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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.

Each season a flu vaccine protects against the 3 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 2025-2026 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/Croatia/10136RV/2023 (H3N2)-like virus (for egg-based vaccines) or an influenza A/District of Columbia/27/2023 (H3N2)-like virus (for cell culture-based and recombinant vaccines)
  • B Victoria: B/Austria/1359417/2021 (Victoria lineage)-like virus (egg-based) or B/Austria/1359417/2021(Victoria lineage)-like virus (for cell culture-based and recombinant vaccines)

All flu vaccines for use in the United States are trivalent (three component) vaccines for the 2025-2026 season.

Know the Difference? Is it a cold or flu or COVID-19?

Understanding the Flu

It is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness and, at times, can lead to death. People infected with the flu often feel some or all of the signs and symptoms, which include:

  • Fever or feeling feverish/chills (It is important to note that not everyone with the flu will have a fever)
  • 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

You could spread influenza to someone else before you know that you are sick and when you have symptoms of being sick. People with the flu are most contagious during the first three days of their illness, with most health adults being contagious from one day prior to symptoms showing up to five to seven days after the illness starts.

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
  • Take over-the-counter medications to relieve the symptoms of the flu if necessary (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.

Treating the Flu

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 how they are given, their dosage (which can vary depending on a person’s age and/or medical conditions), side effects, and who should receive them. It’s important to remember that antibiotics used to treat bacterial infections will not treat influenza infections.

For more information, see CDC Influenza Antiviral Medications: Summary for Clinicians, CDC Treating Flu with Antiviral Drugs, 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.

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