Questioning Offit. Part 14. Mercury and autism
Previous parts:
Part 1: Introduction
Part 2: DTP
Part 3: Encephalopathy
Part 4: Dravet syndrome
Part 5. SIDS
Part 6. West syndrome
Part 7. Hib
Part 8. Hepatitis B
Part 9. Pneumococcus and rotavirus
Part 10. HPV
Part 11. Lawsuits
Part 12. Wakefield
Part 13: MMR and autism
The smart way to keep people passive and obedient is to strictly limit the spectrum of acceptable opinion, but allow very lively debate within that spectrum.”
― Noam Chomsky
Chapter 6 (continued)
All the epidemiological studies that supposedly prove there’s no link between vaccines and autism fall into two categories. The first group claims to show that the MMR vaccine isn’t associated with autism, while the second argues that the mercury-based preservative thimerosal isn’t linked to autism. In the previous section, we examined the studies on the MMR vaccine; in this one, we’ll look at the studies on thimerosal.
Offit:
In 1999, the American Academy of Pediatrics and the Centers for Disease Control and Prevention worried that children might be receiving too much mercury in vaccines, called for pharmaceutical companies to remove a mercury-containing preservative called thimerosal... Now, parents weren’t scared just of MMR; they were scared of any vaccine that contained thimerosal.
Reading this, you might get the impression that the American Academy of Pediatrics and the CDC were genuinely worried about children’s health and decided to remove mercury from vaccines “just to be safe.” That’s not really the case. In fact, no one seemed concerned about mercury in vaccines. What actually happened was that in 1997 a new law required the FDA and other agencies to review mercury levels in various products and issue a public report. Suddenly it turned out that vaccines contained far too much mercury—dozens of times higher than the official safety limit of 0.1 mcg/kg/day.
Back in the 1990s, as the vaccination schedule expanded with new shots for hepatitis B, hepatitis A, and Hib, no one seemed bothered by the fact that all of them contained mercury. When it came to giving children more and more vaccines with thimerosal, no one appeared troubled that the only clinical study of the substance dated back to 1929. That study tested thimerosal on 22 patients with meningitis. Every single one of them died—seven within the first 24 hours after injection.
What’s more, as early as 1982 an FDA advisory panel had recommended removing all mercury-based preservatives, including thimerosal, from topical products such as skin-lightening creams, ear and eye drops, and nasal sprays. The experts concluded that these products should be reclassified as “not generally recognized as safe.” An FDA memo at the time noted that mercury was an unreliable preservative. In fact, thimerosal was described as being “no more effective than water in protecting mice from lethal streptococcal infection.” It turned out to be more toxic to healthy cells than to bacteria. For example, in chick embryonic heart tissue, thimerosal proved to be 35 times more poisonous than it was to Staphylococcus aureus.
By 1992, Scandinavian countries had banned the use of thimerosal in vaccines altogether. But that didn’t stop the American Academy of Pediatrics (AAP) and the CDC from continuing to add new thimerosal-containing vaccines to the schedule. It wasn’t until 1998 that the FDA finally banned thimerosal from topical medications—adopting the very recommendation its own advisory panel had made 16 years earlier. Even then, neither the AAP nor the CDC saw any problem with the fact that mercury was still present in vaccines.
After the mercury-reporting law went into effect, the CDC launched the Verstraeten study, which found an association between mercury in vaccines and autism (we’ll come back to that study later). In other words, the very “groups” that, as Paul Offit puts it, “believed mercury in vaccines caused autism” included the CDC itself.
So how did it happen that, year after year, no expert noticed that with each new vaccine, infants were being exposed to ever-higher doses of mercury—only to “suddenly” notice later on? The answer comes from one of the architects of America’s vaccination program, Neal Halsey, a member of the AAP Committee on Infectious Diseases and also a member of the CDC’s Advisory Committee on Immunization Practices (ACIP). The issue was that manufacturers listed mercury content as a percentage—0.01%. A hundredth of a percent sounded negligible. But once that number was converted into micrograms, the result didn’t look small at all. In fact, it was alarmingly high.
My first reaction was simply disbelief, which was the reaction of almost everybody involved in vaccines,'' Halsey says. ''In most vaccine containers, thimerosal is listed as a mercury derivative, a hundredth of a percent. And what I believed, and what everybody else believed, was that it was truly a trace, a biologically insignificant amount. My honest belief is that if the labels had had the mercury content in micrograms, this would have been uncovered years ago. But the fact is, no one did the calculation.'
