Virus vaccines: principles and prospects.

*Update/Le point Virus vaccines: principles and prospects* J.L. MeInick1 The present status of vaccination for controlling viral diseases is reviewed, and the needs and directions for future investigations are discussed. A survey of viral vaccines now in use has shown that knowledge about the viral agents and about the hosts' responses to infection was essential for their development. The steps needed to demonstrate the efficacy and safety of a viral vaccine are summarized; the final require- ment for a successful vaccine is that it be administered in proper dosage and potency to the target populations. After general remarks on the proper use of current vaccines there follows an overview of various developments in creating new vaccines, along with the predicted time-frames for their coming into general use. Topics considered include vaccines to be administered locally at the portal of entry, subunit vaccines, viruses attenuated by genetic manipulation, use of viral vectors, vaccines developed by means of recombinant DNA, synthetic peptides, and anti-idiotype vaccines, as well as new vaccines being devel- oped by more conventional methods. Vaccine development General remarks This review describes the present situation in con- trolling viral diseases by immunization, and indicates the needs and directions for future investigations. Not all such diseases are equally amenable to pre- vention by immunization, even if sometimes our enthusiasm for a vaccine may overtake present knowledge about the pathogenesis of the disease. Since potential new vaccines will also be discussed, it is useful to consider the time-frame and steps in the development of various successful vaccines. Table 1 lists the dates when the latter were developed and introduced into public health practice. Table 2 presents the following information on vaccines recommended for general use and for use only under special conditions of exposure: (1) the source of each vaccine; (2) whether composed of live attenuated or killed virus, or a subunit of the virus; and (3) the route of administration. Of prime importance is specific identification of the viral pathogen, its nature and its classification. * This article is based on a lecture presented at the Czecho- slovak Academy of Sciences Symposium on Prevention and Treatment of Viral Infections, Bechyne Castle, Bechynb, Czecho- slovakia, 21 June 1988. A French translation will appear in a later issue of the Bulletin. 1 WHO Collaborating Centre for Virus Reference and Research, Department of Virology and Epidemiology, Baylor College of Medicine, Houston, TX 77030, USA. The stability or lability of the viral antigen is critical, as is the presence or absence of multiple serotypes. Thus a single type is sufficient for an effective measles vaccine, but three types are required in poliovaccine. Knowledge must be at hand regarding the cultivation of the agent. It is also essential that effective diagnostic reagents be available, permitting accurate recognition of the targeted disease and its differentiation from other illnesses. Other vital points of information include whether the pathogen is exclusively a human virus or one that infects other species, and whether there are related viruses that infect animal species other than man. Also needed is an understanding of the determinants of infectivity, virulence, antigenicity, and immunogenicity of the virus. Table 1: Chronology of virus vaccines Year Type of vaccine 1721 Variolation with live virus 1798 Smallpox attenuated 1885 Rabies attenuated and inactivated 1936 Yellow fever attenuated 194Os Influenza inactivated (and later subviral) 1955 Poliomyelitis inactivated 1960 Poliomyelitis attenuated 1960s Measles (inactivated) attenuated Mumps attenuated Rubella attenuated 1970s Japanese encephalitis inactivated 1980s Japanese encephalitis attenuated Varicella attenuated 1982 Hepatitis B subviral particles 1986 Hepatitis B recombinant Bulletin of the World Health Organization, 67 (2): 105-112 (1989) © World Health Organization 1989 05 J.L. Meinick Table 2: Principal vaccines used In prevention of virus diseases In man Condition Route of Disease Source of vaccine of virus administration Immunization recommended for everybody: Poliomyelitis Tissue culture (human diploid cell line, monkey kidney) Live attenuated Oral Killed Subcutaneous Measles Tissue culture (chick embryo) Live attenuated Subcutaneous Mumps Tissue culture (chick embryo) Live attenuated Subcutaneous Rubella Tissue culture (duck embryo, rabbit, or human diploid) Live attenuated Subcutaneous ImmunIzaton recommended only under certain conditions (pidemics, exposure, travel, mlitary): Smallpox" Lymph from calf or sheep Live vaccinia Intradermal Yellow fever Tissue cultures and eggs Live attenuated Subcutaneous Viral hepatitis B Purified HBsAg from "healthy" carriers Killed, subunit Subcutaneous Recombinant HBsAg from yeast Subunit Subcutaneous Influenza Highly purified or subviral forms (chick embryo) Killed Subcutaneous Rabies Human diploid cell cultures Killed Subcutaneous Adenoviral