Skip directly to search Skip directly to A to Z list Skip directly to navigation Skip directly to site content Skip directly to page options
CDC Home

This page is a historical archive and is no longer maintained.

For current information, please visit

Press Briefing Transcripts

Early Thimerosal Exposure and Neuropsychological Outcomes at 7 to 10 Years

CDC, New England Journal of Medicine Telebriefing
Moderator: Curtis Allen, September 26, 2007, 1:00 p.m. EST

OPERATOR: Good afternoon, and thank you all for holding. At this time your lines have been placed on listen-only until we open up for questions-and-answers. Please be advised that today′s conference is being recorded. If you have any objections, you may disconnect at this time.

I would now like to turn the conference over to Mr. Curtis Allen.

CURTIS ALLEN, CDC: Thank you for joining us today. My name is Curtis Allen. I′m in the press office at CDC. Joining us today are Anne Schuchat, M.D. She as mistakenly identified in the press – in media advisory as Director of the National Immunization program. She is actually Director of the National Center for Immunization and Respiratory Diseases. That is a National Center for Immunization and Respiratory Diseases.

Also joining us William Thompson, Ph.D. He is an epidemiologist and the lead author of the study. Also joining us today is Dr. John Iskander, M.D. And he is the Acting Director of the Immunization Services Office – Safety Office. Immunization Safety Office, I′m sorry.

As a reminder, the New England Journal of Medicine article is embargoed until five p.m. today. So please, respect the embargo.

We will start with Dr. Anne Schuchat who will have a brief statement and then we will open for questions. Dr. Schuchat.

DR. ANNE SCHUCHAT, DIRECTOR, NATIONAL CENTER FOR IMMUNIZATION AND RESPIRATORY DISEASES, CDC: Hi. Today we′re going to talk about the results of a study that looked at early thimerosal exposure and outcomes in children seven to 10 years of age.

The findings of this study are reassuring about the safety of vaccines. We already have great information about the incredible effectiveness of vaccines in the routine childhood program.

This was a large comprehensive study of about 1000 children conducted with an external consultation and input from a variety of individuals and experts. It looked at a wide range of standardized tests that looked at intelligence, speech, language and motor skills in children between the ages of seven and 10. And it also quantifies how much thimerosal preservative in vaccine or in immunoglobulins these children had exposure to either prenatally or during the first seven months of life.

The study is not an autism study. It did not look at the question of whether thimerosal is associated with autism. The scientific literature to date dose not support a causal link between autism and thimerosal. But it′s important to say this study is not a study of autism. There′s a separate CDC study that′s ongoing that′s going to get at that question to provide more information, but as of now, there is no scientific support for that concern.

I want to tell you a little bit more about this study. It involved identification of about 1000 children and formal standardized testing. The children were brought in for several hours series of tests and the tests that were used were selected based on these external consultants and experts in the field. People with expertise in toxicology, statistics, vaccine safety and also we had representation from an advocacy – an autism advocacy organization on that consultation group.

So with about a three hours worth of testing, there was quite a lot that was looked at, and overall the study results are very reassuring. There were a few statistically significant associations among all of the things that were looked at. And the things that were statistically significant found very small effects and they went in both directions. What I mean by both directions is that some of the results suggested that exposure to higher thimerosal quantities led to better performance. And some of the tests showed that exposure to higher thimerosal content led to worse performance. So the totality of the results are quite reassuring and suggest continued to reaffirm the safety of vaccines.

There is one finding that I wanted to mention in particular that relates to TICS. Motor and phonic TICS are movement disorders. They were looked at in the study in a variety of ways, asking parents whether they had observed these in the past, and also observing the children during the few hours of the testing and having an evaluators report. The parental report of any kind of TIC was not associated with higher thimerosal exposure, but the evaluators observations of the children did suggest in boys, but not in girls a higher risk of TICS among the boys who had higher thimerosal exposure.

The findings for that particular result are similar to findings from two earlier studies. And because of those two earlier studies that issue is being looked at a little bit further with experts here in developmental disability and consultation with some pediatric neurologists.

There are limitations in the way this study was done in interpretation of the TIC finding. Transient TICs are something that come and go that really get better on their own and they are not a helper medical problem. Chronic TICs can only be diagnosed with a longer term observation, and are not the type of thing that could be assessed in a brief exam the way the study was done. So this particular finding may or may not have importance, and it′s being looked at a bit further.