In other words, when deciding which new vaccines to add to the schedule, not a single expert at the FDA, AAP, CDC, or any of the other so-called “world’s leading health agencies” ever bothered to calculate how many micrograms of mercury an infant was actually getting through vaccination. Everyone simply assumed that “one-hundredth of a percent” must be a trivial amount.
Since 2003, children’s vaccines in the U.S. have officially been mercury-free. But that doesn’t mean kids stopped receiving mercury from vaccines. Beginning in 2002, the CDC started recommending flu shots for infants, and the only licensed option for them at the time contained thimerosal. Flu vaccines with thimerosal were also recommended for pregnant women. Starting in 2010, infants were given two doses of the flu vaccine, and then one dose every year after that.
So in practice, even though thimerosal was removed (or almost removed) from most other vaccines, the actual amount of mercury children were getting from vaccines after 2000 stayed about the same—and over the course of a lifetime, it roughly doubled. Thimerosal was also left in one meningococcal vaccine and in one tetanus-diphtheria vaccine. And in much of the rest of the world, thimerosal continued to be used in childhood vaccines. In 2012, the AAP, together with the WHO, persuaded the UN not to ban mercury in vaccines.
Offit:
Again, the academic and public health communities responded, performing six large epidemiological studies examining the risk of autism in children who had or hadn’t received vaccines containing thimerosal. The results were reproducible and clear: thimerosal didn’t cause autism.
Offit cites eight epidemiological studies on thimerosal. Most of the large-scale studies on this subject were carried out in Denmark and other European countries, where thimerosal use in vaccines was far lower than in the U.S., and where autism prevalence was also significantly lower.
The authors analyzed data from Danish psychiatric clinics covering the years 1971 to 2000. They identified 956 children with autism and concluded that after thimerosal was removed from vaccines in 1992, autism rates actually went up. But this study has a number of serious methodological flaws:
- Before 1994, the dataset included only hospitalized patients. Starting in 1995, it also included outpatients. In other words, the inclusion criteria expanded dramatically just two years after thimerosal was removed, which artificially increased the number of autism cases—since outpatients outnumbered inpatients by a factor of 4 to 6. In another paper by the same authors, based on the same dataset, the number of outpatient cases was reported to be as much as 13 times higher.
On top of that, the largest psychiatric clinic in Copenhagen, which accounted for about 20% of all autism cases in Denmark, wasn’t included in the registry until 1993. This, too, created an artificial jump in cases after thimerosal was phased out. The authors never disclosed or adjusted for these factors.
- In 1994, Denmark switched from ICD-8 to ICD-10 diagnostic codes. That change alone could have increased autism diagnoses by as much as 25-fold.
- The original version of the article included data through 2001, which actually showed a decline in autism rates. That version was submitted to The Lancet, but it was rejected. Then the authors tried JAMA, which also turned it down, so they finally sent it to Pediatrics. On top of that, the CDC sent a letter to Pediatrics urging them to publish the study as quickly as possible. One of the Pediatrics reviewers pointed out that the decline in 2001 might indicate a beneficial effect of removing thimerosal. In response, the authors simply deleted the 2001 data from the paper, and it was published. Had those data remained, the results would have matched later research showing a decline in autism prevalence in Denmark after 1992—from a peak of about 1.5% in 1994–1995 down to 1% in 2002–2004.
- Several of the study’s authors worked for a Danish pharmaceutical company that manufactured vaccines, but this conflict of interest wasn’t disclosed in the publication. Unlike in the Wakefield case, nobody seemed bothered by it. In addition, one of the coauthors was Poul Thorsen, later indicted for embezzling more than a million dollars from the CDC.
- Since this is the largest study ever conducted on thimerosal and autism, it has become the centerpiece of the argument that there’s no link between them—despite all the methodological problems, despite the fact that two journals rejected it, despite the authors cutting out data that contradicted their conclusion, and despite the conflicts of interest. This paper is still held up as the key piece of evidence supposedly proving that thimerosal has nothing to do with autism.
2. Stehr-Green (2003, Am J Prev Med)
This study was published in response to a California analysis that had found a correlation between thimerosal exposure and autism. The authors compared data from three countries: Denmark, Sweden, and the U.S. (California).
