Influenza, one of the most common infectious diseases, is a highly contagious airborne disease that occurs in seasonal epidemics and manifests as an acute febrile illness with variable degrees of systemic symptoms, ranging from mild fatigue to respiratory failure and death. Influenza causes significant loss of workdays, human suffering, and mortality.
The US Centers for Disease Control and Prevention (CDC) documented that seasonal influenza was responsible for 24,000-62,000 deaths during the 2019-2020 season.
Mortality is highest in infants and elderly persons.
Signs and symptoms
The presentation of influenza virus infection varies, but it usually includes many of the following signs and symptoms:
Frontal or retro-orbital headache
Weakness and severe fatigue
Cough and other respiratory symptoms
Red, watery eyes
The incubation period of influenza is 2 days long on average but may range from 1 to 4 days in length. Aerosol transmission may occur 1 day before the onset of symptoms
; thus, it may be possible for transmission to occur via asymptomatic persons or persons with subclinical disease, who may be unaware that they have been exposed to the disease.
See Presentation for more detail.
Influenza has traditionally been diagnosed on the basis of clinical criteria, but rapid diagnostic tests, which have a high degree of specificity but only moderate sensitivity, are becoming more widely used. The criterion standard for diagnosing influenza A and B is a viral culture of nasopharyngeal samples or throat samples. In elderly or high-risk patients with pulmonary symptoms, chest radiography should be performed to exclude pneumonia.
Avian influenza (H5N1) is rare in humans in developed countries (see the image below). Unless advised by the CDC or regional health departments, clinicians do not routinely need to test for avian influenza.
Countries where avian influenza has been reported. Image courtesy of the World Health Organization.
See 11 Travel Diseases to Consider Before and After the Trip, a Critical Images slideshow, to help identify and manage infectious travel diseases.
See Workup for more detail.
Prevention of influenza is the most effective management strategy. Influenza A and B vaccine is administered each year before flu season. The CDC analyzes the vaccine subtypes each year and makes any necessary changes on the basis of worldwide trends.
Traditionally, the vaccine is trivalent (ie, designed to provide protection against 3 viral subtypes, generally an A-H1, an A-H3, and a B). The first quadrivalent vaccines, which also provide coverage against a second influenza B subtype, were approved in 2012 and were made available for the 2013-2014 flu season.
The FDA has approved a vaccine for H5N1 influenza. It is available only to government agencies and for stockpiles.
The following are influenza vaccine recommendations by the Advisory Committee on Immunization Practices:
In the Northern Hemisphere, all persons aged 6 months or older should receive influenza vaccine annually by the end of October, if possible. Influenza vaccination should not be delayed to procure a specific vaccine preparation if an appropriate one is already available.
Persons with a history of egg allergy who have experienced only hives after exposure to egg should receive influenza vaccine. Inactivated influenza virus cell culture–based (ccIIV4; Flucelvax) or trivalent or quadrivalent recombinant influenza vaccine (RIV; Flublok) should be used. RIV may be used for persons aged 18 years or older who have no other contraindications.
Regardless of allergy history, all vaccines should be administered in settings in which personnel and equipment for rapid recognition and treatment of anaphylaxis are available.
A previous severe allergic reaction to influenza vaccine, regardless of the component suspected of being responsible for the reaction, is a contraindication to future receipt of the vaccine.
In addition to vaccination, other public health measures are also effective in limiting influenza transmission in closed environments. Enhanced surveillance with daily temperature taking and prompt reporting with isolation through home medical leave and segregation of smaller subgroups decrease the spread of influenza.
In the United States, the following prescription antiviral drugs have been approved for treatment and/or chemoprophylaxis of influenza and are active against recently circulating subtypes of influenza: