Causality

The Immunization Safety Review Committee has adopted the framework for assessing causality developed by previous IOM committees (IOM, 1991, 1994), convened under the congressional mandate of P.L. 99–660 to address questions of immunization safety. The categories of causal conclusions used by the committee are as follows:

1.

No evidence

2.

Evidence is inadequate to accept or reject a causal relationship

3.

Evidence favors rejection of a causal relationship

4.

Evidence favors acceptance of a causal relationship

5.

Evidence establishes a causal relationship.

Assessments begin from a position of neutrality regarding the specific vaccine safety hypothesis under review. That is, there is no presumption that a specific vaccine (or vaccine component) does or does not cause the adverse event in question. The weight of the available clinical and epidemiologic evidence determines whether it is possible to shift from that neutral position to a finding for causality (“the evidence favors acceptance of a causal relationship”) or against causality (“the evidence favors rejection of a causal relationship”). The committee does not conclude that the vaccine does not cause the adverse event merely if the evidence is inadequate to support causality. Instead, it maintains a neutral position, concluding that the “evidence is inadequate to accept or reject a causal relationship.”

Although no firm rules establish the amount of evidence or the quality of the evidence required to support a specific category of causality conclusion, the committee uses standard epidemiologic criteria to guide its decisions. The most definitive category is “establishes causality,” which is reserved for those relationships where the causal link is unequivocal, as with the oral polio vaccine and vaccine-associated paralytic polio or with anaphylactic reactions to vaccine administration. The next category, “favors acceptance” of a causal relationship, reflects evidence that is strong and generally convincing, although not firm enough to be described as unequivocal or established. “Favors rejection” is the strongest category in the negative direction. The category of “establishes no causal relationship” is not used because it is virtually impossible to prove the absence of a relationship with the same surety that is possible in establishing its presence.

If the evidence is not reasonably convincing either in support of or against causality, the category “inadequate to accept or reject a causal relationship” is used. Evidence that is sparse, conflicting, of weak quality, or merely suggestive either toward or away from causality falls into this category. Some authors of similar assessments use phrases such as “the evidence does not presently support a causal association.” The committee believes, however, that such language does not make the important distinction between evidence indicating that a relationship does not exist (category 3) and evidence that is indeterminate with regard to causality (category 2). The category of “no evidence” is reserved for those cases in which there is a complete absence of clinical or epidemiologic evidence.

The sources of evidence considered by the committee in its scientific assessment of causality include epidemiologic and clinical studies directly addressing the question at hand. That is, the data are specifically related to the effects of the vaccine(s) under review and the adverse health outcome(s) under review—in the case of this report, the effects of SV40 contamination of the polio vaccine and the risk for certain types of cancer.

Epidemiologic studies carry the most weight in a causality assessment. These studies measure health-related exposures and outcomes in a defined set of subjects and make inferences about the nature and strength of associations between exposures and outcomes in the overall population from which the study sample was drawn. Epidemiologic studies can be categorized as observational or experimental (clinical trial), and as uncontrolled (descriptive) or controlled (analytic). Among these various study designs, experimental studies generally have the advantage of random assignment to exposures and are therefore the most influential in assessing causality. Uncontrolled observational studies are important but are generally considered less definitive than controlled studies. In uncontrolled observational studies, where observations are made over time, confounding from factors such as changing case definitions or improving case detection may affect the apparent incidence and prevalence of the adverse outcomes studied.

By themselves, case reports and case series are generally inadequate to establish causality. Despite the limitations of case reports, the causality argument for at least one vaccine-related adverse event (the relationship between vaccines containing tetanus toxoid and Guillain-Barré syndrome) was strengthened most by a single, well-documented case report on recurrence of the adverse event following re-administration of the vaccine, a situation referred to as a “rechallenge” (IOM, 1994).

Biological Mechanisms

Evidence considered in the scientific assessment of biological mechanisms¹ includes human, animal, and in vitro studies related to biological or pathophysiological processes by which immunizations could cause an adverse event. When other evidence of causality is available, biological data add supportive evidence, but they cannot prove causality on their own.

This committee is often faced with a set of circumstances in which the epidemiologic evidence is judged inadequate to accept or reject a causal association between a vaccine exposure and an adverse event of concern. It is then left with the task of examining proposed or conceivable biological mechanisms that might be operating if an epidemiologically sound association could be shown between a vaccine exposure and an adverse event.

