Seasonal Influenza


Seasonal influenza is often called the “ flu,” which can be misleading as there are several variations of the influenza virus. Seasonal influenza infections typically occur in the late fall and winter. While many people refer to a variety of illnesses as “flu”, actual influenza is a respiratory illness caused by a viral infection that affects between 5-20% of the US population each year.

What is Seasonal Influenza?

Seasonal influenza is caused by viral infection. It causes respiratory symptoms that generally last for up to a week or two. People can be infected by the virus anytime, but higher rates of infection occur during certain parts of the year.

Flu season in the US is between October and May. The infection rate usually peaks in January or February. Flu season occurs worldwide during the coldest parts of the year.

It is not known exactly why the rate of infection peaks during cooler weather, but there are several observations that are thought to contribute to increased infection:

Colder weather often means that people stay indoors more and are therefore in more contact with one another, increasing the chances they will come in direct contact with an infected person. Less amount of UV radiation during cooler seasons may mean that the virus is not killed by direct radiation or indirectly killed by thing such as ozone concentrations.

Cooler, drier air is thought to enhance the airborne transmission of the virus. Additionally, the aridity of the colder months tends to dehydrate mucous membranes in the human body, which may cause less than effective defense against respiratory diseases.

The Virus

The main virus families that most commonly cause seasonal flu outbreaks are Influenza A and B. Usually each year the outbreak of the illness is associated with a different subtype of the virus. Almost every year, the virus changes, or mutates, due to acquired immunity of vaccinated or previously infected people. Because of this, exposure to a single strain of influenza does not provide adequate antibodies to successive strains.

Influenza A Virus

The influenza A virus is broken down and categorized into subtypes. There are 18 hemagglutinin and 11 neuraminidase subtypes. These subtypes are determined using two of the proteins that are found on the surface of the virus.

The subtypes are further broken down and categorized into different strains. In 2009, the influenza A H1N1 virus emerged and caused the first flu pandemic seen in over 40 years.

Influenza B Virus

The influenza B virus is broken down only into different strains with no subtypes.

Influenza B generally mutates more slowly than influenza A. Abrupt changes in the influenza virus mean that people have little to no immunity from previous infections. The emergence of certain strains from animal populations, especially, can cause high rates of and widespread infection of the flu, which is what occurred during the 2009 pandemic.

The influenza vaccine covers both families of the influenza virus.


The virus can be contracted through direct contact with an infected person, contact with airborne droplets released when an infected person coughs, sneezes or talks and by direct contact with throat or nasal fluids released by an infected person. The virus can also be contracted by touching surfaces with the hand and then touching the eyes, nose or mouth.

Once contracted, the infection usually produces symptoms within one to four days. Infected people can pass the virus to others a day before and seven days after symptoms develop. However, children, may be infectious for longer than seven days after symptoms develop.

Symptoms of Influenza

Common symptoms of influenza include:

  • Cough
  • Sore throat
  • Runny nose or congestion
  • Chills
  • Headaches
  • Pain in the eyes during eye movement
  • Body aches
  • Fatigue
  • Fever over 100, or feeling feverish
  • Nausea, vomiting or diarrhea (more common among children)

These symptoms are similar to those of the common cold; however the flu symptoms are usually more severe.


Generally, most people who contract seasonal influenza recover in a few days to a few weeks. Sometimes, however, the flu can cause severe and even deadly complications.

Complications can range from fairly minor bacterial infections, such as ear infections and bronchitis, to life threatening conditions such as pneumonia. Additionally, people with pre-existing, chronic medical conditions, such as asthma, may experience a worsening those conditions.

The number of deaths caused by seasonal influenza varies each. During regular seasons, however, approximately 90% of deaths caused by flu occur in people aged 65 or older.

Prevention and Vaccination

The CDC recommends that each person over the age of 6 months be vaccinated annually against the flu. It is especially vital that some people with certain risk factors be vaccinated to prevent severe complications however. These people include:

  • Pregnant Women
  • People younger than five years of age and older than 65 years of age
  • People with chronic medical conditions or disorders such as asthma, diabetes and chronic lung diseases
  • People with compromised immune systems such as AIDS and cancer patients
  • People with a body mass index of 40 or higher (considered morbidly obese)

Health care workers, caretakers of chronically ill people and caretakers of infants less than six months of age should be vaccinated as well. The flu vaccine can be administered as an injection or by a nasal spray and is approximately 60% effective in preventing infection. Flu vaccinations should be administered as soon as the vaccine is available, ideally in October. The vaccine is ineffective for the first two weeks after administration. Generally, vaccinations are readily available during the entire flu season and can be given anytime. Other methods of prevention include disinfection of surfaces, linens and eating utensils, especially when an infected person is present. Of course, hand washing is effective in preventing the flu and other illnesses.

Pandemic Prevention

In the case of an influenza pandemic, a vaccine may not be readily available Additional measures are recommended to prevent the spread of the illness. On a community scale, these measures involve increasing and maintaining space between people. Schools may be closed and large gatherings may be postponed or canceled. Additionally, employers will be encouraged to offer more flexible sick leave as well as to offer remote or telecommuting options to employees.

On a personal level, prevention methods include staying in when ill and thoroughly disinfecting surfaces and objects. Of course, as always, people should be conscious of covering coughs and sneezes and washing hands thoroughly and often.


