Recommendations for UseThe infection rates with influenza are highest in children aged 5-9 years but complications and mortality are greatest in children aged less than 2 years, the elderly and in patients with chronic diseases. The vaccine is recommended for use in the following patient groups
In countries like the USA, influenza vaccine is additionally recommended for all women likely to be pregnant in the influenza season, health care workers, all children aged 6 months to 18 years and all contacts of children with high risk conditions/healthy children less than 5 years of age. However due to lack of accurate data on the burden of disease in India and competing health priorities, the IAPCOI does not recommend the use of the vaccine in these groups. The influenza vaccines are given before the peak influenza season. However unlike temperate countries where the peak influenza season is in winters, in tropical countries like India the illness occurs all year round. The vaccine should therefore be given as soon as the new vaccine are released in the market or at the time of presentation to the health care provider. When used for the first time in children 6 months to less than 9 years of age the vaccines (TIV/LAIV) are given as 2 doses 1 month apart; only one dose is sufficient 9 years and above. Revaccination is recommended with a single annual dose (irrespective of age) and even if the vaccine antigenic composition does not change. |
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The influenza virus is an orthomyxovirus and is capable of causing disease in humans, birds and animals. In the industrialized world morbidity, absenteeism, economic burden and mortality due to influenza is well quantified and significant. Unfortunately there is scanty data on the burden of influenza in India. The emergence of avian influenza and a consequent human pandemic is a matter of great concern in the current scenario.
Influenza A and B cause significant human disease. Influenza A is further classified into several subtypes based on hemagglutinin and neuraminidase. The currently circulating influenza virus strains are Type A H1N1 and H3N2 and influenza B virus. Influenza virus is characterized by frequent mutations — antigenic drifts (minor antigenic change, both A and B) and antigenic shifts (major antigenic change, only A).
Vaccines elicit a relatively strain-specific humoral response, have reduced efficacy against antigenically drifted viruses and are ineffective against unrelated strains. It is of the utmost importance, therefore, that the vaccine should incorporate the current strain prevalent during that time. The influenza vaccine is therefore unique as the precise composition has to be changed periodically in anticipation of the prevalent influenza strain expected to circulate in a given year. The WHO reviews data obtained from its chain of reference laboratories from world over and recommends vaccine composition on a biannual basis; in September/October for the Southern Hemisphere and in February/March for the Northern Hemisphere. This gives the vaccine manufacturer’s 4-6 months to manufacture the vaccine in time for the flu season for the respective hemisphere.
Inactivated Influenza Vaccines (TIV)
The inactivated influenza vaccines are produced from virus grown in embryonated hen’s eggs, and are of three types: whole virus, split-product, subunit surface-antigen formulations. Whole-virus vaccines are associated with increased adverse reactions, especially in children, and are currently not used. Most influenza vaccines are split-product vaccines, produced from detergent treated, highly purified influenza virus, or surface-antigen vaccines containing purified hemagglutinin and neuraminidase. Vaccines are trivalent (TIV); containing 15 ìg each of the WHO recommended two influenza A strains (H1N1 and H3N2) and one influenza B strain.
They should be stored at 2 to 8°C and never be frozen. These vaccines are licensed for use in children aged 6 months and older. Provided there is good antigenic match, the vaccines provide 70-90% efficacy against laboratory confirmed disease in healthy adults, 25-40% reduction in hospitalizations in elderly non institutionalized patients and 40-75% reduction in mortality in influenza seasons. The efficacy of vaccine is lower and around 50% in children below 5 years and in individuals with high risk conditions.
The vaccine is administered intramuscularly, the dose being 0.25 ml in children below three years and 0.5 ml thereafter. Side effects are mild and include fever, rash and injection site reactions. The link between currently available influenza vaccines and Gullain Barre Syndrome (GBS) is equivocal and if present is less than 1 case per million people vaccinated. However the vaccine should preferably be avoided in patients with history of GBS and who are not at high risk of severe influenza related complications. The vaccine should be administered with caution in patients with history of severe egg allergy only if expected benefits outweigh risks.
Live Attenuated Influenza Vaccines (LAIV)
The live attenuated influenza vaccine (LAIV) FluMist® has been recently approved by US FDA for use in healthy non pregnant individuals 2-49 years of age. The vaccine is composed of the live attenuated reassortants of the three WHO recommended strains and is administered as a nasal spray. It is stored at 2 to 8° C.
This vaccine has superior efficacy as compared to the inactivated vaccines in young healthy children especially below 5 years of age. However unlike the inactivated vaccine it has not been licensed in individuals with any chronic disease, pregnant women and children aged less than 2 years due to lack of efficacy and safety studies. This vaccine should also be avoided in children less than 5 years of age with history of reactive airway disease, those with history of hypersensitivity to eggs/vaccine components and those with history of GBS in the past.
Side effects include mild fever, runny nose and sore throat. This vaccine is currently not available in India.
A pre-pandemic vaccine is produced in advance of a pandemic.
Such a vaccine is based on the currently circulating avian H5N1
influenza virus likely to cause a pandemic and has the ability
to raise immune protection against potential drift H5N1 strains.
Pre-pandemic vaccines therefore play a critical role in
pandemic preparedness planning, with experts citing that
immunization with such stockpiled vaccines in advance or at
the onset of a pandemic is the most effective strategy for
protecting entire populations. Recently European Commission
has granted license for H5N1 adjuvanted pre-pandemic vaccine
Prepandrix™ for all 27 EU member states.