- The Danish data were the very same used in Madsen’s study (see above), with all the same methodological problems and distortions.
- The Swedish data were based solely on hospital visits, which captured only a fraction of actual autism cases and didn’t reflect real prevalence.
- Some of the original California data were excluded from the final publication, which could have significantly affected the conclusions.
- The study was funded by the CDC and by the same Danish pharmaceutical company that produced vaccines.
Another population-based study from Denmark, this one looked at children born between 1990 and 1996 (440 autism cases and 787 cases of ASD). The authors concluded not only that thimerosal didn’t cause autism, but that it actually seemed to reduce the risk. That conclusion directly contradicts all existing biological research, which consistently shows thimerosal to be neurotoxic even at very low doses.
- The authors note that the average age of autism diagnosis was 4.7 years, yet they still included one-year-olds in their analysis. Obviously, autism is rarely diagnosed at that age. In other words, they knowingly added noise to the dataset in order to dilute any real signal.
- Instead of counting the number of children diagnosed with autism, they calculated person-years of observation. This makes no sense for chronic conditions. With this method, each age group is weighted equally, even though the likelihood of diagnosis is far lower in the younger groups. This artificially inflates the weight of early diagnoses (which are relatively rare) compared to later ones. The result: even more noise, obscuring any true signal.
- Several of the study’s authors worked for a Danish pharmaceutical company that manufactured thimerosal-containing vaccines. This conflict of interest was not disclosed in the publication.
A retrospective cohort study conducted in the UK among 110,000 children (including 104 autism cases). The authors concluded that thimerosal was not associated with autism risk and even appeared to protect against developmental delay—again, findings that flatly contradict biological evidence.
- Instead of using the standard statistical method for such studies (a regression analysis with thimerosal exposure as the independent variable and autism as the dependent variable), the authors used a more complex multivariate regression. In that model, two key independent variables—thimerosal exposure and year of birth—were highly correlated. This introduced a problem of multicollinearity: as birth year increased, so did thimerosal exposure. That made it impossible to reliably distinguish the effect of one variable from the other. Without analyzing these factors separately, the statistical significance of the results cannot be established.
- Despite repeated requests from other researchers for the raw data so it could be properly reanalyzed, the authors refused to share it.
- The study was funded by the WHO. Later, emails obtained through FOIA revealed that the funding had in fact been approved by CDC staff themselves. Several of the authors had conflicts of interest due to ties with UK vaccine manufacturers. One of the coauthors, Elizabeth Miller, was an architect of Britain’s national vaccination program and testified in court on behalf of pharmaceutical companies in lawsuits brought by vaccine-injured plaintiffs.
5. Thompson (2007, N Engl J Med)
A CDC study designed to evaluate whether thimerosal was associated with neuropsychological problems, based on parent surveys. The authors examined 42 different neurological outcomes but, for some reason, excluded autism. The lead author was William Thompson—the same CDC scientist mentioned in the previous chapter who later admitted that his colleagues destroyed data showing a link between the MMR vaccine and autism.
In 2014, in a series of phone conversations with Dr. Brian Hooker (which Hooker recorded), Thompson confessed that CDC leadership had put enormous pressure on him to “water down” the original results, which had shown associations between thimerosal and certain neurological problems—especially motor tics. Among other things, he said: “I have great shame now when I meet families with kids with autism because I have been part of the problem.”
- The study did, in fact, find an association between thimerosal and motor tics (that result couldn’t be fully watered down). The authors even acknowledged that two other studies had also found a thimerosal–tic association. Despite this, they dismissed the finding as a coincidence rather than a causal link.
- Only 30% of the 3,600 children selected for the study actually participated. The majority of parents declined, raising the risk of selection bias. Children born with a birthweight under 2.5 kg were also excluded.
- The study was funded by the CDC, and 7 of its authors had financial ties to pharmaceutical companies.
6. Schechter (2008, Arch Gen Psychiatry), Fombonne (2006, Pediatrics)
Neither of these studies had a control group. Both concluded that since thimerosal use in vaccines declined in California (Schechter) and Quebec (Fombonne), yet autism rates continued to rise, thimerosal could not be a factor.