In any case, the committee’s causality assessments must be guided by the current understanding of biological processes. In fact, the current thinking on a possible biological explanation for a relationship between immunization and an adverse event will influence the design of a good epidemiologic analysis. The essential consideration of “confounders” in epidemiologic studies depends on an understanding of the biological phenomena that could underlie or explain the observed statistical relationship. A statistical observation can be considered as evidence of causality only when important confounders are considered. However, without evidence of a statistical association or convincing clinical evidence, biological mechanisms cannot be invoked as proof of causality.

The identification of sound biological mechanisms can also influence the development of an appropriate research agenda and give support for policymakers, who frequently must make decisions without having complete information regarding causality. In addition, there is often value in investigating and understanding possible biological mechanisms even if the available epidemiologic evidence suggests the absence of a causal association. A review of biological data could give support to the negative causality assessment, for example, or it could prompt a reconsideration or further investigation of the epidemiologic findings. If new epidemiologic studies were to question the existing causality assessment, the biological data could gain prominence in the new assessments.

The committee has established three general categories of evidence on biological mechanisms:

1.

Theory only. A reasonable mechanism can be hypothesized that is commensurate with scientific knowledge and that does not contradict known physical and biological principles, but has not been demonstrated in whole or in part in humans or in animal models. Postulated mechanisms by which a vaccine might cause a specific adverse event but for which no coherent theory exists would not meet the criteria for this category. Thus, “theoretical only” is not a default category, but one that requires thoughtful and biologically meaningful suppositions.

2.

Experimental evidence that the mechanism operates in animals, in vitro systems, or humans. Experimental evidence often describes effects on just one or a few of the steps in the pathological process required for expression of disease. Showing that multiple components of the theoretical pathways operate in reasonable experimental models increases confidence that the mechanisms could possibly result in disease in humans. The evidence can be derived under highly contrived conditions. For example, achieving the results of interest may require extensive manipulation of the genetics of an animal system, or in vivo or in vitro exposures to vaccine antigen that are extreme in terms of dose, route, or duration. Other experimental evidence is derived under less contrived conditions. For example, a compelling animal or in vitro model exists whereby administration of a vaccine antigen under conditions similar to human use results in a pathologic process analogous to a human disease pathology. Mechanistic evidence also could come from studies in humans, but this is distinct from the evidence (about incidence of adverse events following immunization) that derives from randomized controlled trials or other population-based epidemiologic studies, which contribute to the causality assessment.

3.

Evidence that the mechanism results in known disease in humans. For example, the wild-type infection causes the adverse health outcome, or another vaccine has been demonstrated to cause the same adverse outcome by the same or a similar mechanism. Data from population-based studies of the effects of the vaccine administration on the occurrence of the adverse outcomes under review are not considered evidence regarding the biological mechanisms but rather as evidence regarding causality.

If the committee identifies evidence of biological mechanisms that could be operational, it will offer a summary judgment of that body of evidence as weak, moderate, or strong. Although the committee tends to judge biological evidence in humans as “stronger” than biological evidence from highly contrived animal models or in vitro systems, the summary judgment of the strength of the evidence also depends on both the quantity (e.g., number of studies or number of subjects in a study) and quality (e.g., the nature of the experimental system or study design) of the evidence. Obviously, the conclusions drawn from this review depend on both the specific data and on scientific judgment. To ensure that its own summary judgment is defensible, the committee intends to be as explicit as possible regarding the strengths and limitations of the biological data.

Published and Unpublished Data

Published reports carry the most weight in the committee’s assessment because their methods and findings are laid out in enough detail to be assessed. Furthermore, published works, which undergo a rigorous peer review, are subject to comment and criticism by the entire scientific community. In general, the committee cannot rely heavily on unpublished data in making its scientific assessments (regarding either causality or biological mechanisms) because they usually lack comment and criticism and must therefore be interpreted with caution. The committee also relies on editorial and peer review procedures to ensure the disclosure of potential conflicts of interest that might be related to the source of funding for the research study. Immunization safety studies and other data reviewed by the committee are funded by a variety of sources, including—NIH, CDC, vaccine manufacturers, research advocacy organizations, or foundations. The committee does not investigate the source of funding of the published research reports it reviews, nor does the funding source influence the committee’s interpretation of the evidence.

Unpublished data and other reports that have not undergone peer review do have value, however, and are often considered by the committee. They might be used, for example, in support of a body of published, peer-reviewed literature with similar findings. If the committee concluded that the unpublished data were well described, had been obtained using sound methodology, and presented very clear results, the committee could report, with sufficient caveats in the discussion, how the unpublished data fit with the entire body of published literature. Only in extraordinary circumstances, however, could an unpublished study refute a body of published literature.

