Vaccine, attenuated

The varicella vaccine is made up of an attenuated Oka strain of varicella zoster virus. This is a live attenuated vaccine. Attenuation was achieved by performing serial passages through human embryonic lung cells, embryonic guinea pig cells, and human diploid cells. 

Live vaccines are normally weakened strains that do not cause diseases in the host, but stiU can stimulate the immune response. A typical example is the poho vaccine. The weakening of microorganisms or attenuation of the vims or bacteria can be accompHshed by passage through different substrates and/or at different temperatures. Modem genetic engineering techniques can also be used to attenuate a vims or bacterium. 

Poliomyelitis. Two vaccines are Hcensed for the control of poliomyelitis in the United States. The Hve, attenuated oral polio vims (OPV) vaccine can be used for the immunization of normal children. The killed or inactivated vaccine is recommended for immunization of adults at increased... 

Measles, Mumps, Rubella. Live, attenuated vaccines are used for simultaneous or separate immunization against measles, mumps, and mbeUa Hi children from around 15 months of age to puberty. Two doses, one at 12—15 months of age and the second at 4—6 or 11—12 years are recommended Hi the United States. 

Composition and Methods of Manufacture. Vaccine is produced from the Oka attenuated strain. Vacciae is produced in human diploid cells such as MRC-5. After growth in the cell substrate, the cells themselves are harvested into the growth medium and sonicated to release the cell-associated vims. Sucrose and buffering salts are generally in the medium to help stabiLize the vims. The vacciae is presented in a free2e-dried vial to be reconstituted with sterile distilled water before injection (27). 

Standardization and Testing. Potency is determined by titrating the amount of Hve vims using a suitable cell substrate. Safety testing is also performed on seed lots to assure proper attenuation and on vaccine to assure absence of unwanted contaminants. 

Rotavirus. Rotavims causes infant diarrhea, a disease which has major socio-economic impact. In developing countries it is the major cause of death in infants worldwide, causing up to 870,000 deaths per year. In the United States, diarrhea is stiU a primary cause of physician visits and hospitalization, although the mortaUty rate is relatively low. Studies have estimated a substantial cost benefit for a vaccination program in the United States (67—69). Two membrane proteins (VP4 and VP7) of the vims have been identified as protective epitopes and most vaccine development programs are based on these two proteins as antigens. Both Hve attenuated vaccines and subunit vaccines are being developed (68). 

Live attenuated vaccines for RSV are also being developed. Most of these vaccine candidates are derived from cold adaptation, by passing the vims at progressingly lower temperatures than human body temperatures. However, other means of mutagenesis have been used to generate vaccine candidates (72). Several clinical trials of these vaccines are also in progress (73,74). 

Parainfluenza. Parainfluenza vimses (PIV) also causes viral pneumonia in infants. It is similar to RSV, therefore similar approaches are being used for developing a vaccine. A five attenuated PIV-3 vaccine has been in clinical trial (74). 

Influenza. Although current influenza vaccine (subunit spHt vaccine) has been in use yearly for the elderly, it is not recommended for the general population or infants. Improvements to increase or prolong the immunogenicity, reduce the side-effects (due to egg production procedure), and provide mass protection are stiU being pursued. One approach is to use a five, attenuated vims though cold adaptation. A vaccine has been used in Russia and demonstrated to be safe and efficacious for infants (82). Clinical trials for a similar vaccine are being carried out in the United States (83). 

Vaccine A suspension of attenuated or killed bacteria or virus, or portions thereof, injected to produce active immunity. 

Although a humoral immune response is the primaiy protection against most viral and some bacterial diseases, protective defense against other pathogens such as HIV, Plasmodium and Mycobacterium tuberculosis requires a cytotoxic response mediated by CD8+ T-cells (CTL response). Since the introduction of the vaccination concept by Jenner almost 200 yeats ago, only few vaccines have been developed that are able to induce a CTL response. These vaccines are usually attenuated live vaccines that are accompanied by certain risks and are not readily available for most pathogens. The immense appeal of DNA vaccines can be attributed to a considerable part to the fact that they are able to induce... 

An example of the use of an attenuated virus is the administration of the measles vaccine to an individual who has not had measles. The m easles (rubeola) vaccine contains the live, attenuated measles virus. The individual receiving the vaccine develops a mild or modified measles infection, which then produces immunity against the rubeola virus. The measles vaccine protects 95% of the recipients for several years or, for some individuals, for life. An example of a killed virus used for immunization is the cholera vaccine. This vaccine protects those who receive the vacdne for about 3 to 6 months. 

