Vaccination

Many countries are currently gearing-up for distribution of Pandemic H1N1 2009 vaccine. It has been produced quickly, and should be ready for distribution through October and Novemeber in several countries. In Australia, there is enough vaccine to immunise the entire population. Processes have commenced for production of veaccine for developing countries.

Current influenza vaccines contain either inactivated influenza virus antigens or living, attenuated virus. Although there is some progress to registration of vaccines prepared from viruses grown in cell culture, the great majority are prepared from influenza cultivated in embryonated chicken eggs. Currently only inactivated, egg-grown, split-product, sub-unit and an adjuvanted sub-unit vaccine are licensed for use in Australia. The new pandemic vaccine is like thiese usual seasonal vaccines.

Seasonal or annual influenza vaccine formulation follows recommendations made by the World Health Organization and local regulatory bodies, based on a “best guess” of what the likely coming strains of influenza will be. There is a slightly different vaccine for the northern and southern hemispheres, and they are released into the market in the corresponding autumn. The northern and southern hemisphere vaccines are usually very similar, usually sharing at least two subtypes. In recent years, vaccines for children and adults have contained 15 micrograms each of the haemagglutinin antigens of three viruses, representing the two circulating subtypes of influenza A plus influenza B. A reduced antigen dose (half the adult dose) is recommended for children aged two - six years and a quarter of the adult dose for children six months to two years of age. Vaccination is not recommended for children under six months of age.

In immunologically 'primed' populations (ie individuals who have experienced antigenically related viruses or virus antigens of the same type and, in the case of influenza A, subtype) a single vaccine dose is required to provide optimal immunity. This is generally achieved within 2 weeks post-vaccination. In unprimed populations (young children, or in the case of a new pandemic virus, all of the population) two vaccine doses, spaced by an interval of 4 weeks, are required to achieve optimal immunity.

The composition of the influenza vaccine is determined annually by the AIVC.