Vaccines: an overview and update
Vaccines prevent death on a large scale, but only if enough people are immunised. If most of the people within a population are vaccinated against an infection, those who are not will still be protected because spread of the pathogen is blocked. This is known as “herd immunity”. However, this immunity relies on public support of vaccination programmes which, in turn, strongly depends on perception of risk versus benefit. For example, the controversy relating to the safety of the combined measles, mumps and rubella (MMR) vaccine stemming from proposed links with autism has had a detrimental impact on the protection of children from these diseases within the UK. While most of the researchers who initially proposed this link subsequently retracted their speculations, the damage in terms of public confidence was done. A marked reduction in the percentage of the population immunised against MMR (from 92 to 82 per cent) occurred across the UK, with some areas of London reporting an uptake 20 per cent lower than the national average.1 The World Health Organization recommends an immunity level of 95 per cent to prevent disease outbreaks. The reduction in MMR vaccinations has been reflected by increased reports of measles and mumps in the UK and, in response, some health authorities are holding publicity events and training primary care staff to encourage uptake of the combined vaccine.
Basic principles of vaccination
Vaccination is based on two key elements of adaptive (acquired) immunity: specificity and memory. When an infection occurs, the body produces cells capable of fighting it and memory cells specific to the pathogen are generated. Memory cells implement a faster and stronger response on a second encounter with an infectious agent compared with the response on first exposure to the pathogen. Vaccines allow an individual to acquire specific immunity to an infectious agent without having to suffer an initial infection. This is important because the primary response to a natural infection is often too slow to prevent serious symptoms developing.
The aim of vaccine development is to modify a pathogen (or its toxins) so that it is harmless without loss of antigenicity. This is possible because antibodies and memory cells within the immune system recognise particular areas of antigens, known as epitopes, rather than the whole organism or toxin. As long as the body is presented with such epitopes it does not need to encounter the full pathogen for specific immunity to be generated. Generally, the closer the vaccine is to the original infectious agent the better its efficacy. Live attenuated systems tend to be more effective than inactivated ones or cellular extracts (see below for further discussion of vaccine types). However, the enhanced efficacy of live vaccines is accompanied by increased adverse effects.
Risks and benefits of vaccines
What is deemed an acceptable risk of adverse effects should be considered in relation to the risks associated with a disease. Panel 1 (p209) gives examples of the risks of vaccinations compared with their corresponding disease.
It could be argued that some anti-vaccine sentiment stems from the success of immunisation programmes. Developed countries no longer have major incidences of many diseases so people are more likely to question the need for vaccination. In developing countries, the risk versus benefit ratio can be different. For example, in developed countries, access to effective health care significantly limits mortality rates to rotavirus but in countries where access to doctors and standard fluid rehydration treatments are limited these rates soar and vaccination is more important. In 1998, Wyeth Pharmaceuticals introduced a rotavirus vaccine but, less than a year later, this was withdrawn as a result of the associated risk of intussusception which appeared in one in 12,000 children vaccinated. New rotavirus vaccines are now in clinical trials.
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Citation: The Pharmaceutical Journal URI: 10997378