Problems in the control of epidemic influenza by vaccination

Although influenza is a long established and well recognised disease with a known aetiological agent its control by vaccine has proved to be extremely difficult. This is largely the result of the unusual ability of influenza virus to change antigenically thereby avoiding neutralisation by antibody already produced by previous infection. The changes may be of a major nature, with the appearance of a new sub-type to which no one has any immunity; or of a minor nature, during the prevalence of a sub-type, but great enough to allow the virus to overcome waning immunity. Much effort has gone into the preparation of both killed and live attenuated vaccines. The killed vaccines are most widely available and are made from whole virus, disrupted virus or purified viral subunits. Live attenuated vaccines still have some disadvantages and their use is still restricted. A regular policy of immunisation of the general population has rarely been adopted for the prevention of spread of the disease, and the use of vaccine is usually directed towards protecting those people who are likely to become seriously ill or die. Influenza is particularly dangerous where there is pre-existing respiratory or cardiac disease, but the main mortality from influenza is among the elderly. Besides these target groups, vaccine can be effectively used for modifying the attack rates in close communities such as schools and military groups. In the event of pandemic influenza vaccines need to be applied to other target groups besides those above mentioned with the aim of preventing disruption of the social organisation when large numbers of the working population are ill at the same time.