Introduction
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, hemagglutinin, 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.
Influenza Epidemics
The epidemiological behavior of influenza in humans is a result of antigenic drift and antigenic shift. During antigenic drift, new strains of virus evolve by the 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 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.
Transmission of Influenza Virus
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. The 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.
Influenza Vaccines
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.
Who To Give Influenza Vaccine?
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, the annual influenza vaccine is recommended for all children aged 6-59 months.
Contraindications for Influenza Vaccine
Vaccine should still be administered in cases of a mild egg allergy (only rashes) in the child under physician supervision for 30 mins.7
Influenza Vaccine - Side Effects
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.
Inactivated influenza vaccines
- A uniform dosing of 15 mcg (0.5 mL) of inactivated influenza vaccines is recommended for all children older than 6 months.6
- So far, two brands, Influvac TetRa (Abbott) and Fluarix Tetra (Glaxo Smithkline) have received DCGI approval for this uniform dosage recommendation
- Many countries including USA, Finland, Australia, UK, New Zealand and Canada already have accepted the uniform dose regime across all age groups.
1. Isaacs D. Routine Use of Influenza Vaccine. In: A. Finn et al. (eds.), Hot Topics in Infection and Immunity in Children V, Springer. 2009; 95-110.
2. Pickering LK, Orenstein WO. Active Immunization. Principles and Practice of Pediatric Infectious Diseases Revised, 3rd ed. Churchill Livingstone 2009: 48-71.
3. Reichert TA, Sugaya N, Fedson DS, Glezen WP, Simonsen L, Tashiro M. The Japanese experience with vaccinating schoolchildren against influenza. N Engl J Med 2001; 344:889-96.
4. Nguyen-Van-Tam JS. Influenza related hospital admissions in children: evidence about the burden keeps growing but the route to policy change remains uncertain. Arch Dis Child 2006; 91: 5-7.
5. Zangwill KM, Droge J, Mendelman P, et al. Prospective, randomized, placebo-controlled evaluation of the safety and immunogenicity of three lots of intranasal trivalent influenza vaccine among young children. Pediatr Infect Dis J 2001; 20: 740-6.
6. Iapindia.org [Internet] IAP Vaccine Information. ACVIP: 2020-2021 Available from: https://iapindia.org/vaccine-information/
7. Acvip.org (Internet) Influenza Advisory Committee on Vaccines & Immunization Practices. Available from: https://acvip.org/