Home Medical Topics Influenza – Causes, Symptoms, Treatment &

Influenza – Causes, Symptoms, Treatment &



Influenza is a viral respiratory infection causing fever, coryza, cough, headache, and malaise.Seasonal epidemics can result in death, especially in high-risk patients (such as institutionalized people, people who are very old or very young, people who have cardiopulmonary insufficiency, or women who are late in their pregnancies). During pandemics, even young, healthy people might pass away. The diagnosis is typically made clinically and is based on regional epidemiologic trends. Everyone older than 6 months who does not have a contraindication has to receive the influenza vaccine each year. Antiviral medication, which should be specifically taken into consideration for high-risk patients, reduces the length of illness by around one day.

When is flu season?

Flu season — when cases of the flu go up dramatically — in the Northern Hemisphere (which includes the U.S.) is October through May. The highest number of cases (peak) usually happen between December and February.

Influenza antigens

The influenza virus can attach to cellular sialic acid and fuse with the host cell membrane thanks to hemagglutinin (H), a glycoprotein found on the surface of the virus. Another surface glycoprotein called neuraminidase (NA) catalytically breaks down sialic acid, facilitating the release of the virus from the infected host cell. There are 198 potential combinations when 18 H types and 11 NA kinds are combined, although only a few of these are human diseases.

Antigenic drift refers to relatively minor, progressive mutations in preexisting combinations of H and NA antigens, resulting in the frequent emergence of new viral strains. These new strains may cause seasonal epidemics because protection by antibody generated to the previous strain is decreased.

Antigenic shift refers to the relatively rare development of new combinations of H and/or NA antigens, which result from reassortment of subunits of the viral genome. Pandemics can result from antigenic shift because antibodies against other strains (resulting from vaccination or natural infection) provide little or no protection against the new strain.

Who is more susceptible to flu complications?

Certain health conditions can put you at higher risk for severe illness from the flu. This includes life-threatening complications that require hospitalization. You’re at higher risk for serious illness if you:

  • Have asthma, COPD or another chronic lung disease.
  • Have a history of kidney, liver, neurological, heart or blood vessels disease, including stroke.
  • Have a condition that causes issues with muscle function or makes it difficult to cough, swallow or clear fluids from your airways.
  • Have diabetes.
  • Have a weakened immune system (from HIV/AIDScancer or immunosuppressive medications).
  • Have a blood disorder, like sickle cell disease.
  • Have a BMI greater than 40 (have obesity).
  • Are under 5 years old or over 65 years old.
  • Are pregnant.
  • Are under 19 years old and take aspirin regularly.
  • Live in a long-term care facility.


Influenza viruses can be spread by

  • Airborne droplets
  • Person-to-person contact
  • Contact with contaminated items

Airborne spread appears to be the most important mechanism.

High-risk groups

Certain patients are at high risk of complications from influenza:

  • Children < 5 years; children < 2 years are at particularly high risk
  • Adults > 65 years
  • People with chronic medical disorders (eg, cardiopulmonary disease, diabetes mellitus, renal or hepatic insufficiency, hemoglobinopathies, immuodeficiency)
  • Women in the 2nd or 3rd trimester of pregnancy
  • Patients with disorders that impair handling of respiratory secretions (eg, cognitive dysfunction, neuromuscular disorders, stroke, seizure disorders)
  • Patients ≤ 18 years taking aspirin (because Reye syndrome is a risk)

Morbidity and mortality in these patients may be due to exacerbation of underlying illness, acute respiratory distress syndrome, primary influenza pneumonia, or secondary bacterial pneumonia.


Symptoms of the flu usually come on quickly, and can include:

  • Fever.
  • Chills.
  • Body aches.
  • Cough.
  • Headache.
  • Sore throat.
  • Runny or stuffy nose (congestion).
  • Tiredness or feeling run down.
  • Diarrhea or vomiting (usually only in kids).

You may not have all of these symptoms.


  • Clinical evaluation
  • Sometimes rapid antigen or conventional reverse transcriptase-polymerase chain (RT-PCR) tests
  • Pulse oximetry and chest x-ray for patients with severe respiratory symptoms

The diagnosis of influenza is generally made clinically in patients with a typical syndrome when influenza is known to be present in the community.