- First, the authors assumed thimerosal use ended in 2001. That’s not quite true. Production of thimerosal-containing vaccines mostly stopped in 2001 (except for flu shots), but existing stocks continued to be used until 2003.
- Second, and far more important, the vaccination schedule expanded during the study years—something the authors failed to account for.
- Third, Eric Fombonne, author of the Quebec study, testified as an expert witness on behalf of vaccine manufacturers in thimerosal lawsuits. In his study, he examined autism prevalence in several Montreal schools (180 children total). Yet he compared autism prevalence in Montreal to vaccination coverage in Quebec City—265 kilometers away. When this was pointed out to him, Fombonne declined to comment. Pediatrics also refused to publish a letter from the researcher who uncovered this discrepancy.
Another UK study that found no link between thimerosal and developmental problems. But autism wasn’t even examined, which makes it irrelevant to the debate. The study was funded by the WHO.
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It’s striking that nearly all of the authors of these studies had conflicts of interest—conflicts they often failed to disclose. Yet unlike Wakefield, no one accused them of misconduct, and none of their studies were retracted.
Here, I’ve only given a brief outline of the methodological flaws and conflicts of interest in these papers. They are analyzed in more detail here and here.
Verstraeten (2003, Pediatrics)
Offit doesn’t mention one of the key studies on this subject—Verstraeten, 2003—and now we will see why. This was a CDC study that essentially kicked off the whole saga about thimerosal and autism. Although it was finally published in 2003, work on it began at least four years earlier and went through five different drafts. By the time the final version appeared, it had little in common with the original results.
The study analyzed the closed Vaccine Safety Datalink (VSD). In the published 2003 version, the authors reported that they found no link between thimerosal and autism. But internal documents obtained through the Freedom of Information Act told a very different story.
Phase One (Nov–Dec 1999, internal presentation). According to the November analysis, infants who received more than 25 mcg of mercury from vaccines and immunoglobulins in their first month of life were 7.6 times more likely to develop autism than those who received none. In the December analysis, the risk rose to 11.35 times higher. Elevated risks were also found for ADHD, motor tics, sleep disorders, and developmental delays.
Phase Two (Feb 2000). After his colleagues in the CDC suggested methodological changes to reduce the signal, Verstraeten presented a second analysis. Infants who received more than 62 mcg of mercury from vaccines before three months of age had a 2.5-fold higher risk of autism compared to those who got less than 37 mcg. Among other changes, Verstraeten excluded children who had received hepatitis B immunoglobulins (which also contained thimerosal). He admitted he was at a loss for what else to do. On December 17, 1999, he sent an email to colleagues, including Frank DeStefano, titled “It just won’t go away.” In it, he wrote that he couldn’t find any explanation other than thimerosal causing autism, and asked for help coming up with alternative explanations.
Phase Three (June 2000). While working on the third phase, on March 9, 2000, Verstraeten emailed his colleagues again. He wrote that among children who had skipped their first Hib and DTP shots (both thimerosal-containing), the risk of developmental delay began to decrease. This supported his hypothesis that “the key is not to vaccinate before three months of age, after which the effects gradually diminish.”
He also noted that among premature infants who received 200 mcg of mercury, the risk of developmental problems was five times higher than in those who received 100 mcg. By June 2000, after new statistical adjustments, Verstraeten presented Phase Three, showing the autism risk reduced to 1.69 (meaning thimerosal increased autism risk by 69%). Every additional microgram of mercury raised the risk of developmental disorders by 0.7%.
These findings were presented at a closed-door two-day CDC meeting on thimerosal in 2000, known as the Simpsonwood meeting. Dozens of doctors, scientists, and pharma representatives gathered to decide what to do with the results. At the meeting, Dr. Clements of the WHO remarked that "perhaps this study should not have been done at all, because the outcome of it could have, to some extent, been predicted and we have all reached this point now where we are left hanging...". Another participant asked, “What if the lawyers get hold of this? There‘s not a scientist in the world who can refute these findings.”
Professor Stehr-Green, author of one of the studies mentioned earlier, later wrote that despite a “prolonged reanalyses, the data still showed a slight tendency for groups with higher exposure to thimerosal-containing vaccines to have higher rates of the same neurobehavioral outcomes”.
CDC statistician Philip Rhodes suggested a way to “water down the signal” by reinserting into the dataset thousands of children with congenital conditions who had originally been excluded—thus “adding noise” to reduce statistical significance. In the end, that’s exactly what was done.