infectionsb Human diploid cell cultures Live attenuated Oral Japanese B encephalitis Tissue culture (hamster kidney) Killed Subcutaneous Varicella Human diploid cell cultures Live attenuated Subcutaneous 8 Since the smallpox virus seems to have been totally eradicated from the world, vaccination is no longer recommended. However, stocks of vaccine are held for use if cases were to reappear. b Adenovirus vaccine, licensed in the USA, is recommended only for military populations, in which epidemic respiratory disease caused by adenovirus is a frequent occurrence; types 4 and 7 are included in the vaccine. Underlying the possibility of developing a vaccine is our ability to grow the virus in large quan- tities. A live, attenuated virus for vaccine should have clearly been proved to lack virulence, while retaining the ability to establish infection and induce humoral and cellular immunity. For an inactivated vaccine to be successful, the capacity of surviving antigens to immunize should have been demon- strated. Concentration and purification of the killed- virus product is usually required, whether it is the whole virus or a subunit. After these criteria have been fulfilled in laboratory investigations, pilot lots must be prepared and have consistently passed the safety tests in cultures, and in animals if an appropri- ate host exists. Efficacy must be demonstrated in animals both by antibody development and by resistance to infection and disease upon challenge with wild virus. Preliminary dosage schedules should also be established in animals. When the safety and immunogenicity levels are satisfactory and sufficient quantities of pilot lots have been prepared, initial trials in human beings are required. These are conducted in small, carefully controlled groups of volunteers, enlisted from sectors of the population chosen to yield maximum informa- tion while entailing minimum risk. For example, childhood vaccines are tried first in persons who have recently recovered from the disease, to deter- mine if the vaccine acts as an immunogenic booster and to be assured that there is no immunopatho- logical response. Next the vaccine can be tested on very small numbers of seropositive adults, followed by seronegative adults, and only then given to small numbers of children. Before a trial is initiated, it must be carefully planned, with a clear idea of spe- cific questions it is supposed to answer and how the answers can best be obtained. To permit adequate evaluation of the vaccine, there must be accurate and precise record-keeping-including essential data on the vaccinees themselves as well as on the route of administration, adverse reactions, antibody re- sponses, and resistance after natural exposure to wild virus. If the early trials are successful, mass production of the vaccine can be undertaken-with great care to test, at every step, the transfer from laboratory and pilot-scale quantities and procedures to those of mass production. Consecutive lots must pass the safety and potency tests. Field trials on much larger groups of the targeted populations can then be planned and executed-again with care in the selec- tion of populations, using sufficient numbers of persons to permit full statistical analysis and main- taining complete records of all data that will be required. Evaluation of the field trials should include analysis of the results in terms of vaccine coverage, adverse effects (if any), and the epidemiological effect on the incidence of the target disease. Assessment of the benefit/risk ratio is also appropriate at this point. Information gained from the trials should cover not 106 Virus vaccines: principles and prospects only the safety and efficacy of the vaccine but also its acceptability by the target populations to whom it is offered. Acceptance both by medical professionals and by the public at large is an essential feature of any successful vaccine; no vaccine can be effective until it gets into the populations that need its protec- tion. The development and evaluation of vaccines against some viral diseases are made difficult by the nature of the disease itself. Clinical features present- ing special problems in evaluation include a long incubation period, frequent inapparent infections, and difficulty of specific diagnosis of the target disease. When epidemics are unpredictable, the evaluation of the vaccine in the face of natural chal- lenge may be difficult. Evaluation will also present problems, when the disease is endemic but with low or sporadic incidence, or when the first infection occurs early in infancy. Sometimes knowledge is lacking about the pathogenesis of the disease, and its immunology is not adequately understood or the first infection with the agent may not prevent subsequent infections. Alternatively, other means of defence against the disease may be at hand which lessen the urgency of vaccine development. Antiviral drugs are becoming available for some diseases. Protection may be achieved by controlling the virus vector (e.g., in arthropod-borne encephalitides) or by improving wastewater disposal (in viral hepatitis A, viral gas- troenteritis and other enteric viral diseases). The fact that a viral pathogen cannot be culti- vated or will grow only in unacceptable systems- such as cancer cell lines or neural tissues that may themselves be dangerous to recipients-has held up vaccine development in some instances. Or there may be so many serotypes, as with the rhinoviruses, that vaccine development is impracticable. For some viruses, the essential immunizing antigen may not yet be identified, or the virus undergoes wide and frequent antigenic variation. Because of the impor- tance of animal models, the absence of a suitable experimental host or extreme scarcity of the only animal host (e.g., chimpanzee) may also make the development of vaccines difficult. In the past, particular problems, have been pre- sented by the vaccine itself, such as thermolability, genetic reversion of live vaccine viruses, or a need for multiple doses to maintain immunity. Special car- riers or adjuvants may be needed which in them- selves complicate the vaccine preparation and testing. Recombinant vaccines prepared by means of a vector virus, such as vaccinia, or by other methods of gene splicing, are under development. The first recombinant vaccine, hepatitis B vaccine made in yeast, is proving to be acceptable both by the public and by health professionals, perhaps even more than the earlier vaccine made from HBsAg-containing plasma from human carriers of the hepatitis B virus. A second recombinant hepatitis B vaccine, produced in a mammalian continuous cell line, is undergoing field trials, which so far have shown early and strong antibody responses. It is important to consider the target popu- lation. Adults as well as children are susceptible to many viruses, and education of the general public is needed to achieve better protection of the population by wider acceptance and use of vaccines. A vaccine intended to protect infants may require immuniza- tion of the mother or immunization of the infant at birth. In those developing countries where polio- myelitis has not yet been satisfactorily controlled, oral poliovaccine is now recommended to be given at birth, even though it is recognized that only 50% of the infants will be immunized at this early period. However, administration of the vaccine will imme- diately reduce by one-half the overall susceptibility of this vulnerable population. The vaccine manufacturer is often faced with complex technology entailing high costs for both development and production of the vaccine. With some vaccines, the market may scarcely cover the cost of development. This is particularly true if the potential market is relatively small, or the target population is unable to pay the purchase costs. With some expensive vaccines, such as the hepatitis B vac- cines, the countries that need them the most can afford them the least. One of the problems hampering the improve- ment of current vaccines and the development of new ones is that of legal liability. Obviously com- pensation should be awarded to those who, in the course of a vaccine programme that greatly benefits the general population, are themselves harmed by a rare adverse reaction to the vaccine. In effect, the vaccine-induced problems of these few persons have to be weighed against the vaccine's benefits to society as a whole. A number of manufacturers, however, have abandoned the vaccine field because of the high liability costs. Current vaccines General principles Immunity to virus infection is based on the develop- ment of an immune response to specific antigens located on the surface of virus particles or virus- infected cells. For enveloped viruses, the important antigens are the surface glycoproteins. Although infected animals may develop antibodies against virion core proteins or nonstructural proteins involved in virus replication, such antibodies are 107 J.L. Melnick believed to play little or no role in the development of resistance to infection. Vaccines are available for the prevention of several important human diseases. Currently licensed vaccines are summarized in Table 2. Certain general principles apply to virus vaccines for use in the pre- vention of human disease. The pathogenesis of a particular viral infection influences the objectives of immunoprophylaxis. Mucosal immunity (local IgA) is important in resist- ance to infection by viruses that replicate exclusively in mucosal membranes (e.g., rhinoviruses, influenza viruses, rotaviruses). Viruses that have a necessary viraemic mode of spread (polioviruses, hepatitis and measles viruses) are controllable by serum anti- bodies. Cell-mediated immunity also is involved in protection against systemic infections (measles, herpes). Neither vaccination nor recovery from natural infection always results in total protection against a later infection with the same virus. This situation holds true for diseases for which successful control measures are available, including poliomyelitis, smallpox, influenza, rubella, measles, mumps, and adenovirus infections. Control can be achieved by limiting the