The flu vaccine (influenza immunization or flu shot) is a viral respiratory infection primarily developed in the lungs and spreads between people in secretions of the nose and lungs. It causes higher fever, more malaise, and severe body aches and it is a very common illness.
Viruses of influenza can be divided into three types: A and B, which are responsible for epidemics of respiratory illness that occur almost every winter and type C, which usually causes very mild or no respiratory illness. Type A viruses are divided into subtypes H (named by surface protein hemagglutinin, there are 16 known subtypes) and N (named by surface protein neuraminidase, 9 known N subtypes).
In patients with flu, superinfections may occur, which are bacterial infections and in elderly and very young people it can cause death. The flu vaccine can be effective against the influenza virus within two weeks of administration and October and November is considered the best time to do it because the flu season can begin in October and last as late as May.
Vaccination can only be effective against the strains of the virus that match the vaccine so every year scientists predict and prepare an appropriate vaccine because the virus can mutate or change its structure.
Flu vaccine can be injected into the deltoid muscle at the side of the arm. Side effects of the vaccination can include soreness at the site of the injection, muscle aching, fever, and feeling unwell.
Flu vaccine can also be given by nasal spray. It is licensed since 2003 and approved by the U.S.
Food and Drug Administration (FDA) for use in:
- Nonpregnant healthy people between 2-49 years of age,
- People with a medical condition that places them at high risk for complications from influenza (including those with chronic heart or lung disease, such as asthma or reactive airways disease)
- People with medical conditions such as diabetes or kidney failure, or people with illnesses that weaken the immune system, or who take medications that can weaken the immune system, Children or adolescents receiving aspirin
- People who have a severe allergy to chicken eggs or who are allergic to any of the nasal-spray vaccine components
- People with a history of Guillain-Barré syndrome (a severe paralytic illness, also called GBS) that occurred after receiving influenza vaccine and who are not at risk for severe illness from influenza.
- After nasal spray vaccination, side effects can occur and include a runny nose, headache, sore throat, and cough, and in children mild fever and muscle aches.
U.S. Centers for Disease Control and Prevention (CDC) recommends this group of people is vaccinated each year:
- Children aged 6 months through 4 years (59 months) and people aged 50 years and older
- Children are aged 6 months through 18 years and receiving long-term aspirin therapy and who therefore might be at risk for experiencing Reye's syndrome after influenza virus infection
- People who have chronic pulmonary (including asthma), cardiovascular (except hypertension), renal, hepatic, neurologic, hematologic, or metabolic disorders (including diabetes mellitus)
- People who are immunosuppressed (including immune suppression caused by medications or by human immunodeficiency virus)
- Women who are or will be pregnant during the influenza season
- People who are American Indians/Alaska Natives
- People who are morbidly obese (body-mass index is 40 or greater)
- People who are residents of nursing homes and other chronic-care facilities
- People who are health-care personnel
- People who are household contacts and caregivers of persons with medical conditions that put them at higher risk for severe complications from influenza
The flu vaccine should avoid:
- People who have ever had a severe allergic reaction to eggs
- People who have ever had a severe allergic reaction to influenza vaccine
- People with a history of Guillain-Barré syndrome (a severe paralytic illness, also called GBS) that occurred after receiving influenza vaccine and who are not at risk for severe illness from influenza should generally not receive vaccine
- People under 65 years of age should not receive the high-dose flu shot.
The effectiveness of the flu vaccine depends on the match between the virus strains used to prepare the vaccine and those viruses in actual circulation, but it is approximately 70%-90% in preventing influenza respiratory illness, 50%-60% in preventing influenza-related hospitalization or pneumonia and 80% effective in preventing influenza-related death, although the effectiveness in preventing influenza respiratory illness can be as low as from 30%-40%.