After the conference, Verstraeten wrote to colleagues:
I do not wish to be the advocate of the anti-vaccine lobby and sound like being convinced that thimerosal is or was harmful, but at least I feel we should use sound scientific argumentation and not let our standards be dictated by our desire to disprove an unpleasant theory.
Phase Four (July 2001). To further reduce statistical significance, the data were divided by HMO's. In one HMO, the signal disappeared entirely, lowering the autism risk to 1.58. Soon after, Verstraeten left the CDC for GlaxoSmithKline, a manufacturer of thimerosal-containing vaccines. By the time the study was published in 2003, he had already worked at GSK for two years, yet the paper still listed him as a CDC employee. This conflict of interest caused no uproar, and the publication was never retracted (unlike Wakefield’s paper, which The Lancet pulled for far less).
Phase Five (2003). To drive the autism risk down even further, the authors relied on partial datasets and included children aged 0–3 years—even though the average age of autism diagnosis at the time was 4.4 years. Including such young children diluted the risk and weakened significance; about 40% of the sample fell into this group. Moreover, while autism prevalence at the time was 50 per 10,000, the study reported only 11 per 10,000—meaning 80% of cases simply vanished. In addition, developmental disorders were split into separate categories, further reducing statistical significance. The authors also excluded diagnoses made outside of the health plan. If a child saw an outside specialist (very common in the U.S.), the diagnosis wasn’t counted. As a result, 60% of ADHD cases, 50% of speech disorders, and 20% of autism cases were omitted.
Finally, in the published version, the control group was not children who had received no thimerosal at all, but those who had received up to 37.5 mcg—naturally lowering the apparent risk.
Still, despite all the statistical maneuvers, the study couldn’t completely hide the increased risk of speech delays and motor tics.
We only know this whole story because the study was conducted by the CDC, which—under the Freedom of Information Act—was required to release all the correspondence related to it. The FOIA requests came from SafeMinds, a parents’ organization whose children were harmed by vaccines. It’s a rare chance to see, from the inside, how an epidemiological study is actually done—and how data manipulation reduced an eleven-fold autism risk linked to thimerosal down to statistically insignificant levels. It’s reasonable to assume that other studies were carried out in much the same way.
Even setting aside the fact that none of these studies compared vaccinated children with completely unvaccinated ones (a study that could easily have been done using existing data), not one of them offers a simple, straightforward comparison of two groups. Instead, the authors invariably manipulate age categories, use person-years for chronic conditions, analyze only subsets of the data, manipulate the control group, withhold the raw data from outside review, and most likely do this for the same reasons it was done in the Verstraeten study.
A handful of epidemiological studies on MMR and autism, plus a handful on thimerosal and autism—none of which provide access to their raw data (and some of which actually found a link but discarded it)—are the foundation for the media’s twenty-year mantra: “Vaccines don’t cause autism.” Outside of these studies, there’s essentially nothing.
Meanwhile, a 2013 study analyzing the same database as Verstraeten found that the thimerosal-containing hepatitis B vaccine was linked to a threefold higher risk of autism. A 2010 study, based on a different dataset, found the same thing: boys vaccinated against hepatitis B on their first day of life were three times more likely to develop autism compared to those vaccinated later—or not at all. [1] Yet unlike Verstraeten, which is still cited as evidence, these studies are almost never mentioned. After the Wakefield affair, top journals largely stopped publishing any research that questions vaccine safety. As a result, authors are forced to publish in less prestigious journals, where such papers rarely get taken seriously.
There are also several other epidemiological studies—some using the very same VSD database on which the Verstraeten study was based—that concluded thimerosal in vaccines is associated with autism. Their authors, Mark and David Geier, are the only independent researchers who were ever granted access to the VSD. [1] [2] [3] [4] [5] and so on. These studies are simply dismissed. The Geiers also tried to gain access to the raw data from the Verstraeten study, but the CDC told them the data had been lost.
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There is no shortage of animal studies demonstrating the toxicity of thimerosal and its link to neurological disorders. Here are just a few: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12].
The symptoms of autism resemble those of mercury poisoning. A 1996 case study describes a 44-year-old man who contracted hepatitis B, C, and D through a blood transfusion. He underwent a liver transplant and was prescribed both intramuscular and intravenous injections of hepatitis B immunoglobulin.
