Influenza (Flu)

What is influenza (flu)?

Influenza (or flu) is a highly contagious viral respiratory tract infection. About 5% to 20% of people in the U.S. get the flu each year. Influenza usually comes on abruptly, with fever, muscle aches, sore throat, and a nonproductive cough.

Influenza can make people of any age ill. Although most people are ill with the flu for only a few days, some have a much more serious illness and may need to be hospitalized. Influenza can also lead to pneumonia and death.

Influenza viruses are divided into three types, designated as A, B, and C:

  • Influenza types A and B cause epidemics of respiratory illness that occur almost every winter and often lead to increased rates for hospitalization and death. Public health efforts to control the impact of influenza focus on types A and B.

  • Influenza type C usually causes either a very mild respiratory illness or no symptoms at all. It doesn't cause epidemics and doesn't have the severe public health impact that influenza types A and B do.

Influenza viruses continually change (mutate). This helps the virus to evade a person's immune system. Because the virus changes, people can get the flu no matter what their age. The process works like this:

  1. A person infected with an influenza virus develops antibodies against that virus.

  2. The virus changes.

  3. The "older" antibodies no longer recognize the "newer" virus.

  4. The person becomes infected again.

The older antibodies can, however, give some protection against getting the flu again. Currently, three different influenza viruses circulate worldwide: two type A viruses and one type B virus. Vaccines given each year to protect against the flu contain the influenza virus strain from each type that is expected to cause the flu that year.

What are the facts about the flu?

Although each flu season is different, 5% to 20% of the population will get the flu each year. Of those who get the flu, between 3,000 and 49,000 will die from it or from complications, with more than 90% of deaths occurring in people over 65. 

What causes influenza?

The influenza virus is generally passed from person to person through the air - when an infected person sneezes or coughs. But the virus can also live for a short time on objects like doorknobs, pens, pencils, keyboards, telephone receivers, and eating or drinking utensils. So, you can also get the flu by touching something that has been handled by someone infected with the virus and then touching your own mouth, nose, or eyes.

What are the symptoms of the flu?

The following are the most common symptoms of the flu. But each person may experience symptoms differently. Influenza is called a respiratory disease, but the whole body seems to suffer when a person is infected. People usually become acutely ill with several, or all, of the following symptoms:

  • High fever

  • Headache

  • Runny or stuffy nose

  • Sneezing at times

  • Cough, often becoming severe

  • Severe aches and pains

  • Fatigue for several weeks

  • Sometimes a sore throat

  • Extreme exhaustion

Fever and body aches usually last for 3 to 5 days, but cough and fatigue may last for 2 weeks or more. Although nausea, vomiting, and diarrhea may happen with the flu, these gastrointestinal symptoms are rarely prominent. Stomach flu is an incorrect term sometimes used to describe gastrointestinal illnesses caused by other microorganisms.

The symptoms of the flu may resemble other medical conditions. Always consult your doctor for a diagnosis.

What is the treatment for influenza?

Specific treatment for influenza will be determined by your doctor based on:

  • Your age, overall health, and medical history

  • Extent and type of influenza, and severity of symptoms

  • How long you've had symptoms 

  • Your tolerance for specific medications, procedures, or therapies

  • Expectations for the course of the disease

  • Your opinion or preference

The goal of treatment for influenza is to help prevent or decrease the severity of symptoms. Treatment may include:

  • Medications to relieve aches and fever. Aspirin should not be given to children with fever without first consulting a doctor. The drug of choice for children is acetaminophen.

  • Medications for congestion and nasal discharge

  • Bed rest and increased intake of fluids

  • Antiviral medications. When started within the first 2 days of treatment, they can reduce how long you'll have the flu, but they can't cure it. Some side effects, such as nervousness, lightheadedness, or nausea, may result from taking these medications. People with asthma or chronic obstructive pulmonary disease may not be able to take certain of these medications. Viral resistance to these drugs may vary. Some drugs may be ineffective if current viral strains have developed resistance. All of these medications must be prescribed by a doctor.

Consult your doctor for more information.

How can the flu be prevented?

A new influenza vaccine is introduced each September. Everyone who is at least 6 months old should get a flu vaccine this season  It is usually recommended for specific groups of people (see below), as well as for people who want to avoid having the flu. In addition, antiviral medications are approved for use in preventing the flu. All of these medications are available by prescription, and a doctor should be consulted before using any medication to prevent the flu.