The Immunization Safety Review Committee’s scope of work includes consideration of clinical topics for which high-quality experimental studies are rarely available. Many other panels making clinical recommendations using evidence-based methods are able to require that randomized trials be available to reach strong conclusions. However, the IOM Committee was convened specifically to assess topics that are of immediate concern and for which data of any kind may just be emerging. Thus, given the unique nature of this project, the committee decided that it was important to review and consider as much information as possible, including unpublished information. The committee does not perform primary or secondary analyses of unpublished data, however. In reviewing unpublished material, the committee applies generally accepted standards for assessing the quality of scientific evidence, as described above. (All unpublished data reviewed by the committee and cited in this report are available—in the form reviewed by the committee—through the public access files of the National Academies. Information about the public access files is available at 202– 334–3543 or www.national-academies.org/publicaccess.)

Polio Vaccines and SV40

Efforts to develop a vaccine effective against the three distinct types of the poliovirus began in the 1930s, but progress was hindered by the difficulty of producing an adequate supply of virus in the laboratory. The development in 1949 of a technique for growing the virus in tissue cultures (Enders et al., 1949) was soon followed by successful trials of a trivalent killed-virus vaccine. In 1954, 400,000 of 1.8 million children from the United States, Canada, and Finland actually received the potentially contaminated inactivated poliovirus vaccine (IPV) developed by Salk and colleagues (Francis et al., 1957). The vaccine was licensed for use in the United States in 1955. Vaccines that used a live attenuated virus and were administered orally were also developed, initially in a monovalent form. The trivalent oral poliovirus vaccine (OPV) known as the Sabin vaccine was licensed in the United States in 1963.

The tissue cultures used to grow poliovirus for these vaccines came from kidneys of rhesus and cynomolgus macaques.³ In 1960, Sweet and Hilleman (1960) reported that these tissues could be infected with SV40, a previously unknown virus that commonly infects rhesus macaques. Soon after its discovery, SV40 was shown to be able to produce tumors in hamsters and to transform human cells in culture (Eddy et al., 1961, 1962; Girardi et al., 1962; Koprowski et al., 1962; Shein and Enders, 1962a,b). Testing confirmed that some of the tissue cultures used in producing IPV and OPV were contaminated with SV40. In 1961, the U.S. government established requirements for testing to verify that all new lots of polio vaccine are free of SV40 (Egan, 2002). Potentially contaminated vaccine from previously approved lots of IPV was not recalled, however, and might have been used until early 1963.

IPV administered between 1955 and 1963 to about 98 million children and adults is assumed to be the primary source of human exposure to SV40 in the United States.⁴ In addition, experimental lots of OPV contaminated with SV40 was administered to about 10,000 people participating in clinical trials between 1959 and 1961. Recipients of the oral vaccine, in contrast to those receiving contaminated IPV, did not develop an antibody response to SV40 (as reviewed in Shah and Nathanson, 1976). This suggests that IPV, not OPV, resulted in the infection of humans with SV40. Nonetheless, concerns about the validity, and in particular the specificity for SV40, of the serologic testing create some uncertainty about this conclusion.

Details of the level and extent of the contamination of IPV are unavailable. Because the process used to inactivate the poliovirus was less effective against SV40, IPV could have included killed or live SV40. Furthermore, manufacturers used different types of cell cultures, and some were less vulnerable to contamination (Shah and Nathanson, 1976). Tests of stored samples of the vaccine that had been administered in the United States from May through July in 1955 found various levels of SV40 contamination, with some vaccine showing no contamination (Fraumeni et al., 1963). From these data, Shah and Nathanson (1976) estimated that 10% to 30% of IPV contained live SV40 and that similar percentages of the approximately 98 million Americans who had been vaccinated by 1961 were exposed to live SV40.

While it is certain that many people were directly exposed to SV40 through injections of IPV, two related matters remain unresolved. First, it is possible that some portion of the population might have been exposed to SV40 before IPV was introduced (Shah et al., 1972; Geissler et al., 1985). Second, it is unclear whether the SV40 received through the vaccine could be transmitted within the population once contaminated vaccine was no longer in use. A discussion of other possible sources of SV40 exposure is described in the section on biological mechanisms.

Cancers

In rodents, the injection of SV40 has been associated primarily with four types of cancer: mesothelioma, osteosarcoma, ependymoma, and lymphomas. Investigators have also detected SV40 DNA in human forms of these cancers. All but NHL are very rare, and they are described briefly here.

