Influenza Vaccine
Ira Shah
Influenza Virus
Influenza is a contagious illness that causes various respiratory syndromes, including otitis media, croup, bronchiolitis, bronchitis, asthma and pneumonia. About 20% of children and 5% of adults worldwide develop symptomatic influenza A or B each year. Children have very high rates of hospitalization for influenza. World Health Organization (WHO) estimates that in developed countries alone 3 to 5 million severe cases occur due to influenza resulting in 250,000-500,000 deaths every year.

Influenza viruses are RNA viruses. There are three types of influenza viruses: A, B, and C, but only types A and B cause widespread outbreaks. Influenza A viruses are classified into subtypes based on antigenic differences between their two surface glycoproteins, haemagglutinin and neuraminidase. There are 15 described haemagglutinin subtypes (H1-H15) and nine neuraminidase subtypes (N1-N9). Only one subtype of haemagglutinin and one of neuraminidase are recognized for influenza B viruses.

The epidemiological behaviour of influenza in humans is a result of antigenic drift and antigenic shift. During antigenic drift, new strains of virus evolve by accumulation of point mutations in the surface glycoproteins. The new strains are antigenic variants but are related to those circulating during preceding epidemics. This feature enables the virus to evade immune recognition, leading to repeated outbreaks during interpandemic years. Antigenic shift occurs with the emergence of a 'new', potentially pandemic, influenza A virus that possesses a novel haemagglutinin alone or with a novel neuraminidase. The new virus is antigenically distinct from earlier human viruses. Usually the haemagglutinin, are introduced into people by direct transmission of an avian virus from birds 'and' there are mutations which make the avian virus able to replicate in man and transmissible between humans.

Most influenza infections are spread through virus-laden droplets expelled by coughing or sneezing. The North American community studies provide convincing data that children introduce and spread influenza infection in households and communities. The elderly may be particularly vulnerable to infection introduced by children. Vaccination of Japanese children prevented about 37,000-49,000 deaths of elderly persons per year. When the vaccination of schoolchildren was discontinued, the excess mortality rates in Japan increased. Thus, it appears that vaccinating healthy children against influenza has the potential for reducing the impact of influenza epidemics.

Two types of influenza vaccines are licensed for use. One is a vaccine that is inactivated with formalin, (trivalent inactivated vaccine or TIV or quadrivalent inactivated vaccine or QIV). TIV/QIV is recommended for children 6 through 59 months of age, and for older children and adolescents in high-risk groups and their close contacts. Inactivated influenza vaccines are significantly less immunogenic in children. It is recommended to give two doses of vaccine to children <9 years who have not received a previous dose. For previously unvaccinated children 6 months through 8 years of age, two doses of vaccine, given at 4-week intervals, are recommended. For other groups, only a single dose is necessary.

The second vaccine is a live, nasally administered vaccine (live attenuated influenza vaccine or LAIV) licensed for healthy people 2 years through 49 years of age, including close contacts of high-risk people. Live, attenuated, intranasal vaccines may be more immunogenic in infants than inactivated vaccines.

Routine childhood immunization against influenza is currently recommended in most countries for high-risk children. These include children with immunodeficiency, asthma, cystic fibrosis, and congenital heart disease. In the USA, annual influenza vaccine is recommended for all children aged 6-59 months.

Inactivated influenza vaccine should not be administered to people known to have anaphylactic hypersensitivity to eggs or to other components of the influenza vaccine. The following populations should not be vaccinated with LAIV: people less than 2 years of age or people > 50 years of age; people with asthma, reactive airways disease, or other chronic disorders of the pulmonary or cardiovascular systems; people with other underlying medical conditions, including metabolic diseases such as diabetes, renal dysfunction, and hemoglobinopathies; people with known or suspected immunodeficiency diseases or who are receiving immunosuppressive therapies; children or adolescents receiving aspirin or other salicylates (because of the association of Reye syndrome with wild-type influenza virus infection); people with a history of GBS; pregnant women; or people with a history of hypersensitivity, including anaphylaxis, to any of the components of LAIV or to eggs.

About 3% to 5% of recipients experience local tenderness or fever after vaccination. The occurrence of Guillain Barre Syndrome (GBS) within 6 weeks of influenza vaccination was observed in adults after the swine influenza vaccine program campaign (1976) but was not observed subsequently until 1990, when a case-control study suggested a slightly elevated risk in persons 18 to 64 years old but not in people > 65 years of age.

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