4] The dependence on humidity 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 nice hardened shell; therefore, it can survive in the cold weather similar to a spore. In the summer, the coating melts before virus reaches the respiratory tract. ^[5]

Flu vaccinations

Flu vaccinations have been used to diminish the effects of the flu season. 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) makes two vaccine formulations every year; one for the Northern, and one for the Southern Hemisphere.

According to the U.S. Department of Health, a growing number of large companies provide their employees with seasonal flu shots, either at a small cost to the employee or as a free service.

The annually updated trivalent influenza vaccine consists of hemagglutinin (HA) surface glycoprotein components from influenza H3N2, H1N1, and B influenza viruses.^[6] The dominant strain in January 2006 is H3N2. Measured resistance to the standard antiviral drugs amantadine and rimantadine in H3N2 has increased from 1% in 1994 to 12% in 2003 to 91% in 2005. ^[7][8] .

See also

• Influenza_vaccine Year-by-year details
• Bird flu
• Human Flu
• Swine Flu
• Horse Flu
• Dog Flu
• Freshers' Flu

Sources and notes

Further reading

• CDC U.S. influenza season summary with weekly updates ? Contains about a dozen charts and graphs with extensive text