Medical Information

• What is Influenza?
• What is Pandemic Influenza?
• Prevention Strategies
• Vaccines
• Antiviral Drugs

Vaccines

Current human influenza vaccines

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.

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.

Avian influenza vaccine

Currently no vaccine is available to protect humans against the H5N1 virus that is being seen in Asia. However, vaccine development efforts are under way, including in Australia where trials of candidate vaccine is currently being tested in humans. Research studies to test a vaccine to protect humans against H5N1 virus began in April 2005. (Researchers are also working on a vaccine against H9N2, another bird flu virus subtype.)

It is hoped that a suitable vaccine can be produced to commence community-wide immunisation against H5N1 virus. While this remains technically difficult, it is still potentially achievable through time, and priorities will be determined to distribute any suitable vaccine as production volumes allow. Even though any novel pandemic strain may be different to H5N1, it is possible that partial immunity may be gained (particularly against the H5 component), and that the pandemic strain might only produce a milder disease.

Pandemic vaccines

It is planned that any new pandemic strain of virus would be assessed as quickly as possible after identification to determine the ability to create vaccine. Experts around the world will work together to select the virus strain that will offer the best protection against that virus. This would take some time, (several weeks at least), and so a vaccine would not be available in the early stages of a pandemic. Manufacturers then use the selected strain to develop a vaccine and it takes several months before a vaccine will be widely available. This is useful in terminating a prolonged pandemic, by targeting the people who have not been infected. This vaccine is obviously no use for those who have had the disease, and so it has a limited role in the setting of a highly infectious, virulent strain that infects a large part of the world’s population quickly.

WHO has recently summarised progress on the development of pandemic vaccines with a series of questions and answers. Click here for more information

Other vaccines

It is important to consider general vaccination in preparation of a pandemic. In particular it will be important for those individuals who have chronic medical disorders, especially lung disease, or weakened immune systems to be well vaccinated with other vaccines. Pneumococcal vaccine is particularly relevant.

Many deaths and severe infections precipitated by influenza are due to secondary infection with bacterial pathogens such as Streptococcus pneumoniae. The pneumococcal vaccine, administered to high-risk groups of the population, can significantly reduce the incidence of this secondary infection and hence reduce the morbidity and mortality associated with influenza. Increasing pneumococcal vaccine coverage in high risk groups will therefore have a role in potentially lessening the impact of an influenza pandemic.

At present, it is appropriate that the recommended higher-risk categories are considered for this vaccine. The vaccine is available through general practitioners and specialist vaccination clinics.

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