multiplication of wild virulent virus upon subsequent exposure and preventing its spread to the target organs where the pathological damage is done (e.g., preventing poliomyelitis and measles viruses from reaching the brain and spinal cord, and rubella virus from contact with early embryos). Recently, Marek's disease, a widespread lymphoproliferative tumour caused by a herpesvirus of domestic chickens, has been brought under control by an attenuated virus vaccine. The vaccine results in a lifelong active infection of the chicken and does not prevent superinfection of the vacci- nated animal with the virulent virus, but it does prevent the appearance of the tumour. This is the first practical cancer vaccine that has been devel- oped. A second cancer vaccine may be at hand; hepatitis B vaccine, by preventing hepatitis B infec- tion, should also protect against the development of primary hepatocellular carcinoma, but only time will confirm this, after the vaccine has been used in areas of the world where both hepatitis B and liver cancer are prevalent. Killed virus vaccines Killed virus vaccines are made by purifying virus harvests to a certain extent and then inactivating viral infectivity in a way that does minimum damage to the viral structural proteins. Mild formalin treat- ment is most frequently used. These vaccines, pre- pared from whole virions, generally stimulate the development of circulating antibody against the coat proteins of the virus, conferring some degree of resistance. For some diseases, killed virus vaccines are currently the only ones available. The following points apply to killed vaccines: * Extreme care is required during the inactivation process used in their manufacture, to make certain that no residual live virulent virus is present in the vaccine while preserving the immunizing antigen. * The immunity conferred is often brief and must be boosted, which not only introduces a logistic problem of repeatedly reaching the persons in need of immunization but has also caused concern about possible hypersensitivity reactions due to repeated administration of foreign proteins. * Parenteral administration of a killed virus vaccine, even when it stimulates circulating antibody (IgM, IgG) to satisfactory levels, has sometimes given limited protection because local resistance (IgA) is not induced adequately at the natural portal of entry or primary site of multiplication of the wild virus infection (e.g., the nasopharynx for respiratory viruses, and alimentary tract for poliovirus). * The cell-mediated response to inactivated vaccine is generally poor, when compared with that after a live vaccine. * Some killed virus vaccines have induced hypersen- sitivity to a subsequent infection, perhaps owing to an unbalanced immune response to viral surface antigens and failure to mimic the natural infection. Live attenuated virus vaccines Live virus vaccines utilize virus mutants that anti- genically overlap with wild-type virus but are restricted in some critical step necessary in the pathogenesis of disease. Previously the development of virus strains suitable for live virus vaccines was chiefly carried out by selecting naturally attenuated strains or by cultivating the virus serially in various hosts and cultures in the hope of deriving an attenu- ated strain fortuitously. The search for such strains is now being approached by laboratory manipulations aimed at specific, planned, genetic alterations in the virus (e.g., with rabies, dengue, influenza, and respir- atory syncytial viruses). Live attenuated vaccines, as regards their effect on immunity, have the advantage of resembling the natural infection. They multiply in the host and tend to stimulate longer-lasting production of antibodies, to induce a good cell-mediated response, and to induce antibody production and resistance at the portal of entry. The problems with live attenuated vaccines include the following: (1) There is a risk of reversion to greater viru- lence during multiplication within the vaccinee. 108 Virus vaccines: principles and prospects Although reversion has not proved to be a signifi- cant problem in practice, its potential exists, and public health officials must be alert to this possi- bility. (2) Unrecognized adventitious agents latently infecting the culture substrate (eggs, primary cell cultures) may enter the vaccine stocks. Viruses found in vaccines have included avian leukosis virus, simian papovavirus SV40, and simian cytomegalo- virus. The problem of adventitious contaminants may be circumvented through the use of normal cells serially propagated in culture (e.g., human diploid cell lines as substrates for cultivation of vaccine viruses). Vaccines prepared in such cultures have been in use for years and have been safely adminis- tered to many millions of persons. (3) There is the potential problem that the live vaccine virus may produce persistent infections in the vaccinee. The actual risk of this appears to be very low. (4) The storage and limited shelf-life of attenu- ated vaccines present problems, but this can be over- come in some cases