Vaccination

Chapter 18 Vaccination




Summary




Vaccination applies immunological principles to human health. Adaptive immunity and the ability of lymphocytes to develop memory for a pathogen’s antigens underlie vaccination. Active immunization is known as vaccination.


A wide range of antigen preparations are in use as vaccines, from whole organisms to simple peptides and polysaccharides. Living and non-living vaccines have important differences, living vaccines being generally more effective.


Adjuvants enhance antibody production, and are usually required with non-living vaccines. They concentrate antigen at appropriate sites or induce cytokines.


Most vaccines are still given by injection, but other routes are being investigated.


Vaccine efficacy needs to be reviewed from time-to-time.


Vaccine safety is an overriding consideration. When immunization frequencies fall, the population as a whole is not protected. Fears over the safety of the MMR vaccine resulted in measles epidemics and increases in incidence of rubella.


Vaccines in general use have variable success rates. Some vaccines are reserved for special groups only and vaccines for parasites and some other infections are only experimental.


Passive immunization can be life-saving. The direct administration of antibodies still has a role to play in certain circumstances, for example when tetanus toxin is already in the circulation.


Non-specific immunotherapy can boost immune activity. Non-specific immunization, for example by cytokines, may be of use in selected conditions.


Immunization against a variety of non-infectious conditions is being investigated.


Recombinant DNA technology will be the basis for the next generation of vaccines. Most future vaccines will be recombinant subunit vaccines incorporated into viral or bacterial vectors. This should provide enhanced efficacy and safety.



Vaccination



Vaccines apply immunological principles to human health


Vaccination is the best known and most successful application of immunological principles to human health. It exploits the property of immunological memory to provide long lasting protection against infectious disease.


The first vaccine was named after Vaccinia, the cowpox virus. Jenner pioneered its use 200 years ago. It was the first deliberate scientific attempt to prevent an infectious disease and was based on the notion that infection with a mild disease (cowpox) might protect against infection with a similar but much more serious one (smallpox), although it was done in complete ignorance of viruses (or indeed any kind of microbe) and immunology.


It was not until the work of Pasteur 100 years later that the general principle governing vaccination emerged – altered preparations of microbes could be used to generate enhanced immunity against the fully virulent organism. Thus Pasteur’s dried rabies-infected rabbit spinal cords and heated anthrax bacilli were the true forerunners of today’s vaccines, whereas, until very recently, Jenner’s animal-derived (i.e. ‘heterologous’) vaccinia virus had no real successors.


Even Pasteur did not have a proper understanding of immunological memory or the functions of the lymphocyte, which had to wait another half century.


Finally, with Burnet’s clonal selection theory (1957) and the discovery of T and B lymphocytes (1965), the key mechanism became clear.


In any immune response, the antigen(s) induces clonal expansion in specific T and/or B cells, leaving behind a population of memory cells. These enable the next encounter with the same antigen(s) to induce a secondary response, which is more rapid and effective than the normal primary response.


While for many infections the primary response may be too slow to prevent serious disease, if the individual has been exposed to antigens from the organism in a vaccine before encountering the pathogenic organism, the expanded population of memory cells and raised levels of specific antibody are able to protect against disease. The principles of vaccination can be summarized as:





Vaccines can protect populations as well as individuals


Vaccines protect individuals against disease, and if there are sufficient immune individuals in a population, transmission of the infection is prevented. This is known as herd immunity.


The proportion of the population that needs to be immune to prevent epidemics occurring depends on the nature of the infection:



Effective vaccines must be safe to administer, induce the correct type of immunity, and be affordable by the population at which they are aimed. Over the middle of the 20th century for many of the world’s major infectious diseases, this was achieved with brilliant success, culminating in the official eradication of smallpox in 1980. Beyond this era progress was much slower and fears over vaccine safety made development more lengthy and costly. However, the advent of recombinant DNA technology has led to a number of significant advances in the first decade of the 21st century and a number of new, safe and effective vaccines have come onto the market during this period. Despite these successes for many diseases development of an effective vaccine has remained elusive, in particular, parasitic diseases and HIV.


Nevertheless, with the availability of new technologies and a greater understanding of the immunological principles that underlie effective vaccines, the future for new vaccine development looks brighter than it has for some years.



Antigen preparations used in vaccines


A wide variety of preparations are used as vaccines (Fig. 18.1). In general, the more antigens of the microbe retained in the vaccine, the better, and living organisms tend to be more effective than killed organisms. Exceptions to this rule are:





Live vaccines can be natural or attenuated organisms




The new rotavirus vaccines should prevent many infants from dying in developing countries


imageInfantile diarrhea caused by rotavirus (a double stranded RNA virus of the Reoviridae family) infection accounts for approximately 600 000 deaths per annum worldwide, with the majority of these occurring in developing countries. An effective vaccine would therefore be of great value and save many lives.


In 1998 a vaccine based on live rotavirus was tested on large numbers of infants in the USA. Although the vaccine was found to be effective, approximately 1 in 2500 vaccinated infants developed intussusception (a potentially fatal bowel condition) and it was withdrawn by the manufacturer.


