The Johns Hopkins University, Center for Civilian Biodefense Strategies
D.A. Henderson, M.D., M.P.H.
Working paper - WHO Committee meeting on smallpox
In March 1999, a Committee of the U.S. Institute of Medicine published a report setting forth prospective scientific research needs requiring the use of variola virus (1). The Committee had been charged with defining possible research studies which might be undertaken utilizing variola virus without consideration of the public health utility, feasibility, cost or priority of such research relative to other initiatives. Likewise, the Committee was instructed not to weigh the possible benefits of such research against the risks associated with retaining the virus and pursuing such research.
As stated in the report, the Committee's overall conclusion was that "the most compelling need for long-term retention of live variola virus would be for the development of antiviral agents or novel vaccines to protect against a reemergence of smallpox due to accidental or intentional release of variola virus". An important question, hitherto not addressed, is how a new vaccine and/or an antiviral agent would address public health needs.
I. Considerations Pertaining to Vaccine Research
The Decision to Cease Vaccination
Routine vaccination stopped in all countries by or soon after eradication was declared by the 1980 World Health Assembly. Several factors had recommended that vaccination be stopped. First, it was clear from epidemiological data that the risk of recurrent smallpox occurring naturally was close to nil, there being no reservoir of smallpox in nature. The only conceivable source might be a past victim of smallpox, buried in the frozen tundra. Such efforts as have been made to isolate virus from such bodies have been unsuccessful but, even if virus could be recovered, suitable precautions to prevent its spread could be readily effected. As of 1980 it was believed that the risk of an accidental or deliberate release of smallpox from a laboratory was likewise close to nil. So far as was then known, research utilizing the virus had ceased and the virus stocks appeared to be contained under suitable conditions in freezers in Moscow and Atlanta.
Balanced against the seemingly negligible risk of recurrent smallpox were the considerable costs associated with continuing vaccination programs and the small but definite risk of serious, sometimes fatal adverse reactions following vaccination. None dissented when the World Health Assembly voted to recommend the cessation of smallpox vaccination.
Vaccination and quarantine of patients and close contacts represent the only tools presently available to stem an epidemic should smallpox reappear. Although an antiviral drug, methisazone, used prophylactically, was first reported in the 1960s to be highly effective in non-placebo control trials (2), more rigorous studies revealed its protective efficacy to be in the range of 30 to 40% (3;4). Whatever its efficacy, the occurrence of severe nausea and vomiting among more than half of all recipients made it impracticable for general use (5). The drug is not now commercially available.
Because of the fact that vaccination everywhere ceased 20 or more years ago, there exists in every country a large population that has never been exposed to either variola or vaccinia antigen. Moreover, some substantial proportion of those vaccinated before 1980 are now susceptible to smallpox because of waning immunity. Thus, the potential for a rapidly spreading epidemic is great. Large-scale vaccination would undoubtedly be required to effect control should smallpox recur. Half or more would be primary vaccinations.
Current Vaccine Supply and Vaccine Strains
Vaccine supplies are scarce and little, if any, vaccine is now being produced. Countries replying to a 1998 WHO survey report having reserves of vaccine amounting to less than 70 million doses. However, the usable amount may be less than this because not all stocks have been properly refrigerated and some have not been regularly retitred to assure that vaccine potency is sustained. Virtually all of the available vaccine was produced in the mid 1970s by the traditional method of virus growth on the scarified flank of a calf. No country reports having an operational vaccine production facility at this time.
Virtually all of the vaccine is from one of four strains of vaccinia which were used during the smallpox eradication program -- Patwadanger (India); New York City Board of Health (Americas); EM-63 (Russian Federation); Lister (other countries) (5). Two strains have been adapted to tissue cell culture for production purposes. The Lister strain was produced in rabbit kidney cells by the National Institute of Public Health of the Netherlands (6). A more attenuated strain (Lc16m8) derived by numerous low temperature passages of the Lister strain was produced also in rabbit kidney cells by the Chiba Serum Institute of Japan (7).
Of the strains noted above, all except the Japanese strain were used in endemic areas and were found to be highly efficacious. The strains, however, varied in their pathogenicity and in the severity of the reactions they induced (5;8).The least reactogenic is the LC16m8 strain; next most reactogenic are the New York Board of Health and EM-63 strains; and finally, the Lister and Patwadanger strains.
Production of New Supplies of Vaccine
Because of the limited supplies of vaccine available, several countries are considering producing vaccine for reserve stockpiles. A stockpile, once created, would probably never need to be renewed because freeze-dried vaccine retains its potency almost indefinitely if stored at -20ºC.
In the United States, a number of governmental meetings have been held to determine which strain of vaccine should be used in production and what cell substrate would be most appropriate. Growth in tissue culture is preferred so as to exclude the inadvertent introduction of adventitious agents such as might be introduced by traditional methods of harvesting vaccine grown on a calf, sheep or water buffalo. Because vaccinia grows so well in such a wide variety of tissue cell culture systems, there are many options. For the U.S., chick embryo fibroblast is preferred simply because of the extensive experience already acquired with this cell substrate.
