There is the potential to develop a protective vaccine/ immunization programme for all infectious diseases, although some pathogens are considerably more challenging candidates than others.
OBJECTIVES OF A VACCINE / IMMUNIZATION PROGRAMME
There is the potential to develop a protective vaccine/ immunization
programme for all infectious diseases, although some pathogens are considerably
more challenging candidates than others. Whether or not such vaccines are
developed and deployed is related to the severity and economic impact of the
disease on the community as well as the effects upon the individual. Various
factors governing the likelihood of an immunization programme being adopted are
discussed below.
The severity of the disease in terms of its morbidity and mortality, the
probability of permanent injury to its survivors and the likelihood of
infection must be sufficient to warrant the costly development of a vaccine and
its subsequent use. Thus, although influenza vaccines are constantly reviewed
and stocks maintained, the control of influenza epidemics through vaccination
is not recommended. Rather, those groups of individuals, such as elderly
people, who are at special risk from the infection, are protected.
Vaccines to be included within national immunization and vaccination
programmes should be chosen to reflect the infection risks within that country.
Additional immunizations, appropriate for persons travelling abroad, are
intended to protect the at-risk individual, but also to prevent importation of
the disease into an unprotected home community.
Vaccination and immunization programmes seldom confer 100% protection
against the target disease. More commonly the degree of protection is 60-95%.
In such instances, although individuals receiving treatment have a high
probability of becoming immune, virtually all members of a community must be
treated in order to reduce the actual proportion of susceptible individuals to
below the threshold for epidemic spread of the disease. Anti-diphtheria and anti-tetanus
prophylaxes, which utilize toxoids, are among the most efficient immunization
programmes, whereas the performance of BCG is highly variable.
No medical or therapeutic procedure comes without some risk to the
patient, but all possible steps are taken to ensure safety, quality and
efficacy of vaccines and immunological products (see Chapter 24). The risks
associated with immunization procedures are constantly reviewed and balanced
against the risks associated with contracting the disease. In this respect, the
incidence of paralytic poliomyelitis in the USA and UK in the late 1990s was
low, with the majority of cases being related to vaccine use
(vaccine-associated paralytic polio or VAPP). As the worldwide elimination of
poliomyelitis approaches, there is debate as to the value of the live (Sabin)
vaccine outside endemic areas, and the inactivated polio vaccine (IPV) is now
the vaccine of choice in the UK for prophylaxis against paralytic polio.
Public confidence in the safety of vaccines and immunization procedures
is essential if compliance is to match the needs of the community. The
correlation between actual risk and perception of risk is not always reliable,
however. In this respect, public concern and anxiety in the mid-1970s over the
perceived safety of pertussis vaccine led to a reduction in coverage of the
target group from about 80% to 30%. Major epidemics of whooping cough, with
over 100 000 notified cases, followed in the late 1970s and early 1980s. By 1992,
public confidence had returned and coverage had increased to 92%, with a
considerable associated decrease in disease incidence. Similarly, links have
been claimed between the incidence of autism in children and the change in the
UK from single measles and German measles vaccines to the combined measles,
mumps and rubella (MMR) vaccine. Such claims have been proved to be unfounded
beyond reasonable doubt but have nevertheless decreased the uptake of the MMR
vaccine and thereby increased the likelihood and magnitude of measles
epidemics.
Cheap, effective vaccines are an essential component of the global
battle against infectious disease. It was estimated that the 1996 costs of the
USA childhood vaccination programme, directed against polio, diphtheria,
pertussis, tetanus, measles and tuberculosis, was $1 for the vaccines and $14
for the programme costs. The newer vaccines, particularly those that have been
genetically engineered, are considerably more expensive, putting the costs
beyond the budgets of many developing countries.
The ideal of any vaccine is to provide lifelong protection of the
individual against disease. Immunological memory depends on the survival of
cloned populations of B-and T-lymphocytes (memory cells). Although these
lymphocytes can persist in the body for many decades, the duration of
protection varies from one individual to another and depends on the vaccine;
commonly ranging between 10 and 20 years. Thus, if the immune system is not
boosted, either by natural exposure to the organism or by re-immunization,
protective immunity gained in childhood may be lost by the age of 30. Those
vaccines that provide only poor protection against disease may have proportionately
reduced timespans of effectiveness. Equally, vaccines may be less effective and
have a shorter duration when administered to neonates. Yellow fever
vaccination, which is highly effective, must there-fore be repeated at 10-year
intervals, while the typhoid vaccine is only effective for up to 3 years.
Whether or not immunization in childhood is boosted at adolescence or in
adulthood depends on the relative risks associated with the infection as a
function of age and the longevity of immunity conferred by the vaccine.
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