by the use of viral stabilizers (e.g., MgCl2 for poliovaccine), plus a cold chain for delivery of vaccine to the field. (5) Interference by co-infection with a naturally occurring, wild-type virus may inhibit the replication of the vaccine virus and decrease its effectiveness. This has been noted with the vaccine strains of poliovirus, which can be inhibited by concurrent infections by certain other enteroviruses. Proper use ofpresent vaccines One fact cannot be overemphasized: an effective vaccine does not protect against disease until it is administered in the proper dosage to susceptible individuals. The failure to provide all sectors of the population with complete courses of immunization is reflected in the continued occurrence of large numbers of cases of paralytic poliomyelitis and measles in unvaccinated persons in many developing countries around the world. There was a theoretical possibility that the anti- body response might be diminished or that inter- ference might occur if two or more live vaccines were given at the same time. In practice, however, simulta- neous administration of live vaccines can be safe and effective. Thus, trivalent live oral poliovaccine (when given in three doses) or a combined live measles, mumps, and rubella vaccine (given by injection), is effective. The antibody response to each component of these combination vaccines is comparable with the antibody response to the individual vaccines given separately. As indicated in Table 2, certain virus vaccines are recommended for use by the general public. Other vaccines are recommended only for use by persons at special risk due to occupation, travel, or life-style. Prospective vaccines Molecular biology and modern technologies are combining to make possible novel approaches to vaccine development. New vaccines are anticipated in the next few years (Table 3). Local admInistration of vaccine Intranasally administered aerosol vaccines are being developed, particularly for respiratory disease viruses Table 3: New and forthcoming vaccines: predlcted time before licensure Time Disease and vaccines (years) 1. Cytomegaloviral disease: Live attenuated rDNA glycoprotein 2. Dengue: Live attenuated cDNA in live vector virus 3. Viral hepatitis A: Live attenuated Subunit 4. Viral hepatitis B: rDNA 5. Alphaherpesviral disease: rDNA glycoprotein Live attenuated 6. Influenza A and B: Purified haemagglutinin and neuraminidase Live attenuated 7. Japanese encephalitis: Cell-culture-grown; inactivated 8. Parainfluenza: Trivalent, subunit 9. Rabies: Vero-cell-grown; inactivated rDNA glycoprotein cDNA in live vector virus 10. Respiratory syncytial viral disease: rDNA glycoprotein Live attenuated 11. Rotaviral enteritis: Live bovine Live human or animal-human reassortant 12. Varicella: Live attenuated 13. Yellow fever: Cell-culture-grown; attenuated 14. AIDS (HIV infection): rDNA glycoprotein cDNA in live vector virus Subunit or synthetic antigen Anti-idiotype 3 7 8 4 5 5 8 4 6 6 5 2 3 3 5 5 3 4 2 ?7 ' Caused by human (alpha) herpesvirus 1 and 2 (herpes simplex virus type 1 and 2, respectively). 109 J.L. Melnick and also for measles virus. Like orally administered attenuated poliovirus, their use should stimulate local antibodies at the portal of entry. Subunit vaccines Subviral components can be obtained by breaking apart the virion so that the vaccine may contain only those components needed to stimulate protective antibody. This approach is coupled with better puri- fication procedures. This can eliminate nonviral pro- teins and reduce the possibility of adverse reactions to the vaccine. Purified material can be administered in more concentrated form, containing greatly increased amounts of the specifically desired immu- nogen. Attenuation of viruses by genetic manipulation Recombinants or mutants are being produced that can then serve as live virus vaccines. The develop- ment of specific deletion mutants that alter the virus but do not inactivate it are expected to yield vaccines that can replicate but cannot revert to virulence. Recombinant DNA Use of aviruient virus vectors. Recombinant DNA techniques are used to insert the gene coding for the immunizing protein of one virus into the genome of a second, avirulent virus that can be administered as the vaccine. The prototype vector under study is vac- cinia virus. The gene for hepatitis B surface antigen (HBsAg) has been introduced into a gene non- essential for vaccinia replication. The resulting recombinant virus has elicited an immune response to hepatitis B virus in test animals. Other virus vectors possessing large genomes (e.g., herpesvirus) are also under study. Oral adenovirus vaccine has been modified so that it carries the HBsAg immu- nizing gene of hepatitis B virus. Chimeric poliovirus vaccines have been constructed in which the com- pletely avirulent type 1 virus acts as a vector for the gene carrying the immunizing VP1 gene of type 3. Vectors include not only avirulent viruses but bacteria as well. A live recombinant hepatitis A vaccine has been constructed using attenuated Sal- monella typhimurium as the