Flu is a contagious respiratory illness that affects the nose, throat and sometimes the lungs. It can cause mild to severe illness and can lead to death. In the United States, flu season usually occurs in the fall and winter. Last flu season, there were 41 million flu illnesses, 490,000 people were hospitalized with the flu, and there were 25,000 flu related deaths.
During flu season, the U.S. Centers for Disease Control and Prevention (CDC) provides a weekly surveillance report.
Transmission
Influenza (flu) is primarily transmitted in households and community settings. Flu spreads from person to person predominantly through large-particle respiratory droplets. Indirect transmission via hand transfer of the flu virus from contaminated surfaces or objects to mucosal surfaces of the face (e.g., nose, mouth) is possible. Airborne transmission in the vicinity of the infectious individual may also occur.
Signs & symptoms
Signs and symptoms of flu can vary by age, immune status, and underlying medical conditions. Signs and symptoms of uncomplicated flu include:
- Fever
- Muscle aches
- Headache
- Lack of energy
- Dry cough
- Sore throat
- Nasal congestion
- Possible runny nose
Diagnosis
Flu can be difficult to diagnose based on clinical signs and symptoms alone because they can be like those caused by other viruses or bacteria. A number of tests can help in the diagnosis of flu. Available diagnostic tests include rapid antigen testing and reverse transcriptase polymerase chain reaction (RT-PCR). In addition, home influenza tests were authorized by the Food and Drug Administration (FDA) in early 2024.
Prevention strategies
Health care settings require multi-faceted approaches to prevent the transmission of flu. Core prevention strategies include vaccination and early treatment, hand hygiene, environmental cleaning with approved products and engineering controls.
Vaccination
Routine, annual flu vaccination is recommended for all people aged 6 months or older who do not have contraindications. Ideally, people should be vaccinated in September or October. However, vaccination should continue throughout the season as long as flu viruses are circulating.
CDC does not recommend any licensed flu vaccine over another for people under 65 years of age. For people 65 years and older, CDC recommends high-dose flu vaccines over standard-dose, unadjuvanted flu vaccines.
Coadministration with other vaccines
Flu, COVID-19, and RSV vaccines may be given at the same visit for eligible patients. If the patient prefers to receive each vaccine at a separate visit, there is no minimum waiting period between vaccines.
Treatment
Flu antiviral drugs should be started as soon as possible to treat patients who are hospitalized with flu, very sick with flu but do not need to be hospitalized, and those at higher risk of serious flu complications based on their age or health. People with mild illness not at higher risk of flu complications do not need to be treated with antiviral drugs.
CDC recommends four FDA approved antiviral drugs:
- Oseltamivir phosphate (Tamiflu®)
- Zanamivir (Relenza®)
- Peramivir (Rapivab®)
- Baloxavir marboxil (Xofluza®)
Doctors should not wait for laboratory confirmation to start antiviral treatment. Follow prescribing data for each antiviral medication and consider the potential for drug-drug interactions.
Infection prevention & control
In health care facilities, Personal Protective Equipment (PPE) for influenza includes wearing a face mask when caring for the patient, in addition to standard precautions. If the patient needs to leave their room, they must wear a mask.