Causality

For its review of the epidemiologic evidence on the association between exposure to polio vaccines containing SV40 and cancer, the committee found studies examining cancer incidence or mortality. Also included in the committee’s review are studies of cancers occurring in children who may have had a prenatal exposure to SV40 through vaccination of their mothers.

In examining the possible association between cancers and exposure to SV40-containing polio vaccines, the committee reviewed several ecologic studies. In an ecologic study, the unit of analysis is a group. Because the data on exposure and disease are available only on a group level, it is difficult to make any causal inferences regarding the association between an exposure and disease at the individual level (Kleinbaum et al., 1982). Thus, although ecologic studies can make only a limited contribution to assessments of causality, only a few other epidemiologic studies of cancer and exposure to the contaminated polio vaccine are available. In contrast to the practice in the committee’s previous reports, the ecologic studies are summarized in the appropriate evidence tables (Tables 1 and 2).

TABLE 1

Evidence Table: Exposure to Polio Vaccine Containing SV40 and Incidence of Cancer.

TABLE 2.

Evidence Table: Exposure to Polio Vaccine Containing SV40 and Cancer Mortality.

The available studies are reviewed in following three categories: cancer incidence, cancer mortality, and cancers following prenatal exposure to SV40-containing vaccine.

Causality Argument

All of the studies that the committee reviewed concerning cancer incidence or cancer mortality and exposure to polio vaccine containing SV40 have substantial limitations. Many of these studies were ecologic in design. In previous reports, the committee included only controlled observational studies in tables summarizing the evidence. Ecologic studies were excluded based on their inherent limitations in assessing causality. In general, it is difficult to make causal inferences from ecologic studies, mainly because: 1) exposure and outcome data are based on measurements averaged over a population, 2) exposure information is based on proxy measures (e.g., SV40 exposure based on birth cohort), and 3) data to control for confounding are unavailable (Morgenstern, 1998). For this report, however, because of the few controlled observational studies available and the public health importance of the issue at hand, the committee has included all of the studies it reviewed in the relevant evidence tables (Tables 1, 2, and 3).

TABLE 3

Evidence Table: Prenatal Exposure to Polio Vaccine Containing SV40 and Incidence of Cancer.

Most of the epidemiologic studies on polio vaccine containing SV40 and cancer are subject to misclassification bias because they rely on year of birth to designate exposure status. Even though polio vaccine known to contain SV40 was in use from 1955 to 1963, it is difficult to accurately determine which individuals received the vaccine without individual vaccination records. The studies may also be subject to misclassification bias because of lack of detailed and specific information about the presence of SV40 in individual vaccine doses. Information reported by Fraumeni and colleagues (1963) suggests that at least some vaccine used in 1955 was not contaminated. Further, samples of polio vaccine produced during the remainder of the period of likely contamination (1955 to 1961) were not available for testing, leaving little means to identify which vaccine lots were contaminated and who was exposed to SV40 contaminated polio vaccine. In addition, the assumption that persons who received polio vaccine after 1963 were unexposed to SV40 may not be accurate if sources of exposure other than contaminated IPV exist.

The studies were also limited by the rarity of the tumors thought to be associated with exposure to SV40. The effect estimates calculated from a small number of tumors are more sensitive to distortion from confounders, bias, and chance. The cohort exposed to contaminated vaccine has not yet reached the age when the cancers of interest are of high incidence and the associations in particular cannot be ruled out by the evidence to date. Studies of cancer mortality are also subject to confounding due to improvements over time in the effectiveness of treatments, which may produce a decline in mortality rates that is unrelated to the incidence of the cancer.

Even if the associations suggested by some studies in this body of weak epidemiologic evidence are true, the absolute risks for additional cancer cases or deaths are small and cannot necessarily be attributed solely to exposure to SV40-contaminated polio vaccine. For example, the 178% increase in the incidence of mesothelioma reported by Fisher and colleagues (1999) implies an increase of 2.2 cases of mesothelioma per 10,000,000 population per year. The increased mortality risk from leukemia in the report by Fraumeni and colleagues (1963) was 3 deaths per 1,000,000 population.

Based on these limitations, the committee concludes that the evidence is inadequate to accept or reject a causal relationship between SV40-containing polio vaccines and cancer.

Biological Mechanisms

Although biological data do not provide an independent basis for evaluating causality, they can help validate epidemiologically based conclusions for or against causal associations. Such data can also guide further investigation when epidemiologic evidence is inconclusive.