Although many rapid diagnostic molecular tests (antigen detection tests) are available and most have good specificity, their sensitivities vary widely, and they usually add little to patient management. Diagnostic tests should be done when results will affect clinical decisions.


Antiviral drugs can be used by healthcare professionals in some situations to treat the flu. Antivirals can shorten the length of your illness and lower your risk of developing a serious disease. Many people are able to treat the flu without using prescription drugs. If the following applies to you:

  • Have had symptoms for under 48 hours. Antivirals are less likely to work if you start them after two days of symptoms. The virus has already made more copies of itself and your body has started to fight it off with its own antibodies.
  • Have an underlying condition or are at risk for severe illness. Providers may prescribe antivirals even if you’ve had symptoms for longer than 48 hours.
  • Have severe symptoms, even if you’ve been sick for longer than 48 hours.
  • Live with or care for people who are at risk for severe complications of the flu.
Drugs for influenza

Antiviral medications taken within 1 to 2 days of the onset of symptoms lessen the length of the fever, the intensity of the symptoms, and the time required to resume regular activity. For high-risk patients (including all hospitalized patients) who experience influenza-like symptoms, treatment with antiviral medications is advised; this advice is based on research demonstrating that early treatment may avoid complications in these patients.

Drugs for influenza include the following:

  • Oseltamivir, zanamivir, and peramivir (neuraminidase inhibitors)
  • Baloxavir (endonuclease inhibitor)

Neuraminidase inhibitors interfere with release of influenza virus from infected cells and thus halt spread of infection.

The endonuclease inhibitor baloxavir interferes with viral replication by blocking viral RNA transcription. It is active against influenza A and B and may be an important new treatment option should resistance to neuraminidase inhibitors develop.

Zanamivir is given by an inhaler, 2 puffs (10 mg) 2 times a day; it can be used in adults and children ≥ 7 years. Zanamivir sometimes causes bronchospasm and should not be given to patients with reactive airway disease; some people cannot use the inhalation device.

Oseltamivir 75 mg orally 2 times a day is given to patients > 12 years; lower doses may be used in children as young as 1 year. Oseltamivir may cause occasional nausea and vomiting. In children, oseltamivir may decrease the incidence of otitis media; however, no other data clearly show that treatment of influenza prevents complications.

Peramivir is given IV as a single dose and can be used in patients > 2 years who cannot tolerate oral or inhaled drugs. Studies of its use for influenza B are limited.

Baloxavir is given as a single 40 mg dose orally to patients ≥ 12 years and 40 to 80 kg or a single 80 mg dose for patients >80 kg. It can be used in patients ≥ 12 years with uncomplicated influenza who have been symptomatic for ≤ 48 hours and who are otherwise healthy or at high-risk of developing complications (1, 2).

Adamantanes (amantadine and rimantadine) were previously used; however, more than 99% of current and recent circulating influenza viruses are resistant to adamantanes, so these drugs are currently not recommended for treatment. Adamantanes block the M2 ion channel and thus interfere with viral uncoating inside the cell. They were effective only against influenza A viruses (influenza B viruses lack the M2 protein).


Getting the flu shot annually is the most effective approach to avoid getting the flu. Immune system training from vaccinations teaches your body to spot diseases and fight them off before you become ill. You should get vaccinated annually since the influenza virus can alter (mutate) slightly each year.

The flu vaccine lowers your risk of developing a serious illness, even if you end up becoming sick with a different strain of the virus than the one in the vaccine. Your healthcare professional can administer the flu shot or nasal spray forms of the vaccine.

A nurse prepares an injection of the influenza vaccine at Massachusetts General Hospital

Other ways to reduce your risk of getting the flu include:

  • Wash your hands often with soap and water. If you aren’t able to use soap and water, use an alcohol-based hand sanitizer.
  • Cover your nose and mouth when you sneeze or cough. Cough or sneeze into your elbow or a tissue rather than your bare hand.
  • Avoid being around other people when you or they are sick with the flu or other infectious diseases.
  • Consider wearing a mask if you’re sick and can’t avoid being around others.
  • Avoid touching your face, eyes, nose and mouth.
  • Don’t share food or eating utensils (forks, spoons, cups) with others.

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