Pregnant women especially should consider getting vaccinated against influenza. The only suggested method for vaccination during pregnancy in an injection. Pregnant women should not receive the nasal spray form of the vaccine.

Not only are pregnant women more prone to complications caused by the flu, vaccination may offer the baby some protection against influenza. This is especially important because the flu vaccine is not approved for use in infants under the age of six months.


Because of the changing nature of the virus, even vaccinated people may become infected. Often times, home treatment to ease the symptoms and prevent complications of the flu is all that is required. Home treatment involves the following:

Bed Rest

Extra rest is often necessitated by the fatigue, muscle aches and general unwell feeling caused by the flu. Bed rest not only helps the body to heal more quickly, it also prevents an infected person from spreading the virus.


Fever is a common symptom of the flu. It is vital to replace the fluids that are lost due to fever to avoid dehydration. Fluids such as hot tea, soups, water and juices are recommended not only to replace fluid, but also to relieve scratchy or sore throats. Drinking plenty of fluids also helps to thin mucous, making it easier to expel. This can help to prevent bacterial infections such as bronchitis.

OTC Medication

Over the counter medications such as ibuprofen or acetaminophen can be used to relieve the general discomfort as well as reduce fever.

Decongestants and cough drops, lozenges or syrups can relieve cough, help expel mucous and ease sore or scratchy throats.

Additional Home Treatments

Elevation of the head during the night can help relieve coughing and ease sleep. The use of a vaporizer or humidifier can help moisten mucous membranes and thin mucous. Smoking and second hand smoke should be avoided always, but especially when a respiratory infection is present.

Antiviral Medication

Sometimes antiviral medications that make it more difficult for the influenza virus to reproduce in the body are prescribed. These medications often shorten the duration and lessen the severity of flu symptoms when taken within 48 hours of the development of symptoms. These medications can be used to prevent the flu after exposure and to treat it. They are not, however, suitable substitutes for vaccination. Antiviral medications can be given to prevent further outbreaks of the illness. They are also given to people with increased risks of developing serious complications.

Not everyone should take antiviral medications. Side effects of the medications include diarrhea, vomiting, headache, dizziness and occasionally, behavioral side effects such as confusion and increased risk of self harm.

Other Influenza Viruses

Different strains of influenza viruses infect certain species. Typically, these strains of viruses cannot be spread from animals to humans. There have been limited reports of certain strains that spread from birds and swine to humans however.

Avian Flu

Avian influenza, or bird flu, has caused some concern in the medical community. The influenza A H5N1 virus is common in wild birds. It generally does not cause wild birds to become ill, however, they are able to pass the virus through saliva and droppings to domesticated birds raised for food.

People who come into contact with infected domestic birds can contract the virus. The virus cannot be contracted by eating fully cooked birds. The cause for concern among the medical community focuses on the belief that there have been instances of human to human, rather than bird to human transmission of the virus.

Most instances of human avian flu infections have occurred in Asian countries. Avian influenza infection remains rare in North America.

Swine Flu

Swine flu viruses are similar to human flu viruses in that they cause respiratory infection and circulate among swine populations. Also similar, there are many subtypes and strains of swine flu. Typically, swine flu does not affect humans, but sporadic cases have been reported.

When a human becomes infected with a swine flu, the virus is called a variant virus. Most often these variant viruses are limited to swine to human transmission. Occasionally, however, there has been limited spread of variant influenza viruses between humans. To date, there has been no sustained or wide-spread incidence of human to human infection of swine variant influenza virus.

History of Influenza Pandemic

Pandemic is defined as a widespread outbreak of disease that affects exceptionally high proportions of the population. Because the influenza virus can change suddenly, people may have little to no immunity to newer strains. This can cause more severe symptoms, more frequent complications and more widespread infections.

The CDC has a scale for representing the severity of a pandemic. These categories are mainly based on case-fatality ratios, or CFR. Category 1, for example, is determined by a CFR of less than 0.1 percent. Category 5, the highest category, is determined by a CFR of 2 percent or higher.

1918-1919 Spanish Flu Pandemic

Between 1918 and 1919 a strain of influenza, called the Spanish Flu, infected between around 20 to 40 percent of the world’s population. This was classified as a category 5 pandemic. Unusually severe and fatal, this is the worst example of pandemic in recent history.

For reasons still known, the highest rate of infection and death occurred between people between the ages of 20 to 50 years old. Unlike previous outbreaks, this particular strain saw high mortality rates among healthy adults. An estimated 50 million people died because of the Spanish Flu and complications thereof.

1957-1958 Asian Flu Pandemic

Not nearly as severe as the Spanish Flu pandemic, the Asian Flu pandemic of 1957-58 was categorized as a category 2 pandemic that originated in China. Estimates of worldwide fatalities range from 1 to 4 million. Deaths in the United States were estimated at around 69,800. The elderly in particular were vulnerable to this strain of virus.

1968-1969 Hong Kong Flu

Also categorized as a category 2 pandemic, the Hong Kong Flu of 1968-69 killed an estimated one million people worldwide. US deaths were estimated at 33,000. People over the age of 65 were particularly vulnerable to this flu.

The uniquely unpredictable nature of influenza viruses means that a pandemic is possible at any time. For this reason, organizations such as the CDC keep close watch on outbreaks and mutations. Fortunately, annual vaccinations and other preventative measures have proven to be successful weapons for fighting seasonal influenza.

Health | Respiratory

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