The committee’s task was to consider the evidence regarding the potential for SV40-contamination of polio vaccines in the late 1950s and early 1960s to cause cancer. Concern has focused on the four cancers discussed above: mesothelioma, ependymoma, osteosarcoma, and NHL. The committee reviewed the evidence on biological mechanisms related to this hypothesis through three key questions:

1.

Is SV40 a transforming virus?

2.

Can SV40 cause cancer in humans under conditions of natural exposure?

3.

Is contamination of the polio vaccine with SV40 responsible for SV40 infection in humans?

Given that the epidemiologic evidence regarding a causal relationship was inconclusive, the committee reviewed the biological evidence with an eye towards additional research that might be needed to better understand the putative role that exposure to SV40 from polio vaccines might have in cancer.

A wealth of literature exists on topics such as the presence of SV40 in tumors and the effects of the virus or its gene products (particularly the large tumor antigen, Tag) in cell cultures. Several large scientific conferences have also been held to review progress in understanding the role of SV40 in human cancers. Because the Immunization Safety Review Committee was not charged with resolving the full range of uncertainties about the biology of SV40 and the role of this virus in human cancers, the review that follows provides only highlights of the key arguments on these issues. More detailed discussion is available in several excellent and comprehensive reviews (Brown and Lewis, 1998; Butel and Lednicky, 1999; Carbone et al., 1997b; Klein et al., 2002; Strickler, 2001b).

Can SV40 Cause Cancer in Humans under Conditions of Natural Exposure?

Although there is a theoretical basis for mechanisms by which SV40 could cause cancer in humans and mechanistic evidence is accumulating to support the theory, it is not clear which specific mechanisms might be operable. The principal lines of evidence for the operation of specific mechanisms are that SV40 acts in ways consistent with tumorigenesis and that DNA sequences consistent with SV40 have been detected in several types of human tumors.

Evidence that SV40 could be tumorigenic comes from in vitro studies and studies in animals. These studies, some of which were reviewed above, point to the critical role of SV40 Tag, which is found in the nuclei of transformed cells. As noted, substantial evidence suggests that Tag binds and inactivates the products of tumor-suppressor genes, especially the p53 and Rb proteins. The inactivation of these proteins allows for unregulated cell division (Butel and Lednicky, 1999; Klein et al., 2002). Because continuous expression of Tag is required to maintain a transformed cell line in hamsters (Carbone et al., 1997b), the failure to detect SV40 Tag in some human tumors of interest has been cited as a counter argument to a tumorigenic role for SV40 (Strickler, 2001b). However, studies with transgenic mice and with human cells in culture appear to suggest that under certain circumstances, SV40-induced cell transformations can persist with little or no expression of Tag (Carbone et al., 1997b), presumably reflecting the development of additional transforming mutations or genetic instability as a result of earlier abolition of cell cycle checkpoint fidelity by Tag.

Data on the association between SV40 and human tumors are inconsistent. A growing body of clinical studies reports the detection of SV40 DNA in several types of tumors. The most notable and well-studied of these is mesothelioma (Carbone et al., 1999, 2002). In addition, SV40 DNA has been detected in bone cancers (Carbone et al., 1996), ependymomas (Bergsagel et al., 1992; Lednicky et al., 1995), and in non-Hodgkin’s lymphoma (Shivapurkar et al., 2002; Vilchez et al., 2002). Other studies, however, report an inconsistency or absence of SV40 in mesotheliomas, osteosarcomas, and brain tumors (Engels et al., 2002; Heinsohn et al., 2000; Strickler, 2001a; Strickler et al., 1996). Shown in Table 4 are the reports from some studies in which mesotheliomas, the tumor in which SV40 has been most strongly implicated, were evaluated for the presence of SV40 by a variety of techniques, most commonly by PCR. Due to the potential for PCR to detect sequences from other polyoma viruses found in humans (BK and JC), in most studies the investigators used one or more tests to enhance the specificity for SV40, the most stringent of which is sequencing of the PCR products. Most studies report the detection of SV40 sequences in some of the mesotheliomas, but few also examined normal tissues in parallel. In some cases, the presence or absence of SV40 in tumors was attributed to geographic differences in exposure to SV40 (e.g., in Finland and Turkey, SV40 containing polio vaccines were never used) or to the multifactorial nature of some cases (DeRienzo et al., 2002; Hirvonen et al., 1999; Reuther et al., 2001). The association of Epstein-Barr virus infection with Burkitt’s lymphoma in Africa but not in European countries provides a precedent for geographic variability in outcomes. When SV40 viral fragments or Tag is detected in a tumor sample, it is often present in only a fraction of the cells. Although EBV is reported in only a portion of tumor cells with which it is associated (Grinstein et al., 2002), HPV is found in all cells of the cervical tumors.