The bulk of the study really found very, very similar performance in children who were exposed to high amounts of thimerosal and children who were exposed to low or no thimerosal. And suggests that the higher thimerosal content that vaccines had back in the 1990s did not lead to harmful effects in children in performance on standardized testing at age seven to 10. That′s very reassuring.

A couple of other comments about the study. There was a transparent process in carrying out this study with where CDC sought input from a variety of outsiders before during and after, trying to help make sure that there was ability for people to participate in the content of how this study was carried out.

Another comment is that the study is about exposure to thimerosal in childhood vaccines. Today in 2007, there are 14 different diseases that children age two can be protected against through immunization.

All but one of the vaccines that children in that age group can get have zero thimerosal in them. One type of vaccine, the influenza vaccine is available in a variety of formulations, thimerosal free and some with thimerosal. And so the vast majority of the pediatric vaccine supply in 2007 does not contain thimerosal.

For parents it′s really important to know that CDC believes vaccines are safe and effective and we have quantified the value that they are providing. Every year′s worth of children that′s immunized in America prevents 33,000 deaths, 14 million infections and saves the country $43 billion. Nevertheless it′s very important for us to evaluate the safety of vaccines before they are ever licensed and to continue to monitor safety and unexpected events adverse consequences or concerns following their wide spread use. CDC is strongly committed to the health of children and we′re also strongly committed to a strong vaccine safety and evaluation effort.

I think we can probably answer specific questions about the study if some of the reporters have them.

CURTIS ALLEN: We′ll open it up for questions now, please.

OPERATOR: Thank you. And at this time, if you would like to ask a question, please press star followed by one on your touch-tone phone. You will be prompted to record your name for proper registration. Your name is required to introduce your question. To withdraw your request, you may press star two. And once again, please press star one to ask a question.

One moment for the first question, please.

Thank you. Our first question come from Judith Graham with the Chicago Tribune, please go ahead.

JUDITH GRAHAM, CHICAGO TRIBUNE: This is – thank you for the press conference and for taking questions. A multipart question.

First is, I note the finding that speech articulation issues are associated with the thimerosal, and you didn′t comment upon that. They are found both among all children and with lower IQ findings among girls. Is this a finding of concern or not? Has it been documented in other studies? What is your comment upon the speech articulation issue?

DR. ANNE SCHUCHAT: Yes, thank you for that question. Let me begin, and then I′ll let Dr. Thompson expand. A rather technical aspect of this study is the very large number of tests that were done and the very large number of statistical comparisons that were made. There were 42 different kinds of tests that were carried out and there were 378 individual statistical comparisons. When you do a very large number of statistical comparisons, the chance of finding things that are abnormal, that are higher or lower in a high thimerosal group is pretty high.

So we would predict by chance alone that 19 of the first 378 individual statistical tests that we ran would be abnormal just because of this chance situation. And, in fact, we found 19 individual tests or just exactly five percent of the very large number of tests that were abnormal.

Now, just 12 of the tests suggested better results in the standardized testing from kids who had high thimerosal exposure. And seven of the results suggested worse performance in the kids who had high thimerosal exposure. The individual tests or the individual statistical comparisons don′t necessarily tell us as much (INAUDIBLE) the picture together because these neuropsychological tests are really a tapestry and interpretation is looking at consistency. So our interpretation of, you know, the speech articulation finding is that it is a random chance event.

The reason that I made additional comments about the motor and phonic TICs was because even though that could easily be just that statistical chance thing, that particular results had been identified in earlier studies. And so that led us to feel additional consideration may be needed. When our scientists did the fancy version of statistical testing of these data where you control for the multiple comparisons that are being made, nothing was actually statistically significantly associated with worse outcomes in the high thimerosal group.

DR. WILLIAM THOMPSON, EPIDEMIOLOGIST, NATIONAL CENTER FOR IMMUNIZATIONS AND RESPIRATORY DISEASES, CDC: Yes, I would just mention that we did mention the speech articulation finding in the discussion, and show that it′s a very small effect from a two standard deviation change in your exposure variable to thimerosal causes 0.29 – an increase of 0.29 articulation errors. So that′s less – that′s one-third of the speech articulation error.