Studies on S-layers present on the cell envelopes of a great variety of pathogenic organisms [100] revealed that these crystalhne arrays can represent important virulence factors. Most detailed studies have been performed on the fish pathogenic bacteria Aeromonas salmonicida and Aeromonas hydrophila [102] and the human pathogen Campylobacter fetus uh p. fetus [103] and Bacillus anthracis [104]. For example, whole-cell preparations or partially purified cell products are currently used as attenuated vaccines against various fish pathogens [102,105]. 

The harvest is a very eomplex mixture of bacterial cells, metabolic products and exhausted medium. In the ease of a live attenuated vaccine it is innocuous and all that is necessary is for the baeteria to be separated and resuspended in an appropriate menstmum, possibly for freeze-drying. In a vaccine made Irom a pathogen the harvest may be intensely dangerous and great care is necessary in the following procedures. 

Cell cultures provide infeeted fluids that eontain little debris and can generally be satisfactorily clarified by filtration. Beeause most viral vaccines made fiom cell cultures consist of live attenuated vims, there is no inaetivation stage in their manufacture. There are, however, two important exeeptions inaetivated poliomyelitis vims vaccine is inactivated with dilute formalin or /3-propiolaotone and rabies vaccine is inactivated with /3-propiolactone. The preparation of these inaetivated vaccines also involves a concentration stage, by adsorption and elution of the vims in the case of poliomyelitis vaccine and by ultrafiltration in the case of rabies vaceine. When processing is complete the bulk materials may be stored until needed for blending into final vaccine. Because of the lability of many vimses, however, it is necessary to store most purified materials at temperatures of-70°C. 

Poliomyelitis (live or oral) (Sabin type) Cell cultures infected with attenuated poliovirus of each of the three serotypes 1 Clarification 2 Blending of virus of three serotypes in stabilizing medium Infectivity titration of each of three virus serotypes Test for attenuation by Inoculation of spinal cords of monkeys and comparison of lesions with those produced by a reference vaccine... 

With attenuated viral vaccines the potential hazards are those associated with reversion of the virus during production to a degree of virulence capable of causing disease in vaccinees. To a large extent this possibility is controlled by very careful selection of a stable seed but, especially with live attenuated poliomyelitis vaccine, it is usual to compare the neurovirulence of the vaccine with that of a vaccine known to be safe in field use. The technique involves the intraspinal inoculation of monkeys with a reference vaccine and with the test vaccine and a comparison ofthe neurological lesions and symptoms, if any, that are caused. If the vaccine causes abnormalities in excess of those caused by the reference it fails the test. 

The ideal of any vaccine is to provide life-long protection to the individual against disease. Immunological memory (Chapter 14) depends upon the survival of cloned populations of small B and T lymphocytes (memory cells). These small lymphocytes have a lifespan in the body of ca. 15-20 years. Thus, if the immune system is not boosted, either by natural exposure to the organism or by re-immunization, then immunity gained in childhood will be attenuated or lost completely by the age of 30. Those vaccines which provide only poor protection against disease have proportionately reduced time-spans of effectiveness. Yellow fever vaccination, which is highly effective, must therefore be repeated at 10-year intervals, whilst typhoid vaccines are only effective for 1-3 years. Whether or not immunization in childhood is boosted at adolescence or in adult life depends on the relative risks associated with the infection as a function of age. 

Disadvantages associated with the use of live vaccines are also apparent. Live attenuated vaccines, administered through the natural route of infection, will be replicated in the patient and could be transmitted to others, ff attenuation is lost during this replicative process then irrfectiorrs rrtight result (see poliomyelitis, below). A second, major disadvantage of live vaccines is that the course of their action might be affected by the infeetion and immunological status of the patient. 

Polio is the only disease, at present, for which both hve and killed vaccines compete. Since the introduction of the killed vims (Salk) in 1956 and the live attenuated virus (Sabin) in 1962 there has been a remaikable decline in the incidence of poliomyelitis (Fig. 16.1). The inactivated polio vaccine (TPV) contains formalin-killed poliovirus of all three serotypes. On injection, the vaccine stimulates the production of antibodies of the IgM and IgG class which neutrahze the vims in the second stage of infection. A course of three injections at monthly intervals produces long-lasting immunity to all three poliovirus types. 

Faecal excretion of vaeeine virus will oeeur and may last for up to 6 weeks. Such released virus will spread to elose eontaets and infect/(re)immunize them. Since the introduction of OPV, notifications of paralytic poliomyelitis in the UK have dropped speetacularly. From 1985-95, 19 of the 28 notified cases of paralytic poliomyelitis were associated with vaeeine strains (14 reeipients, 5 contacts). Vaccine-associated poliomyelitis may occur through reversion of the attenuated strains to the virulent wild-... 

Under no circumstances should living cultures of microorganisms, whether they be for vaccine preparation (Chapter 16) or for use in monitoring sterilization processes (Chapter 23), be taken into aseptic areas. As already pointed out, separate premises are needed for the aseptic filling of live or of attenuated vaccines. 

---