Table 4.

SV40 Sequences in Human Mesothelioma.

The conflicting results in the detection of SV40 have also led to questions about technical aspects of the detection of the virus. It is unclear whether positive findings are the result of overly sensitive but nonspecific tests that are detecting other viruses (i.e., BK or JC) or SV40 from laboratory contamination, or whether negative findings arise from a lack of sensitivity in the detection methods used. Two multicenter studies have attempted to resolve some of the uncertainty regarding the detection of SV40 in human mesothelioma samples.

The first of these studies (Testa et al., 1998) was conducted with the goals of determining the presence or absence of SV40 in mesothelioma samples and investigating whether a correlation exists between SV40 and asbestos exposure.¹¹ Participating in the study were four independent laboratories, including one that works with the BK and JC viruses, but not with SV40. Twelve frozen mesothelioma samples, along with positive and negative controls, were analyzed for the presence of SV40 DNA with two different PCR primer sets. Positive results were confirmed by Southern blot hybridization and by DNA sequencing of the PCR products. For 9 of the 12 samples, the results from each of the four laboratories were positive for the presence of SV40 DNA. But the laboratories’ results for the other three samples were inconsistent. The positive and negative controls gave consistently appropriate results. Further study with immunohistochemical techniques in one of the laboratories found that 10 of the 12 samples, including the 9 that were consistently positive for SV40 DNA, contained cells that were positive for SV40 Tag. The authors could not account for the inconsistencies in the detection of SV40 DNA in three of the samples, but they suggested that those samples might have had a smaller proportion of cells with SV40, compared with the other samples.

The formal goals of the second multicenter study were to compare methods for the detection of SV40 DNA, to examine the ability of these methods to detect SV40 in human mesothelioma, and to understand possible reasons for the conflicting findings in past investigations (Strickler, 2001 a). Each of the nine participating laboratories received masked, paired replicate DNA samples from 25 fresh-frozen mesothelioma specimens and one DNA sample from each of 25 normal lungs. Masked positive and negative controls were also included, but contamination of the initial set of negative control samples by the central processing laboratory led to a number of false-positives. Although the problem of falsely positive negative controls was largely, but not completely, eliminated by the generation of a second set of control samples, this raises concerns in the minds of some regarding study reliability. SV40 was detected in both paired samples from one of 25 mesothelioma specimens in one of the laboratories and from a different one of the 25 mesotheliomas in another laboratory. The other participating laboratories did not detect SV40 in either of these two samples, and either did not detect SV40 DNA in any sample or did so on only one of the two replicates of one of the other 25 mesothelioma samples. Because these collaborative efforts failed to resolve why some laboratories detect SV40 and others do not, the presence, specificity and strength of the association between SV40 and certain types of human tumors remain uncertain.

The detection of SV40 in tumors does not, by itself, demonstrate a causal relationship. SV40 could be a passenger virus, infecting the cells but causing no pathology. Findings from studies examining SV40 in mesothelioma (Table 4) demonstrated a great deal of variability, which preclude the ability at present to draw firm conclusions regarding the frequency with which SV40 can be detected in specific neoplasms and/or normal tissues in humans. Some studies have detected SV40 in a variety of normal tissue, blood and cells shed into the urine from healthy subjects (Li et al., 2002; Martini et al., 1996; Woloschak et al., 1995). Its detection in multiple types of tumors indicates a lack of specificity for a single type of cancer, and also leads to doubts about a causal link (Strickler, 2001b). Although not unheard of, infection of such a broad range of tissues is unusual. In healthy rhesus monkeys, SV40’s natural host, the virus infects primarily kidney cells. HPV, a related papovavirus, has been linked only with a single type of tumor—cervical cancer. However, some viruses are associated with more than one type of tumor. Epstein-Barr virus, for example, is associated with Burkitt’s lymphoma and nasopharyngeal cancer. Additionally, because one of the receptors for a SV40 gene product is a widely distributed MHC protein, effects on multiple tissue types might be expected.

The committee concludes that the biological evidence is moderate that SV40 exposure could lead to cancer in humans under natural conditions.