The – regarding the IQ effect, it was difficult to interpret because we found that among girls increasing thimerosal exposure was associated with lower verbal performance. But with boys increasing thimerosal was associated with increased performance IQ. So again, reiterating what Dr. Schuchat said we interpreted it as random associations that we found by chance. In addition, for that birth to one month exposure period we found five significant associations where it looked like there was a beneficial association of higher thimerosal exposure.

CURTIS ALLEN: And, next question, please.

OPERATOR: Thank you. Our next question comes from Peggy O′Crowley with the Star-Ledger.

PEGGY O′CROWLEY, STAR-LEDGER: Hello, thanks for having the conference. I was interested to know if you looked at the – amongst the children who did have this higher incidence of TICs, did you then try to find out if they had other symptoms that might indicate let′s say Tourettes or something like that?

In other words were the TICs symptomatic of something – a more serious condition?

DR. ANNE SCHUCHAT: So that wasn′t the nature of the testing. The evaluators were trained to recognize tests – I′m sorry – the evaluators were trained to recognize what TICs look like or sounded like and to note them, to document whether they were present or not. But there was not a linkage with follow up in those children.

I also want to just stress the relatively small numbers of children in any of these groups with the abnormalities. You know, the group with TICs is not a very, very large to follow up on. And then to reiterate what I said earlier that observing TICs during the few hour exam does not indicate a chronic TIC problem. A child who had Tourettes syndrome, for instance and was observed would probably be noted to have TICs on this exam. But more of the TICs that people have during one of these exams could easily be transient. So we don′t have information from this study about the full severity of the TICs that were recognized by the evaluators.

We do know that the parent who reported their children having TICs, where a parental report would have been over a longer observation period that there was no association at all between the parent report of ticks and higher thimerosal exposure.

CURTIS ALLEN: Next question, please.

OPERATOR: Thank you. Again, as a reminder, if you would like to ask a question, please press star followed by one on your touch-tone phone.

Our next question will come from Jennifer Smith with FDA Week, please go ahead.

JENNIFER SMITH, FDA WEEK: Hello. (INAUDIBLE), you mentioned about the previous study, I mean overall is this information really that much different from previous findings? I mean is there anything that really is different in anyway, even slightly from previous findings?

DR. ANNE SCHUCHAT: Let me begin and then we′ll have Dr. Iskander supplement. These findings, in general, are consistent with earlier studies about a lack of association between thimerosal exposure and bad outcomes.


DR. ANNE SCHUCHAT: There had been a previous study that had – two previous studies that had – suggested a possible link between the thimerosal exposure and TICs. And so that – the finding that there may be an association in this is also consistent with those earlier studies. So there′s nothing here that′s completely shocking, if that′s what the question is about.

DR. JOHN ISKANDER, ACTING DIRECTOR, IMMUNIZATION SAFETY OFFICE, CDC: Right. And this is John Iskander. The main distinction I would make is that, you know, this, compared to some of the earlier studies that were done, this is really a very methodologically rigorous very well designed and conducted study that was done with great care and patience and foresight. But the bottom line answer is the direction of the findings are consistent with previous well conducted epidemiologic studies in this general area.

CURTIS ALLEN: Next question, please.

OPERATOR: Thank you. Our next question comes from Sheryl Ubelacker with the Canadian Press.

SHERYL UBELACKER, CANADIAN PRESS: Yes hi. Thank you so much for doing this. I wonder if you could just explain clinically what TICs are and phonic TICs, please.

DR. ANNE SCHUCHAT: Yes. TICs are movement disorders. And phonic TICs are disorders of the – movement of the tongue actually, so they are utterances that are abnormal. The place in the brain that contributes to TICs or movement disorders is galled the basal ganglia. And so TICs are really abnormal movements that, you know, you might think of them as something like twitches. But they are basically muscle or movement abnormalities.


OPERATOR: Thank you. Our next question comes from Elizabeth McCaddy (ph) with the Pediatric News.

ELIZABETH MECHATIE (ph), PEDIATRIC NEWS: Hi. Thank you. What was the name of the autism group that was involved with the consulting?

DR. ANNE SCHUCHAT: There was an individual representative of autism concerns.


DR. ANNE SCHUCHAT: OK. So the organization is SafeMinds and the individual was Sallie Bernard.

CURTIS ALLEN: Is that it? Next question, please.