A nasal spray flu vaccine is currently approved to prevent flu caused by influenza A and B viruses in healthy children and adolescents ages 2 to 17, and healthy adults ages 18 to 49. As with other live virus vaccines, the nasal spray vaccine should not be given for any reason to pregnant women or people with immune suppression, including those with immune deficiency diseases, such as AIDS or cancer, or people who are being treated with medications that cause immunosuppression. The nasal spray vaccine also should not be given to these groups of people:

  • Children younger than 2 years old

  • Any person with asthma

  • Children younger than 5 years who have wheezing

  • Adults ages 50 and older

  • Children and adolescents who are taking aspirin as long-term treatment

  • Children and adults who have a chronic disorder of the lung, heart, kidney, liver, nerves, blood, or metabolism 

Following these precautions may also be helpful:

  • When possible, avoid or limit contact with infected people.

  • Frequent handwashing may reduce, but not eliminate, the risk of infection.

  • A person who is coughing or sneezing should cover his or her nose and mouth with a handkerchief to limit spread of the virus.

How effective is the flu vaccine?

Vaccine effectiveness varies from year to year, depending on the degree of similarity between the influenza virus strains included in the vaccine and the strain or strains that circulate during the influenza season. The 2013–2014 traditional flu vaccine (called trivalent vaccine) will protect you against three different flu strains, two influenza A viruses and an influenza B virus. Also, the seasonal flu vaccine (called quadrivalent vaccine) that protects against four strains of the flu virus will also be available. The quadrivalent vaccine will protect against two influenza A viruses and two influenza B viruses. Vaccine strains must be chosen nine to 10 months before the influenza season, and sometimes mutations occur in the circulating strains of viruses between the time vaccine strains are chosen and the next influenza season. These mutations sometimes reduce the ability of the vaccine-induced antibody to inhibit the newly mutated virus, thereby reducing vaccine effectiveness.

Vaccine effectiveness also varies from one person to another, depending on factors such as age and overall health.

What are the side effects of the flu vaccine?

The flu vaccine is safe. Vaccine safety is closely watched by the CDC and the FDA, and hundreds of millions of flu vaccines have been safely given across the country for decades.

The flu vaccine can't give you the flu. The most common side effects from a flu shot are soreness where the shot was given and maybe a low fever or achiness. The nasal spray flu vaccine might cause congestion, runny nose, sore throat, or cough. If you do have them at all, these side effects are usually mild and short-lived.

Who should immunize against the flu?

The flu causes complications that may develop into a more serious disease or become dangerous to some groups, such as older adults and those with chronic medical conditions. For these reasons, the CDC recommends that the following groups get a vaccine each year. Always consult your doctor for more information regarding who should receive the flu vaccine:

  • People ages 50 and older. Vaccine effectiveness may be lower for older adults, but it can significantly reduce their chances of serious illness or death from influenza.

  • Children and adolescents 6 months to 19 years old

  • Residents of nursing homes and any other chronic care facilities that house people of any age who have chronic medical conditions

  • Adults and children who have chronic disorders of the lungs or heart, including children with asthma

  • Adults and children who have these medical conditions:

    • Chronic metabolic diseases, such as diabetes

    • Kidney dysfunction

    • Immunosuppression

    • Hemoglobinopathies, such as sickle cell disease 

  • Children and teenagers ages 6 months to 19 years who take aspirin as long-term therapy

  • Women who will be pregnant during flu season

  • Health care providers

  • Employees of nursing homes and chronic care facilities who have contact with patients or residents

  • Providers of home care to people at high risk

  • Household members, including children, of people high-risk groups

  • People of any age who wish to lower their chances of influenza infection

Who should not be immunized against the flu? 

  • People who are allergic to eggs may be told not to get the vaccine

  • People who have had a severe reaction in the past after getting the flu vaccination

  • People who are sick with a fever (these people should get vaccinated after they have recovered)

  • Babies who are 6 months old or younger

  • People who have a history of Guillain-Barré syndrome, a severe paralyzing illness, after getting the flu vaccination 

When should I get a flu shot?

The CDC recommends getting the flu shot every year, as soon as it becomes available in your community. Flu season can begin as early as October and most commonly peaks in the U.S. in January or February, but flu seasons are unpredictable. The flu shot takes 1 to 2 weeks to become effective.

Although there are many new medications designed to treat flu symptoms and even shorten the duration of the illness, the flu vaccine still offers the best protection against the flu.

If I get the flu shot, can I still get the flu?

Every year, the flu shot cocktail changes to combat the current strains of influenza affecting the population. The World Health Organization monitors flu outbreaks worldwide and recommends appropriate vaccine compositions to be used for the next year. But sometimes a strain may appear that was not included in the flu vaccine. People who have had the flu shot tend to have milder symptoms if they contract the flu.

Is traveling safe during the flu season?

Because the flu is a highly contagious infection usually spread by droplets made by an infected person who is coughing or sneezing, travelers are very susceptible to getting the flu.

The CDC recommends that travelers have the flu vaccine at least 2 weeks before planned travel to allow time to develop immunity. Other antiviral drugs are available to help prevent viral infections and complications. Consult your doctor for more information.