Summary of the Scientific Assessment

In summary, the committee’s scientific assessment concludes that moderate to strong lines of biological evidence support the theory that SV40 contamination of polio vaccine could contribute to human cancers. Specifically, the evidence is strong that:

• SV40 contaminated some polio vaccine used from 1955–1963, and
• SV40 has transforming properties in several experimental systems.

In addition, evidence has accumulated suggesting that SV40¹² is likely present in some human tumors. The data regarding detection of SV40 in many but not all mesothelioma samples, coupled with the evidence for the oncogenic potential of SV40, suggest that SV40 could contribute to cancers in humans¹³. However, it is not clear

• what proportion (if any) of the people exposed to the SV40-contaminated vaccine were infected,
• what proportion (if any) of the human cancers in which SV40 is detected are caused by the SV40,
• that the sole source of SV40 is due to the contaminated polio vaccine, or
• that SV40-contaminated polio vaccine did or did not cause cancer in the vaccine recipients.

Policy Review

Since 1961, FDA has required testing to ensure that polio vaccines used in the United States are free of SV40 contamination. The Advisory Committee on Immunization Practices, the American Academy of Pediatrics, and the American Academy of Family Physicians all now recommend exclusive use of IPV to immunize children and adults in the United States (CDC, 2000). The committee found no indication of any concern that SV40 contamination of IPV in the United States in use today might have escaped detection. Therefore, the committee does not recommend a policy review of polio vaccine by any of the national or federal vaccine advisory bodies on the basis of concerns about cancer risks that might be associated with exposure to SV40, because the vaccine in current use is free of SV40.

Claims have been made that some oral polio vaccines might have been contaminated after 1963 (Kops, 2000). The committee urges that FDA or other agencies address these claims to try to resolve the uncertainty regarding the possibility of exposure to SV40 after 1963. Appropriate assumptions about exposure are essential for conducting valid epidemiologic analyses of the risks that might be associated with contaminated OPV.

Policy Analysis and Communication

The ability of researchers to use epidemiologic studies to assess the possible association of SV40 with certain cancers has been hindered in part by the small numbers of cases for most of the cancers of concern, but also by substantial uncertainty in classifying individuals as exposed or unexposed.

The uncertainty as to exposure arises in large part because of lack of detailed information on the levels of SV40 in the vaccine and on who received the vaccine. Testing of stored samples of IPV used in 1955 showed that the levels of SV40 varied among vaccine lots and that some lots were not contaminated (Fraumeni et al., 1963). But samples of vaccine produced through the remainder of the period of likely contamination (1955–1961) were not available for testing, leaving investigators with little basis for identifying differences in SV40 exposure among the vaccinated population. Misclassification of exposure would result in a bias toward the null hypothesis of no difference between the “exposed” and “unexposed” groups.

The committee hopes that contamination of a vaccine never occurs again, but also considers it prudent to have a comprehensive plan in place for prevention of contamination, as well as for response and communication should such an event occur. Pieces of such a plan already exist within the various agencies with responsibility for assuring the safety of vaccines. For example, FDA has regulatory authority over the production of vaccines. Currently, all vaccines licensed by the FDA are required to fulfill general safety, sterility, and purity requirements (Code of Federal Regulations, 2001). For example, the cell substrates used to produced the MMR vaccine are from flocks free of the avian leukosis virus. The MMR vaccine also undergoes testing for adventitious viral activity. Cell lines used to produce the poliovirus vaccine are tested for (in addition to SV40) tuberculosis, herpes viruses, measles, and other infectious agents (CDC, 1997). However, the committee is not aware of a comprehensive system that is transparent and clearly understandable to, and accessible by, the public. The most recent comprehensive plan put together by the federal government on vaccine safety does not address contamination issues (NIH, 1998).

The committee recommends that the appropriate federal agencies develop a Vaccine Contamination Prevention and Response Plan. The appropriate agencies should be given the authority and resources to implement the plan once it is in place. This plan should identify the procedures already in place or those that need to be developed, for example, to prevent contamination of vaccine during the manufacturing process. In addition, the plan should include strategies for routine assessment of vaccine for possible contamination; notification of public health officials, health care providers, and the public if contamination occurs; identification of recipients of contaminated vaccine; and surveillance and research to assess health outcomes associated with the contamination. Clearly the plan will need to allow for the scientific and technical uncertainties surrounding an assertion that contamination has occurred or is possible. Implementation of the plan will require considerable judgment as to the level of response required to deal with a specific contamination concern. Because the plan will involve multiple agencies and offices, the National Vaccine Program Office is probably the best positioned to organize and coordinate the development of the plan.

