INFLUENZA (FLU) VACCINE

Influenza (flu) is a highly infectious illness caused by a virus. Influenza is a single-stranded, helically shaped, RNA virus of the orthomyxovirus family. Basic antigen types A, B, and

C are determined by the nuclear material. Influenza A causes moderate to severe illness and affects all age groups. Influenza A viruses are perpetuated in nature by wild birds, predominantly waterfowl.

Influenza B generally causes milder disease than type A and primarily affects children. Influenza B is more stable than influenza A, with less antigenic drift and consequent immunologic stability. It affects only humans. Influenza C is rarely reported as a cause of human illness, probably because most cases are subclinical. It has not been associated with epidemic disease.

It is passed on by breathing in droplets of air from infected people. Following respiratory transmission, the virus attaches to and penetrates respiratory epithelial cells in the trachea

and bronchi. Virus is shed in respiratory secretions for 5–10 days. The incubation period for influenza is usually 2 days, but can vary from 1 to 4 days.

“Classic” influenza disease is characterized by the abrupt onset of fever, myalgia, sore throat, nonproductive cough, and headache. The fever is usually 101°–102°F, and accompanied by prostration. The onset of fever is often so abrupt that the exact hour is recalled by the patient.

Myalgias mainly affect the back muscles. Cough is believed to be a result of tracheal epithelial destruction. Additional symptoms may include rhinorrhea (runny nose), headache, substernal chest burning and ocular symptoms (e.g., eye pain and sensitivity to light).

Although many people recover, usually within a few days, the illness can be serious in older people or people with other illnesses such as diabetes, lung, kidney and heart disease.

The most frequent complication of influenza is pneumonia. Reye syndrome is a complication that occurs almost exclusively in children taking aspirin, primarily in association with influenza B (or varicella zoster), and presents with severe vomiting and confusion, which may progress to coma due to swelling of

the brain.

Other complications include myocarditis (inflammation of the heart) and worsening of chronic bronchitis and other chronic pulmonary diseases.

Antivirals are available and are effective in reducing both the intensity and duration of symptoms provided they are administered early in the course of the disease.