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Influenza (Flu)

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Influenza Viruses

Influenza viruses are divided into three types, designated A, B, and C. Influenza types A and B are responsible for epidemics of respiratory illness that occur almost every winter and are often associated with increased rates for hospitalization and death. Influenza type C differs from types A and B in some important ways. Type C infection usually causes either a very mild respiratory illness or no symptoms at all. It does not cause epidemics and does not have the severe public health impact that influenza types A and B do. Efforts to control the impact of influenza are aimed at types A and B.

While there are many different flu viruses, each season a flu vaccine protects against the 3 or 4 viruses that research suggests will be most common. Three kinds of flu viruses commonly circulate among people today: Influenza A (H1N1) viruses, influenza A (H3N2) viruses, and influenza B viruses.

The 2023-2024 influenza vaccine is made to protect against the following:

  • H1N1: A/Victoria/4897/2022(H1N1)pdm09-like virus (egg-based) or A/Wisconsin/67/2022(H1N1)pdm09-like virus (for cell-culture and recombinant vaccines)
  • H3N2: A/Darwin/9/2021 (H3N2)-like virus (for egg-based vaccines) or an influenza A/Darwin/6/2021 (H3N2)-like virus (for cell culture-based and recombinant vaccines)
  • B Victoria: B/Austria/1359417/2021 (Victoria lineage)-like virus
  • B Yamagata: B/Phuket/3073/2013 (Yamagata lineage)-like virus

All vaccines for the 2023-2024 season are quadrivalent.

If You Get Sick

The flu is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness and, at times, can lead to death. Symptoms of flu include:

  • Fever* or feeling feverish/chills
  • Headaches
  • Fatigue (tiredness)
  • Cough
  • Sore throat
  • Runny or stuffy nose
  • Muscle aches
  • Some people may have vomiting and diarrhea, though this is more common in children than adults.

*It is important to note that not everyone with the flu will have a fever.

While getting a flu vaccine each year is the best way to protect against flu, influenza antiviral drugs can fight against influenza, offering a second line of defense.

Antiviral Drugs

Antiviral drugs are an important second line of defense against the flu.

  • If you do get the flu, antiviral drugs are an important treatment option. However, they are not a substitute for vaccination.
  • Antiviral drugs are prescription medicines (pills, liquid, an inhaled powder, or an intravenous solution) that fight against the flu by keeping flu viruses from reproducing in your body.
  • Antiviral drugs can make your illness milder and make you feel better faster. They may also prevent serious flu complications. This could be especially important for people at high risk.
  • For treatment, antiviral drugs work best if started soon after getting sick (within two days of symptoms).

There are four FDA-approved antiviral drugs recommended by the Centers for Disease Control and Prevention (CDC) to treat flu this season.

Antiviral drugs differ in terms of who can take them, how they are given, their dose (which can vary depending on a person’s age or medical conditions), and side effects.

For more information, see CDC Influenza Antiviral Medications: Summary for Clinicians or consult the package insert for each drug. Your doctor can help decide whether you should take an antiviral drug this flu season and which one you should use.

If You Get Sick

Most healthy people recover from the flu without complications. If you get the flu:

  • Stay home from work or school.
  • Get lots of rest, drink plenty of liquids, and avoid using alcohol and tobacco.
  • There are over-the-counter medications to relieve the symptoms of the flu (but never give aspirin to children or teenagers who have flu-like symptoms, particularly fever).
  • Remember that serious illness from the flu is more likely in certain groups of people, including people 65 years of age and older, pregnant individuals, people with certain chronic medical conditions, and young children.
  • Consult your doctor early on for the best treatment, but also be aware of emergency warning signs that require urgent medical attention.

Emergency Warning Signs

In children, emergency warning signs that need urgent medical attention include

  • Fast breathing or trouble breathing
  • Bluish skin color
  • Not drinking enough fluids
  • Not waking up or not interacting
  • Being so irritable that the child does not want to be held
  • Fever with a rash

In adults, emergency warning signs that need urgent medical attention include

  • Difficulty breathing or shortness of breath
  • Pain or pressure in the chest or abdomen
  • Sudden dizziness
  • Confusion
  • Severe or persistent vomiting

Seek emergency medical care if you or someone you know is experiencing any of the signs above.

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