Prevention of Influenza The best approach at the present time lies in prophylactic immunization. It is essential that the vaccine must contain the H and N components of the prevalent strain or strains. The WHO makes recommendations every year as to what strains should be included in the vaccine. To be effective, the vaccine must be administered at least two weeks before the onset of an epidemic, or preferably two to three months before influenza is expected. Since epidemics of influenza are unpredictable, the hope of preventing influenza epidemics by prophylactic mass vaccination is remote.
Since influenza vaccines will not control epidemics, they are recommended only in certain select population groups – e.g. in industry, to reduce absenteeism and in public services, to prevent disruption of critical public services such as the police, fire protection, transport and medical care. Moreover, certain groups e.g. the elderly and individuals in any age group who have a known underlying chronic or debilitating disease are selectively immunized because of the high risk of severe complications including death.
Treatment of Influenza Most influenza vaccination programs make use of inactivated vaccines. The recommended vaccine strains for vaccine production are grown in the allantoic cavity of developing chick embryos, harvested, purified, killed by formalin or beta – propiolactone, and standardized according to the hemaglutinin content.
The vaccine is conventionally formulated in aqueous or saline suspension. One dose of the vaccine contains approximately 15 micrograms of HA. The vaccine is administered by the subcutaneous route. A single inoculation (0.5ml) is usually given. However, in persons with no previous immunological experience (unprimed individuals) two doses of the vaccine, separated by an interval of three to four weeks are considered necessary to induce satisfactory antibody levels. After vaccination, there is an increase in serum antibodies in about one week, which reach a maximum in about two weeks. The protective value of the vaccine varies between 70 to 90 per cent and immunity lasts for only three to six months. Re–vaccination on an annual basis is recommended. The killed vaccine can produce fever, local inflammation at the site of injection, and very rarely Guillain–Barre syndrome (an ascending paralysis). Since the vaccine strains are grown in eggs, persons allergic to eggs may develop the symptoms and signs of hypersensitivity.
Live Attenuated Vaccines Live attenuated vaccines based on temperature–sensitive (ts) mutants have been extensively used in the USSR. They may be administered as “Nose drops” into the respiratory tract. They stimulate local as well as systemic immunity. The frequent antigenic mutations of the influenza virus present difficulties in the production of effective vaccines, particularly live vaccines.
Newer Vaccines Split–virus Vaccine It is also known as the sub–virion vaccine. It is a highly purified vaccine, producing fewer side effects than the “Whole virus” vaccine. Due to its lower antigenicity, it requires several injections instead of a single one. It is recommended for children.
Neuraminidase–specific Vaccine It is a sub–unit vaccine containing only the N antigen, which induces antibodies only to the neuraminidase antigen of the prevailing influenza virus. The antibody to neuraminidase reduces both the amount of virus replicating in the respiratory tract and the ability to transmit virus to contacts. It reduces clinical symptoms in the infected person quite significantly, but permits sub–clinical infection that may give rise to lasting immunity.
Recombinant Vaccine By recombinant techniques, the desirable antigenic properties of a virulent strain can be transferred to another strain known to be of low virulence. Efforts to improve the influenza vaccine have been continuing.