polio vaccine scandal

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Scandals - 10/25/02

The Institute of Medicine Review Of SV40 Contamination of

Polio Vaccine and Cancer

 

The Institute of Medicine (IOM) just released its report on the possible

relationship between SV40, known to have once contaminated the polio vaccine,

and cancer. The committee got a lot right in this mostly excellent review in

which they concluded that the " evidence is inadequate to accept or reject a

causal relationship " . Nevertheless, some aspects of the report warrant

discussion.

 

Although the committee left open, and rightly so, the possibility that a

relationship might yet be proven to exist, and admitted to supportive biological

evidence, in my opinion they did not go far enough in communicating what is

known about the strength and weight of the evidence in favor of there being a

connection.

 

In the abstract to their executive summary, they reported that " Although SV40

has biological properties consistent with a cancer-causing virus, it has not

been conclusively established whether it might have caused cancer in humans. "

While they are probably right to say that it has not been proved that SV40

causes cancer in humans, it is a huge understatement to say that " SV40 has

biological properties consistent with a cancer-causing virus " . There is a large

and considerable body of scientific evidence supporting the notion that SV40 is

associated with cancer, in both animals and humans. As leading SV40 scientist

Carbone titled one of his studies, referring to the preponderance of the

evidence, " Simian virus 40 and human tumors: It is time to study mechanisms. "

 

They then go on to say, " Studies of groups of people who received polio vaccine

during 1955-1963 provide evidence of no increased cancer risk " .

 

Talk about sins of omission! While it is true that there are studies which

provide evidence of no increased cancer risk, it is equally true that there are

studies which provide evidence that such a relationship exists, some of them

even having been discussed in the report (e.g.., Fisher et al, Farwell et al),

although apparently ignored in order to be able to make that statement.

 

One seemingly relevant article was not even included in their review, a 1984

article by Farwell et al, in which it was concluded that " ...an excessive

number of children born in the period 1954 to 1958 have developed

medulloblastomas. " Why was this article omitted?

 

Saying the epidemiologic studies in this review " provide evidence of no

increased cancer risk " seems particularly arguable, however, given that the

committee found every single epidemiologic study they reviewed to be weak, on

both sides of the argument. I have no problem with their conclusion that the

" evidence was inadequate " , given the apparent paucity of good epidemiologic

data, but it would have been better to have made just that point, to have

reported that the few results that exist are mixed and why, than to have made it

incorrectly seem that all available evidence was against there being a

relationship.

 

Equally important are the reasons why some studies, especially more recent ones,

show no evidence of increased risk. Among the most important of them is the

fact that SV40 has now spread beyond those who received the polio vaccine. As

noted, " ...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... A limited number of people are known to have been

exposed to SV40 through other vaccines...Evidence of SV40 exposure has also been

detected in serologic samples obtained before 1955 and from studies of persons

too young to have received contaminated polio vaccine. Detection of SV40 in

persons too young to have received contaminated polio vaccine suggests the

possibility of continuing transmission of SV40 through means other than the

polio vaccine. Possible sources of exposure to SV40 are person-to-person

transmission, animal-to-person transmission, and laboratory exposure to SV40. "

 

Consequently, unless study participants are accurately tested for evidence of

exposure to SV40, they might well be in the wrong study group, thus confounding

the results and making them meaningless. (Which is one of the reasons the IOM

correctly recommended no epidemiologic studies until the methods of detection

and measurement improved.)

 

(By the way, had the question been assiduously studied when it was first

discovered that SV40 contaminated the vaccine, before it had spread beyond the

polio-vaccinated, epidemiologic studies would not have faced this particular

obstacle to the degree they do now.)

 

And while the committee did provide the caveat that " The presence of antibody

might also indicate protective immunity against SV-40 induced oncogenesis " ,

nowhere in their recommendations is there a call for better understanding of the

meaning and significance of SV40 antibody production, including whether or not

the assumption that antibody production necessarily follows exposure is correct,

the possible misunderstanding of which could well result in additional

confounding.

 

Perhaps most troubling, however, is the large number of references not included

in the review by the IOM, including many cited in my March column on SV40 and

cancer.

 

Why were so many relevant articles not included in the IOM review? Shouldn't

any review by the IOM be exhaustive? Given the import attributed to any of

their findings, what justification could the IOM possibly have for leaving out

even one relevant study?

 

In the end, the committee concluded that because of the " flawed " epidemiologic

evidence, and in spite of the " moderate " to " strong " biological evidence,

" evidence was inadequate to conclude whether or not the contaminated polio

vaccine caused cancer " .

 

This conclusion may be reasonable, based on the state of the evidence.

Regardless, a point needs to be made about the weight they give to the two types

of evidence. Their failure to give more weight to biological evidence was at

least partly based on their stated position that " Epidemiologic studies carry

the most weight in a causality assessment " , and their apparent view that the

problems with the studies examined in this review generally revolved around

their poor design (i.e., being " flawed " ), rather than in anything inherently

weak in that kind of study. But all epidemiologic studies are not created

equal. According to Epidemiology in Medicine, " Intervention studies (a.k.a.

" experimental " , " clinical " , or " prospective cohort " trials) are often

considered as providing the most reliable evidence from epidemiologic research. "

Case-control and non-prospective cohort studies do not carry the same weight,

nor are they necessarily superior to evidence gleaned from studies of biological

mechanisms.

 

In an earlier column, a similar point was made about problems associated with

rejecting or minimizing biological evidence due to an alleged lack of

epidemiologic supportive evidence. And while in this case biological evidence

was not rejected outright in favor of epidemiologic evidence, the committee

underplayed the strength of the association, even though they readily

acknowledged the " wealth of (biological mechanism) literature " available to

them. The sheer size of the biological evidence in favor of there being a

relationship between SV40 and both human and animal cancers should probably have

counted for more than was evidenced in their conclusion.

 

Also worth noting is the fact that while no member allegedly had a financial

interest in the issues discussed in this review, it was disclosed that " all the

committee members share the view that immunization is generally beneficial " .

Perhaps different conclusions would have been drawn if they had had a better

balance of members, including a few who were neutral about the benefits of

vaccination, as well as others who leaned against them, in order to avoid the

obvious potential for bias. Maybe then we might have received a report

completely worthy of the attention this one will, in all likelihood, receive.

 

The truth is, the only way to conclusively prove causality in humans is through

human experimentation, preferably through prospective, randomized, double-blind,

placebo-controlled longitudinal studies. That would mean administering SV40 to

some and not to others, and comparing the results over time. Clearly, this is

not going to happen, nor should it. The next best thing would be to study over

time otherwise matched groups with knowledge of and/or evidence of exposure to

SV40 virus, comparing the outcomes, and determining what, if any, biological

mechanisms explain any differences, to whatever extent they occur.

 

The strength of the biological mechanism evidence definitely raises a red flag,

regardless of the weakness of the epidemiological evidence. The recommendations

the committee made will go a long way toward correcting what is missing. But

until the problems are resolved, and resolved they must be, which detract from

our ability to conduct meaningful epidemiologic studies, all we are left with is

the " moderate " to " strong " biological mechanism evidence, and our conclusion,

until then, should be one of guarded but heightened concern.

 

Sandy Mintz

 

 

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