Which should be the preferred strain has generated considerable discussion. A strain that produces fewer complications than the New York City Board of Health strain but which provides comparable protection would be ideal. An attractive candidate is the LC16m8 strain developed by Hashizume at the Chiba Serum Institute in 1975 (7). A summary of its characteristics, prepared by Hirayama (9), indicates that in controlled studies, the strain induces fewer local and febrile reactions than the standard Japanese strain (Ikeda), Lister and EM-63 (Russian strain derived from the New York City Board of Health strain). HI and neutralizing antibodies appear to be equivalent. When the different strains were inoculated into the thalamus of monkeys, the Japanese strain proved to be the least pathogenic. Some 50,000 Japanese children have been vaccinated with this strain; no serious adverse reactions were detected.
However attractive the LC16m8 strain appears to be, the fact that it had not been tested under actual field challenge is a drawback. The U.S. group decided, after extended discussions, that given the dire circumstances under which a smallpox vaccine was apt to be used, it was essential to employ tested strains that had provided a high order of protection under actual challenge and about which there could be no question as to efficacy.
Further definition was required as to what constituted a distinctive strain and which of those strains met the criterion of demonstrated efficacy under field challenge. Until 1958 when a WHO Study Group on Vaccines (5;10) recommended that a "seed lot" system be used for vaccine production, manufacturers regularly used material from the last batch of vaccine prepared to inoculate the next group of calves. During repeated serial passages, virus strains can be expected to change in character. Moreover, in the past, when manufacturers felt that the human cutaneous response to vaccination had become less marked, it was customary to pass the vaccine several times through one or several species, including rabbit, donkey, monkey and sometimes human before again inoculating calves. Thus, even though strains of two producers might bear the same name, the strains could actually be quite different and the comparative efficacy under field challenge might well differ. Thus, if the common genetic ancestors of a vaccine strain antedate the inauguration of the seed lot system for production (about 1961-65), differences have to be assumed. For the U.S., there is only one variant of the New York City Board of Health strain that actually has been tested under field conditions and that is the Wyeth production strain. It was widely and successfully used throughout Africa and parts of Asia during the eradication campaign. This is the strain that will be used for future vaccine production.
Some have suggested that by retaining variola virus, suitable antiviral substances might be found which would permit better treatment of the more severe complications than is now offered by Vaccinia Immune Globulin. Although an alternative solution to VIG would be desirable, it is clear that the research that is needed requires use of vaccinia virus, not variola virus.
From the public health vantage point, it would seem clear that, at least in the U.S., there is no reason to pursue a vaccine research agenda given the fact that its efficacy could not be demonstrated under circumstances of a natural challenge.
II. Considerations Pertaining to Research on Antiviral Drugs
Historical Studies of Antivariola Agents
During the past 40 years, four different drugs have been evaluated to determine their chemotherapeutic value: two thiosemicarbazones, cytosine arabinoside and adenine arabinoside. None proved to be effective. The two thiosemicarbazones were given as tablets or in a syrup (11). There were no differences in case-fatality rates between the treated and untreated and no differences as to disease severity. In an uncontrolled trial in Bangladesh, Hossain reported highly promising results with cytosine arabinoside (12) but when properly evaluated in clinical trials, it was found to be of no benefit whatsoever (13). Other studies utilizing adenine arabinoside also yielded disappointing results (14).
For possible chemoprophylactic use, two drugs have been evaluated: methisazone and cidofovir. As described earlier, methisazone appeared to have a modest prophylactic effect but induced so much severe nausea and vomiting as to preclude its being considered for field use. Recently, Huggins has reported studies in which cidofovir was administered intravenously to monkeys who had been challenged by aerosol with monkeypox virus at 24 and 48 hours before treatment (15). Protection was provided to those who were treated within 24 hours: the effect when given at 48 hours was less certain.
Potential Public Health Use for a Chemotherapeutic Agent
It would be useful to have an effective chemical agent that could be administered after the onset of clinical smallpox to prevent a fatal outcome. Most such cases would be among individuals infected with smallpox before it was recognized that the virus had been released plus those in a second generation of cases who had been exposed more than 3 to 4 days before vaccine could be administered (note that smallpox vaccine administered up to 3-4 days post exposure is protective). Large scale vaccination-containment programs, begun as soon as smallpox was recognized to be extant, should be able to stop most chains of transmission before a third generation of cases would occur.
How many persons might benefit from drug treatment? This would depend, of course, on the efficacy of the drug and how late in the disease course the drug could be satisfactorily administered. The epidemic first generation cases would be a special challenge because diagnosis of the epidemic would almost certainly be delayed, thus making case treatment late in the course of illness. Overall, one wonders if the number potentially benefiting from therapeutic treatment would number more than a few thousand in all following a release which, for example, infected 500 or so persons in a first generation of cases.