Just three days after beginning treatment, he developed paranoid thoughts. By the fourth day, he was struggling to walk and speak. Soon after, he experienced gastritis, tremors, violent arm jerks, and complete loss of speech—though he still understood everything. In a matter of days, the man had essentially become autistic.
At that time, all hepatitis B immunoglobulins contained thimerosal as a preservative. By the onset of symptoms, the man had received a total of 20 mg of thimerosal. He was diagnosed with mercury poisoning, which was believed to be the cause of his neurological decline. Once chelation therapy was started, his condition gradually improved, and within a few weeks his neurological symptoms disappeared.
The dose he received—20 milligrams of thimerosal—was about 400 times the amount found in a single vaccine. On the other hand, he weighed roughly 40 times more than an infant. Adjusted for body weight, his exposure was the equivalent of about 10 “infant-sized” vaccine doses. But it’s important to remember two things: first, infants don’t receive just one dose, but several; and second, their blood-brain barrier is far more permeable than that of an adult. In that light, the man’s exposure is comparable to what many infants received.
Institute of Medicine Reports
Following the secret CDC conference, the agency commissioned the Institute of Medicine to produce a report on thimerosal and autism, funding it with $2 million. The first report, released in 2001, concluded that mercury is a known neurotoxin, but that existing data were insufficient to establish a causal link between mercury in vaccines and neurologic developmental disorders of the nervous system. The committee recommended further research, but in the meantime advised that children should continue to be vaccinated with the remaining stocks of thimerosal-containing vaccines.
Offit:
In 2004, after the Institute of Medicine concluded that evidence clearly refuted Wakefield’s hypothesis, Fisher again saw foul play. “This report is a case of political immunology masquerading as real science,” she said. “With it, the Institute of Medicine takes a step toward weakening its reputation as an independent body capable of making an objective scientific analysis [that isn’t] influenced by government policy and industry profits.” As had been the case with diabetes and multiple sclerosis, Barbara Loe Fisher refused to believe that scientific studies exonerating vaccines were anything other than a vast international conspiracy to hide the truth.
In reality, the CDC—the very agency that commissioned and paid for the report—was actively pressuring the Institute of Medicine. Internal meeting transcripts, later obtained by attorneys for families of autistic children, show that as early as 2001, before the committee had even begun its full work, the chair, Dr. Marie McCormick, stated bluntly:
CDC wants us to declare, well, these things (vaccines) are pretty safe on a population basis...
We are not ever going to come down that [autism] is a true side effect.
And the study director, Kathleen Stratton, was even more explicit:
The point of no return, the line we will not cross in public policy, is pull the vaccine, change the schedule. We could say it is time to revisit this, but we would never recommend that level. Even recommending research is recommendations for policy. We wouldn't say compensate, we wouldn't say pull the vaccine, we wouldn't say stop the program.
And sure enough, in 2004, after reviewing the very epidemiological studies described above, the Institute of Medicine reached its final conclusion: there was no causal link between vaccination and autism, and further research in this direction was deemed unnecessary. The IOM dismissed a vast body of toxicological, clinical, and pharmacological evidence pointing to connections between thimerosal and neurological problems, and instead focused solely on the handful of epidemiological studies. Unlike the 2001 report, the 2004 IOM report focused exclusively on autism, which conveniently allowed it to disregard the entire body of evidence linking thimerosal to other neurological disorders. The report concluded that any potential biological mechanisms linking vaccination to autism remained purely “theoretical.”
Barbara Loe Fisher had been right. The Institute of Medicine had indeed undermined its own credibility as an independent scientific institution.
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For a more detailed post on thimerosal in vaccines, see: en.scibook.org/content/vaccines/mercury
An even deeper dive into the subject can be found in the books Evidence of Harm by David Kirby (2005) and Thimerosal: Let
the Science Speak by Robert F. Kennedy, Jr. (2015).
*****
Chapter Six includes a total of 72 footnotes which include 19 studies:
The studies cited tell us that:
Vaccines as unsafe, parents refuse them, or disease outbreaks (7 footnotes)
Studies unrelated to vaccination (3 footnotes)
Studies already discussed in previous chapters (1 footnote)
8 studies are presented as evidence of vaccine effectiveness or safety and were discussed in this and previous entries.