What is Influenza?

Influenza is a respiratory virus that affects the lungs, throat and nose. It is a seasonal virus that impacts the northern hemisphere from October to May, and the southern hemisphere from May to October. While influenza can often be mild, severe cases can lead to hospitalization or death. The CDC estimates that influenza causes about 36,000 deaths per year in the United States alone.

Who is at Risk for Influenza

Populations at Risk for Influenza
  • Seniors 65 and older
  • Young children, especially those under 2 years old
  • Those with chronic medical conditions that compromise the immune system, including asthma, heart disease, and diabetes, among others.
  • Pregnant women; influenza can also cross the placental barrier and infect fetuses in utero.
  • Research from the CDC also indicates that people of Native American or Native Alaskan heritage are at higher risk.

Infants & Pregnant Women

Infants and children under 2 are especially at risk for complications from influenza. If you develop the flu, limit interactions with your infant or toddler. If your child develops the flu, watch closely for symptoms, and see a doctor as soon as possible.

Pregnant women are significantly more likely to have severe illness from influenza than their peers.

Concerning symptoms in young children include:

  • Rapid breathing
  • Bluish tint to skin (may be caused by a lack of oxygenation)
  • Dehydration
  • Fever with a rash

Emergent symptoms for infants include*:

  • Not eating
  • Trouble breathing
  • No tears when crying
  • Significantly fewer wet diapers than normal
*If your child exhibits any of these symptoms, get help right away!

Pregnant women are significantly more likely to have severe illness from influenza than their peers. High fever from the viral infection can affect the fetus, and has been connected with neural tube defects and other developmental issues. Additionally, influenza is capable of crossing the placental barrier and infecting the fetus in utero.

Pregnant women can and should receive influenza vaccine. The vaccine is safe for pregnant women, and can also protect the baby both in utero and in the first 6 months of life. However, if you are pregnant and have any symptoms of influenza, you should see a doctor right away.

Flu Transmission

Influenza has an incubation period of up to 4 days, and you may be contagious up to a day before you exhibit symptoms. When an infected person coughs or sneezes, mucus and saliva carry the virus through the air, where they can land on another person’s nose or mouth. Less often, people can contract the virus by touching a surface that has come in contact with influenza, and then touching their own mouth or nose. Infected persons are contagious for up to a week after symptom onset.

Symptoms of the Flu

Although many people react in different ways to influenza, there are several overarching symptoms that can help you determine if you’ve been infected.

Symptoms of Influenza

  • Fever
  • Sore throat
  • Dry, persistent coughing
  • Runny or stuffy nose
  • Muscle or body aches
  • Headaches
  • Chills
  • Fatigue and weakness

Though vomiting and diarrhea may occur, these are more common in young children than in adults.

Diagnosing Influenza

Some doctors will diagnose a patient with influenza based on symptoms alone, especially if there is a known outbreak in the area. However, there are several tests that can confirm an influenza diagnosis.

“Rapid” Testing

The “rapid” influenza test is given out by some doctors and is also available in most emergency departments. These are commonly referred to as Influenza Rapid Diagnostic Tests (RIDTs) or point-of-care tests (POCT). First, mucus is collected, usually via a nose or throat swab. Then, the test changes color to show the presence of viral antigens- the part of the virus that triggers your immune system.

Some points to note about “rapid” tests:

  • They usually produce results within 30 minutes.
  • In a region where there is a known outbreak, a positive test is a good indicator of influenza disease.
  • These tests are more likely to produce “false negatives” than other diagnostic tests. A negative result on a rapid test isn’t always correct. Your doctor may diagnose you based on your symptoms, or order a more reliable “overnight” test.
  • The rapid test appears to be more reliable when used for children.