Potential Public Health Use for a Chemoprophylactic Agent
A chemoprophylactic agent would be useful for the prevention of smallpox among those exposed to the disease but who were at risk of developing progressive vaccinia because of having immune response mechanisms which were inadequate to deal with the vaccinial infection.
How many such individuals might there be? This is impossible to know but some idea of risk may be derived from the frequency of this complication as measured during a time when vaccine was widely applied, a time before the era of AIDS. The best available data were compiled in the course of a national survey for vaccinial complications in the U.S. in 1968 (16). Case finding for progressive vaccinia was believed to be reasonably complete because Vaccinia Immune Globulin (VIG) was recognized to be essential to therapy; all supplies of VIG were distributed through specially designated consultants who registered all patients for whom VIG was requested. Likewise, all patients for whom physicians requested the antiviral drug, marboran, were registered. In all, there were 11 cases, 4 of whom died. Three of the cases (2 fatal) were among children less than 10 years of age and were considered to be individuals with congenital defects. The remaining 7 cases were 15 years of age or older, 6 of whom were experiencing leukemia, Hodgkin's disease or lymphoma. Overall, the incidence was somewhat less than one case per million vaccinations.
Four cases are on record of individuals with HIV/AIDS who developed progressive vaccinia. All had very low levels of T4 cells.
There is no information available as to how HIV-infected persons without severely depressed T4 cells might handle a vaccinial infection. It should be noted, however, that live vaccines against a number of viruses (polio, yellow fever, measles and rubella) are in widespread use throughout many countries with high rates of HIV infection. Few problems have been reported, a not unexpected finding in view of the fact that HIV -infected patients generally handle virus infections reasonably well.
Although the availability of a chemoprophylactic agent might be useful, it is difficult to assess how effective or important a role it might play. Presumably, such a drug would need to be taken over a period of days, perhaps weeks, depending on the persistence of the threat. Cost would almost certainly be a major consideration, especially for developing countries. To be noted is the fact that during the eradication campaign, another agent, VIG, was available for treatment of complications in only a few of the industrialized countries. Yet, in view of the risk of the disease itself, vaccination was recommended for everyone whatever their health status.
From the public health perspective, a less reactogenic vaccine providing immunity comparable to that provided by the Lister or New York City Board of Health strains would be desirable. The obstacle to developing such a vaccine is that one could not know with certainty that the vaccine would protect under the circumstances of natural challenge. Given that the vaccine, if needed, would be utilized under emergency circumstances and probably in large quantity, due consideration has to be given to utilizing those strains already tried and tested. This has been the decision taken by the U.S. government.
An effective chemoprophylactic drug could have a role in coping with an epidemic. However, it is far from clear as to how extensively it might be used. For all who could possibly receive vaccine, that would be the prophylactic of choice, given the fact that one would have to weigh the efficacy of a proven vaccine against that of a prophylactic drug whose actual efficacy under the conditions of natural challenge would be entirely speculative. Moreover, one wonders if the projected costs ($300+ million) for development of a new drug and the costs of the actual product might not be better invested. For example, might it not be more practical, less costly and more certain of success to develop chemotherapeutic agents or monoclonals to deal with those few patients who might experience progressive vaccinia.
A chemotherapeutic agent that sharply diminished the 30% case-fatality rate would be a useful product in coping with an epidemic. Two questions, however, need to be considered; 1) The feasibility of developing such a product given the pathogenesis of the disease; and 2) The extent to which the drug might be used.
Treatment of a case of smallpox would presumably begin only after it was determined that the patient had the disease, i.e. after the rash has begun to emerge. By that time, the virus has already sequestered itself in epidermal cells of the middle and upper layers of the stratum spinosum. Vesicles form almost immediately and, with migration of polymorphonuclear cells into the vesicle, pustules are created. One would surmise that penetration of a therapeutic agent into the pustule would be as unlikely as the penetration of antibiotic agents into an abscess or boil. Since organs other than the skin are minimally affected, even in fatal cases (20;21), it is difficult to conceive of the potential mechanism of action of a drug.
The possible extent to which a therapeutic drug might be used would depend, of course, on the number of smallpox cases occurring over time. Following the reintroduction of smallpox into the population, whether accidental or deliberate, one could foresee an intensive containment program and the early and very wide application of vaccine throughout the affected region. Should a continuing or recurrent threat of smallpox be foreseen, it is likely that all countries would again undertake routine vaccination programs, even if the naturally occurring disease were again eradicated. In brief, it is doubtful that a therapeutic drug would be very much used beyond the initial few weeks or months of a reintroduction.
In brief, from a public health perspective, it is difficult to justify a research program whose objective is either a new vaccine or a prophylactic or therapeutic drug.
Back to Smallpox Virus Destruction Page
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