delivery vector via the oral route. This vector and its means of delivery hold considerable promise for the development of vac- cines for other diseases as well. Purified proteins produced by use of cloned genes. Viral genes can now be easily cloned into plasmids. The cloned DNA can then be expressed in prokaryotic or eukaryotic cells if appropriately engineered construc- tions are used. The immunizing antigens of hepatitis B virus, rabies virus, herpes simplex virus (or human (alpha) herpesvirus), foot-and-mouth disease virus, and the influenza virus have been successfully syn- thesized in bacteria or yeast cells. If the bacteria can be made to produce the antigen in sufficient quantity and with immunogenicity, this will facilitate the pro- duction of a purified vaccine containing only the immunizing antigen. It is already apparent that the glycosylation of viral surface glycoproteins is not always essential for antigenicity. Unglycosylated herpesvirus proteins synthesized in bacteria have been able to induce neutralizing antibodies in test animals. Synthetic peptides Viral nucleic acids can readily be sequenced and the amino acid sequence of the gene products predicted. It is now technically possible to synthesize short pep- tides that correspond to antigenic determinants on a viral protein. Much of the pioneer study on synthetic virus vaccines has been carried out in the 1980s with foot-and-mouth disease virus. The location of the antigenic site was predicted by comparing the amino acid sequences of the VP1 of three different isolates of the virus, belonging to two different serotypes. Long tracts of VP1 were found to be highly con- served whereas some regions were highly variable. Reasoning that the variability was due to these regions being available for interaction with neutral- izing antibody and hence subject to high selective pressure, investigators synthesized peptides corre- sponding to these regions and tested them for the ability to elicit neutralizing and protective antibody. These experiments showed the amino acid sequence 141-160 to be very active in eliciting neutralizing antibody; similar results were obtained in another laboratory with a hexadecapeptide encompassing residues 144-159. The synthetic vaccine, containing three different viral antigens, is immunogenic in small animals, and is now being tested in cattle. Antigenically active polypeptides have also been synthesized for hepatitis B, influenza, rabies, polio- myelitis, and scrapie viruses, and for Epstein-Barr virus, human (alpha) herpesvirus types 1 and 2, and human immunodeficiency virus. These synthetic pep- tides have induced neutralizing antibodies in animals. However, they have been weak antigens, and a search for potent and safe adjuvants to be incorporated in the vaccine continues in several laboratories. Synthetic peptides may play another role in immunization. Some peptides have failed to induce neutralizing antibodies in experimental animals, but apparently "primed" the animals' immune systems so that a subsequent subimmunizing dose of viral 110 Virus vaccines: principles and prospects antigen induced a type-specific neutralizing antibody response. One problem with synthetic peptides is that a number of antigenic determinants are not contin- uous sets of amino acids, but rather amino acids brought together by folding of the proteins Such sequences cannot be readily mimicked by synthetic peptides. It seems that in some instances conforma- tion may be even more important than amino acid composition and sequence, so far as antigenic deter- minants are concerned. In some instances, cyclized peptides have proved to be more immunogenic than their linear counterparts. Conformation must be the reason why anti-idiotype antibodies (see below) are able to (1) enhance antibody formation upon sub- sequent injection of a subimmunogenic dose of the natural antigen, and (2) even induce an antibody response (albeit often a weak one) by themselves. Chemical synthesis would preclude exposure of the vaccinees to viral nucleic acid, thereby avoiding any possibility of reversion to virulence. The problem of contamination by cellular proteins also would be avoided. Although synthetic vaccines hold promise, there are several obstacles yet to be over- come. The immune response induced by synthetic peptides is considerably weaker than that induced by intact protein or inactivated virus, but the weak response may be overcome by the use of the newer adjuvants. It is not easy to identify peptide sequences that are able to induce a protective immune response. A single peptide representing a single epitope may not be able to induce resistance against a viral protein containing multiple antigenic determi- nants, and it may be that two epitopes of the viral coat protein will have to be combined into a single peptide in order that the synthetic vaccine will be synergistically more antigenic. Furthermore, not all antigenic determinants are sequential and, as men- tioned, it may be very difficult to simulate conforma- tional determinants (i.e., those determined by the tertiary