“Overnight” Cultures

More accurate tests may take a longer time to produce results. The most commonly used method is called a Reverse Transcription-Polymerase Chain Reaction (RT-PCR) molecular test. This test detects proteins called RNA, which are unique to influenza virus, with both high sensitivity and specificity. Some molecular tests can determine if the patient has Influenza A or B, while others can detect even the subtypes Influenza A. These tests take approximately 1-8 hours to provide results, and are commonly referred to as “overnight” cultures.

Tests Available for Influenza
Expand Resource
Test NameSpecimen NeededHow it WorksSensitivitySpecificityTime Required
Influenza Rapid Diagnostic Tests (RIDTs)Nasal or throat swabDetects viral antigensLow- ModerateHigh<30 minutes
ImmunofluorescenceNasal or throat swabDetects antigens via fluorescent antibody testsModerateHigh2-4 hours
Rapid Molecular TestNasal or throat swabDetects virus via isothermal nucleic acid amplification or RT-PCRHighHigh<20 minutes
Reverse Transcription-Polymerase Chain Reaction (RT-PCR) molecular TestsNasal or throat SwabDetects viral RNA. Some cat detect subtypes of Influenza A.HighHigh1-8 hours
Viral Cell CultureNasal or throat swabDetects virus by growing a live culture.HighHigh1-10 Days (several different types available)

Sensitivity and Specificity

The accuracy of these tests is measured by sensitivity and specificity.

Sensitivity refers to the test’s ability to detect the presence disease.

  • A test with low sensitivity is more likely to produce a “false negative”.

Specificity it refers to the ability of a test to identify a lack of the disease.

  • The more “specific” a test is, the less likely you will have a “false positive.”

For example: The rapid influenza test has only moderate sensitivity, but high specificity. This means that you may have a higher incidence of “false negatives,” but that those with a “positive” result are highly likely to have influenza.

How Influenza is Treated

Some people who get influenza may not need to see a doctor and may only have symptoms for a few days. Others may have severe reactions to the virus that require hospitalization and close care. If you begin to feel unwell during the influenza season, keep a watchful eye on your symptoms. Visit a doctor if you experience persistent vomiting, a fever of about 104 degrees, dizziness or confusion, or are concerned about your health. Remember, it’s better to visit the doctor and receive good news than to skip the visit and allow an illness to get worse!


Mild cases of influenza may not require medication. However, anyone with underlying conditions or a more serious case should go to the doctor for diagnosis and treatment. Your doctor will likely prescribe an antiviral medication for you to take. These medications are available by prescription only. Antiviral medications should be started within 48 hours of symptom onset in order to be most effective, however, they can be taken later than 48 hours after symptom onset to help prevent serious complications. Some quick points about antiviral medications:

  • Antiviral medications can shorten the length of your influenza symptoms by 1-2 days.
  • Antiviral medications are not a substitute for influenza vaccine.
  • Antiviral medications are different from antibiotics, which treat bacterial infections.
  • Antiviral medications will not stop you from being able to spread the disease.
  • Taking antibiotics will not help treat influenza.

The FDA has approved 3 medications for the treatment of influenza:

  • Oseltamivir (available as a generic version or under the trade name Tamiflu)
  • Zanamivir (trade name Relenza)
  • Peramivir (trade name Rapivab)

Brand-Name Flu Medications

Tamiflu is the most widely used antiviral medication for treating influenza and is available as a pill or a liquid. It is approved for anyone older than 2 weeks of age. As a treatment, Tamiflu is taken for 5 days. It works by prohibiting the ability of the virus to multiply while reducing symptoms. In general, Tamiflu will only shorten the length of your illness by a day or so, however, some people do see effects more rapidly.

Tamiflu can also be used to prevent influenza illness. This is often recommended if one member of a family has fallen ill to help protect other family members. In this case, a lower dose of Tamiflu is taken for 10 days rather than 5.