The development of the plan should be the subject of broad and open discussion by the regulators, policymakers, manufacturers, researchers, vaccine administrators, and the public. Once a plan is developed, a communication campaign should be undertaken to inform the public and medical practitioners. Open discussion and communication are important to ensure that trust in the vaccine supply is deserved and is widespread. The committee noted several specific issues that should be considered in developing a Vaccine Contamination Prevention and Response Plan.

A program to store samples from each vaccine lot approved for release should be considered. This would make it possible to test for contaminants should safety questions arise well after the vaccine was used or as new detection techniques become available. Manufacturers are currently required to store a sample of each released lot only until the expiration date of that lot.

Consideration should also be given to developing better mechanisms to identify the recipients of vaccine from specific lots. The committee recognizes that this is increasingly difficult. With a more mobile population and less continuity of medical care, an individual’s immunization records are often scattered or incomplete. Vaccinations provided at public clinics or other sites that are not connected with usual sources of medical care may not be recorded. Moreover, regulations intended to protect individual privacy restrict the use of medical records. The large cohorts that were included in the some of the epidemiologic studies reviewed here (e.g., Heinonen et al., 1973; Fraumeni et al., 1970; Innis, 1968; Stewart and Hewitt, 1965) would be almost impossible to establish without laying the appropriate groundwork (e.g., consent for contact for future research studies) at the time of vaccination.

The Vaccine Safety Datalink (VSD) program has been one attempt to improve access to data linking health outcomes with specific vaccine exposures. This collaboration between CDC and several health maintenance organizations covers about 2.5% of the U.S. population (CDC, 2001). It has been used to investigate certain questions, including intusseption following rotavirus vaccine, and the risk of seizures following vaccination, but larger populations are needed to study rare outcomes, like most of the cancers of concern in this report.

Immunization registries may be another way to meet the need for information on vaccine recipients, but they are still evolving. Most are currently designed to capture information on immunizations given to children under 6 years of age. However, registries are not yet available throughout the country and where they are operating, some immunization providers do not submit data (CDC, 2002b). Even with the benefit of registry data, the challenges of tracing individuals remain. And in their current form, immunization registries do not capture information on vaccine given to adolescents or adults.

Research

The committee notes that data are accumulating rapidly on many fronts on the potential for viruses to cause cancers in humans, particularly if exposure is early in life. The committee supports continued research in viral oncogenesis broadly, independent of the question of SV40-contamination of polio vaccine. However, the research recommendations are focused specifically on the question of the polio vaccine, in keeping with the charge to the committee.

The committee recommends development of sensitive and specific serologic tests for SV40. These would be helpful to resolve the question as to whether or not the SV40 exposure led to infection.

The committee recommends the development and use of sensitive and specific standardized techniques for SV40 detection. Included in these efforts should be a documentation that: 1) all test specimens are masked, 2) positive and negative control tissues are both used and are both subjected to the same processing procedures as test specimens, 3) samples are tested in replicate, and 4) an adequate sample of tissue is available.

The committee recommends that once there is agreement in the scientific community as to the best detection methods and protocols, pre-1955 samples of human tissues should be assayed for the presence or absence of SV40 in rigorous, multicenter studies. These tests would not address the question of whether or not SV40 can cause cancer, but they could influence the interpretation of some epidemiologic and clinical analyses. They also would be relevant for discussion of the relative contribution of contaminated polio vaccine to the burden of SV40 infection in humans.

The committee recommends further study of the transmissibility of SV40 in humans. This will help confirm whether and why SV40 or antibodies specific for SV40 are detected in individuals who have no known exposure to potentially contaminated polio vaccine, animals or laboratory contact. Resolving the issue of transmissibility of SV40 is not directly relevant for a causality assessment, but it would be useful in some of the ancillary debates about the role of the polio vaccine contamination in the cancer burden.

In addition to the research recommended above, it is important to resolve the extent of SV40 contamination of past polio vaccine. The uncertainty of exposure makes interpretation of the epidemiologic studies very problematic. In addition, concerns that the oral polio vaccine had been more widely contaminated than assumed require attention. The issue should be resolved publicly to the satisfaction of the public as well as the scientific and policy-making communities.

If researchers can pursue these strategies and obtain a better understanding of SV40 exposure and methods of detection, more meaningful case-control studies can be undertaken to help resolve the question of causality. Until some of the technical issues are resolved, the committee does not recommend additional epidemiologic studies of people potentially exposed to the contaminated polio vaccine.