configuration of the protein, juxtaposing amino acids that may be widely separated in the primary sequence). Antldiotype vaccines Anti-idiotype antibodies mimic the form of a surface antigen, but contain no protein or synthetic peptide that is derived from a pathogenic agent. To prepare such vaccines, first a monoclonal antibody (Ab-1) is produced specifically against the neutralizing antigen of the virus. Then this monoclonal antibody is injected into an animal, inducing an antibody (Ab-2) that is specific for the initial Ab-1. If Ab-2 binds to the antigen-recognizing portion (idiotype) of Ab-1, then the idiotype of Ab-2 should be structurally ana- logous to the original neutralization antigen of the virus (a so-called internal image of the viral antigen). If Ab-2 is then injected as a vaccine, the antibody (Ab-3) induced in the vaccinee should be specific for the internal image of Ab-2, and thus also should be specific for the original viral antigen. Anti-idiotypes have been shown to induce anti- bodies against hepatitis B virus not only in mice but also in chimpanzees. Furthermore, the chimpanzees were shown to resist an active challenge dose of this virus. The work on anti-idiotypes is being extended. Experimental anti-idiotype vaccines have protected mice partially or completely against a variety of infectious agents: herpesvirus, reovirus, rabies virus, poliovirus, and several disease-causing bacteria and protozoa. Such vaccines would have several advantages: (a) cross-reactive anti-idiotype antibodies can be pre- pared so as to react against several different anti- genic variants of a virus instead of a single strain; (b) an antigenic analogue (Ab-2 above) may induce immunity in some instances when the natural antigen cannot; (c) in many instances only small amounts of natural immunizing antigen can be obtained and purified to provide vaccine stocks, but only small amounts of antigen are needed to prepare anti-idiotype antibody; and (d) an anti-idiotype anti- body carries no live virus or viral nucleic acid, and thus should be of assured safety. Such an approach is being used in the hope of developing vaccines against the human immuno- deficiency viruses (HIV), the retroviruses which trigger the acquired immunodeficiency syndrome (AIDS). New conventionally prepared vaccines While these new approaches are being pursued, efforts to develop vaccines along the lines that have been used in the past are also continuing. Consider- able success has been achieved for a varicella-zoster vaccine where none has existed before, although there is still some concern about the possibility of vaccinated subjects contracting zoster in later life. Vaccines are also under development for cyto- megalovirus (or human (beta) herpesvirus 5) and Epstein-Barr virus, but, as with the varicella-zoster vaccine, there is concern about the long-term effects. Vaccine trials in humans are also under way for dengue virus, herpes virus, rotavirus, and respiratory syncytial virus. Time-frames for new vaccines The approximate schedules, as estimated by W. S. Jordan (Progress in medical virology, 35: 2-20 (1988)), for the availability of new viral vaccines in the near future are listed in Table 3. 111 J.L. Melnick At present, it appears that vaccines for some viral diseases are not feasible. These are listed in Table 4. Table 4: Examples of virus diseases for which vaccines are not likely to be available In the foreseeable future 1. Ocular infections (conjunctivitis): Adenoviruses Enterovirus 70 2. Acute upper respiratory infections: Coronaviruses Coxsackieviruses Rhinoviruses 3. Gastrointestinal infections (diarrhoea): Norwalk virus 4. Liver infections: Non-A non-B hepatitis viruses: Enterically transmitted hepatitis E virus Post-transfusion hepatitis C virus 5. Genitourinary infections: Papilloma viruses 6. Meningitis: Coxsackieviruses Echoviruses Higher-numbered enteroviruses 7. Encephalitis: Arboviruses: Togaviruses Flaviviruses (except Japanese encephalitis virus) 8. Systemic infections: Epstein-Barr (EB) virus Suggested reading Extensive bibliographies to the viral vaccine literature can be found in: 1. Bell, R. & Torriglanl, G., ed. New approaches to vaccine development. Proceedings of a Meeting organized by the World Health Organization, Geneva, 17-20 October 1983. Basle, Schwabe & Co., 1984. 2. Oxford, J.S. & Oberg, B. Conquest of viral diseases: a topical review of drugs and vaccines. Amsterdam, Elsevier, 1985. (Vol. 1 of Perspectives in medical virology). 3. Arnon, R., ed. Synthetic vaccines. Boca Raton, CRC Press, 1987. 4. Plotkin, S.A. & Mortimer, E.A. Jr., ed. Vaccines. Phila- delphia, W.B. Saunders, 1988. 5. Fields, B.N. et al., ed. Virology, 2nd edition. New York, Raven Press (in press). 6. Evans, A.S., ed. Viral infections of humans: epidemi- ology and control, 3rd edition. New York, Plenum PubI. Corp. (in press). 7. Vaccine (periodical published by Butterworths, London, six times a year since 1983). 8. Progress in medical virology (edited by J.L. Melnick; volumes published by S. Karger, Basle, once or twice a year since 1958). Reprint No. 4957 112