Greater knowledge of the viral molecular biology and techniques manipulating the virus in vitro, have now led to the development of two new and highly effective vaccines against rotavirus infection. These vaccines are marketed under the names Rotarix™ and RotaTeq™. The former is a live attenuated virus produced by repeated passage in animal cell lines in the laboratory (see below). The latter is a complex of 5 different hybrid viruses providing immunity to the 5 most prevalent viral strains. These viruses are based on the bovine rotavirus which is naturally attenuated in human hosts. Into this backbone have been incorporated the human rotavirus viral capsid proteins VP4 or VP7, which are known to be the targets of natural immunity in human rotavirus infection (Fig. 18.w1).



The vaccine has proven to be safe and well tolerated in testing in Europe and the USA and provides high levels of protection against rotavirus gastroenteritis. It is hoped that future experience will prove it to be equally effective in developing countries.



Attenuated live vaccines have been highly successful


Historically, the preferred strategy for vaccine development has been to attenuate a human pathogen, with the aim of diminishing its virulence while retaining the desired antigens.


This was first done successfully by Calmette and Guérin with a bovine strain (Mycobacterium bovis) of Mycobacterium tuberculosis, which during 13 years (1908–1921) of culture in vitro changed to the much less virulent form now known as BCG (bacille Calmette–Guérin), which has at least some protective effect against tuberculosis.


The real successes were with viruses, starting with the 17D strain of yellow fever virus obtained by passage in mice and chicken embryos (1937), and followed by a roughly similar approach with polio, measles, mumps, and rubella (Fig. 18.2)




Just how successful the vaccines for polio, measles, mumps, and rubella are is shown by the decline in these four diseases between 1950 and 1980 (Fig. 18.3).




Attenuated microorganisms are less able to cause disease in their natural host


Attenuation ‘changes’ microorganisms to make them less able to grow and cause disease in their natural host. In early attenuated organisms, ‘changed’ meant a purely random set of mutations induced by adverse conditions of growth. Vaccine candidates were selected by constantly monitoring for retention of antigenicity and loss of virulence – a tedious process.


When viral gene sequencing became possible it emerged that the results of attenuation were widely divergent. An example is the divergence between the three types of live (Sabin) polio vaccine:



Those genes not essential for replication of the virus are mostly concerned with evasion of host responses and virulence, that is the ability to replicate efficiently and disseminate widely within the body, with pathological consequences. Many pathogenic viruses contain virulence genes that mimic or interfere with cytokine and chemokine function. Some of these have sequence homology to their mammalian counterparts and others do not.




Killed vaccines are intact but non-living organisms


Killed vaccines are the successors of Pasteur’s killed vaccines mentioned above:



Figure 18.4 lists the main killed vaccines in use today. These are gradually being replaced by attenuated or subunit vaccines. However, in the case of polio, some countries are reverting to the use of killed vaccine which is safer than the attenuated vaccine, even though it is less effective. This choice only becomes relevant when the risk of contracting the disease is low in comparison with the risk of developing adverse reactions to the vaccine.





Subunit vaccines and carriers


Aside from the toxin-based vaccines, which are subunits of their respective microorganisms, a number of other vaccines are in use which employ antigens either purified from microorganisms or produced by recombinant DNA technology (Fig. 18.6). For example, a recombinant Hepatitis B surface antigen synthesized in baker’s yeast, has been in use since 1986.



Acellular pertussis vaccine consisting of a small number of proteins purified from the bacterium has been available for some years now, and has been shown to be effective, safer and less toxic than the whole killed-organism vaccine. It is usually administered as part of a DTaP (Diphtheria, Tetanus, Pertussis) combination vaccine routinely given to infants.



Conjugate vaccines are effective at inducing antibodies to carbohydrate antigens


Although protein antigens such as hepatitis B surface antigen are immunogenic when given with alum adjuvant (see below), for many types of bacteria, virulence is determined by the bacterial capsular polysaccharide, prime examples being Neisseria meningitidis, Streptococcus pneumoniae, and Haemophilus influenzae B. Such carbohydrate antigens, though they can be isolated and have been used for vaccination, are poorly immunogenic, particularly in infants under 2 years, and often do not induce IgG responses or long-lasting protection. Attempts to boost immunity by repeat administration of these vaccines can actually compromise immunity by depleting the pool of antibody-producing B cells.



A major advance in the efficacy of subunit vaccines has been obtained by conjugating the purified polysaccharides to carrier proteins such as tetanus or diphtheria toxoid. These protein carriers, which can now be produced in highly purified form by recombinant DNA techniques, are presumed to recruit TH cells and the conjugates induce IgG antibody responses and more effective long lasting protection.


Starting with Haemophilus influenzae (Hib) in the early 1990s, conjugate vaccines for Neisseria meningitis strains A, C, Y and W-135 are also now in widespread usage. In the UK up until 1992 when the vaccine was introduced, Hib was the major cause of infantile meningitis leading to many hundreds of cases per year. The introduction of the vaccine led to a very rapid decline making Hib meningitis now a very rare occurrence (Fig. 18.7).image



Jun 18, 2016 | Posted by in IMMUNOLOGY | Comments Off on Vaccination

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