Relenza is approved for patients ages 7 and up, and is taken as an inhalant using a device called a diskhaler. This medication can also be used both as treatment and prophylaxis, much in the same way as Tamiflu. However, because it is inhaled, Relenza is not recommended for patients with asthma or other chronic respiratory issues like Chronic Obstructive Pulmonary Disease (COPD).
Rapivab is administered intravenously, and can only be administered by a healthcare professional. It was approved in 2014 and is only used on patients 18 and older.  Rapivab treatment takes about 15-30 minutes.

Home Remedies

In addition to medication, there are a few things you can do at home to help ease symptoms of influenza and prevent other family members from contracting the disease. The CDC recommends that ill family members remain at home and minimize any contact with other people to limit the possibility of passing on the virus. Ill family members should remain home until they have gone at least 24 hours without fever. During this 24 hour period, you also shouldn’t use over-the-counter medications to reduce fever. If you need to leave your home, be sure to wash your hands frequently and cover your mouth when coughing or sneezing.

Two simple steps to help ease influenza symptoms:

  1. Get rest. This helps your body focus on fighting the viral infection.
  2. Stay hydrated and make sure to drink plenty of fluids.

Complications of Influenza

The most common and serious complication of influenza is pneumonia. In fact, one third of pneumonia cases yearly are caused by influenza. The lung becomes infected and swells, impairing oxygen flow and making breathing difficult. Influenza and pneumonia is the 8th leading cause of death in the United States.

Viral pneumonia caused by influenza can cause rapid deterioration in patients, as the swelling in the lungs combines with the flu’s respiratory symptoms. The resulting lack of oxygenation in the body can be deadly if not treated. If influenza is causing difficulty breathing, see a healthcare professional immediately.

Other Influenza Complications
  • Sinus and ear infections
  • Myocarditis (inflammation of the heart)
  • Encephalitis (inflammation of the brain)
  • Myositis (inflammation of the muscle tissue)

In especially severe cases, influenza can trigger multi-organ failure. Influenza can also trigger a massive inflammatory response which can result in sepsis. Both these conditions are life threatening and should be treated by medical professionals immediately.

The Flu Vaccine

Influenza vaccine is the only way to prevent contracting the seasonal influenza. Influenza vaccine can be administered by a healthcare professional at your local doctor’s office, hospital clinic, pharmacy or a registered seasonal event. Influenza vaccine is recommended for everyone over 6 months old and is administered via injection.

Can the flue vaccine give me the flu?

No. The flu vaccination does not contain a live virus, and cannot give you influenza.

What is Influenza Vaccine?

Influenza vaccine changes from year to year. It is created by an international committee headed by the World Health Organization, represented in the US by the Food and Drug Administration and Centers for Disease Control. Using 365 days/year of surveillance, this committee predicts the dominant strains of influenza expected to spread in the coming months. Prototypical strains of the virus are then killed, broken up, and combined with other ingredients for injection administration.

How does the influenza vaccine work?

The broken pieces of the virus in the vaccine elicit an immune response in your body after your receive the shot. This process encourages your body to create antibodies that specifically target influenza, so that if the virus tries to infect you, your body can more easily fight it off.

When should I get the influenza vaccine?

Medical professionals suggest getting the vaccine in September, October or November. However, in the U.S., the peak of influenza season may not come until February, so it is still useful to get the vaccine in December or January.

How long does the vaccine take to be effective?

The influenza vaccine can take up to 10-14 days to take effect.

How effective is the influenza vaccine?

In young, healthy adults, the vaccine is estimated to reach up to 70% total protection. In older adults, efficacy has been measured in recent years at approximately 50%. However, efficacy is rated regarding total protection only. It does not address partial protection, which can make a influenza illness much milder if you still contract the disease.

What about the nasal spray?

In the 2017-2018 season, the nasal spray vaccine will not be recommended. In recent years, there were concerns about the efficacy of the nasal spray, especially as it is primarily used to protect children. Research is ongoing to improve and bring back the nasal spray option.

This Page has been Expert Verified

By Dr. William Schaffner, Professor of MedicineVanderbilt University Medical Center