OPERATOR: Thank you. Our next question comes from Dan Olmsted with the

DAN OLMSTED, RESCUEPOST.COM: Hi. I saw that you had a few never vaccinated kids in there, I think, 16, and my question is why didn′t you just compare a cohort of never vaccinated kids with those with high thimerosal exposure? I mean it seems to me that that′s the question that parents keep asking and the CDC keeps not answering.

DR. ANNE SCHUCHAT: You know, I′ll begin the answer, and then I′m going to let our lead scientist continue. You know, the way this study was done tried to overcome biases that can be introduced through enrollment of children. So this study took several managed care organizations entire enrolled populations and did not seek one kid but not another kid, but instead used a systematic computer way to identify children as eligible for the study.

This actually helps you control for potential biases in why a person might be getting into the study. It was also very important to have the same kind of time period in terms of the exposures because we were looking at children seven to 10 years of age. And we needed to be able to find them, bring them in and do all of this testing. And if you do something like a historical control period and take kids who were born in a different time period there are many, many factors that change over time. So it was important to enroll children born around the same time and then evaluated around the same time as development at the seven to 10 year of age.

So imagine if there was some kind of problem with cell phones or some kind of problem with computers, things that have changed quite a bit over time, there could be big problems with looking – enrolling children from an earlier time period and comparing them with children from a later time period.

DR. WILLIAM THOMPSON: Yes, and so I′ll just add – this is Bill Thompson, in addition to that, there were very few lower exposure or no exposure kids to sample from. So the study couldn′t be easily designed to have a low exposure group. We essentially sampled all of the low exposure kids that were available in the study.

DR. ANNE SCHUCHAT: Yes, and then the other issue is, for example, if you try to look at a group like a group of Amish children or a group of children who are living in a very different way, only one of the factors being that they don′t have their children vaccinated, you could lead to misleading conclusions. So the idea here was to have a very systematic way of identifying eligible children without knowing what kind of vaccines they got. And then very carefully quantify how much they were exposed to and you saw in the paper this range from zero to higher amounts. And then very systematically measure their performance on these standardized tests where the performance and the observations was scored without knowledge of how much thimerosal they had been exposed to.

CURTIS ALLEN: Next question, please.

OPERATOR: Thank you. Our next question is from Susan Sataline with the Wall Street Journal.

SUSAN SATALINE: Hi, thanks again. I wanted to ask how do we describe what percentage of kids had these TICs? Or what – how much thimerosal was the turning point for showing that they had a greater likelihood of having these TICs? I′m struggling with some of the charts here?

DR. ANNE SCHUCHAT: Yes. There′s not really a threshold. There′s a high versus middle versus low group that kids were grouped into. The percentage – I mean I think the best thing to describe is that there was about twice as many TICs in the group that had the high exposure versus the group with the low exposure. In any – the actual percentages are a bit misleading because the numbers are fairly small. But that′s – the table has sort of the relative risk and it′s about twice as high a risk of TICs of those – of these TICs that were observed by the evaluator.

Of course, no difference in TICs in terms of the parents report of TICs. OK.

CURTIS ALLEN: Next question, please.

OPERATOR: Thank you. Our next question comes from Matthew Herper with Forbes, please go ahead.

MATTHEW HERPER, FORBES: So I was just wondering in a follow up question about why not compare to unvaccinated kids. And you – if I understand right, you included all children from several managed care organizations in order to enroll?

DR. ANNE SCHUCHAT: (INAUDIBLE) totally unvaccinated they would be in this, and in fact, there were some that were not vaccinated. So yes, so this was – we didn′t enroll all. We basically identified a time period and did a sample and then offered enrollment. I think, it′s also important to say not everyone who was offered enrollment accepted to participate in the study. So the study did not reach a high enrollment proportion. There were many who didn′t want to be part of the study, primarily because of time.

DR. WILLIAM THOMPSON: And I would follow up with that, that among the HMO populations it′s very unusual to have children that aren′t vaccinated.

DR. ANNE SCHUCHAT: And I guess, I′ll just follow up with, in the United States we′re very fortunate to have immunization coverage and our national surveys of two year olds 0.3 percent of children have no vaccines – have received no vaccines at all. So one of the reasons that we have such low rates of vaccine preventable diseases in this country is because we have high acceptance of vaccines and usage by parents and providers.

So basically there are not a lot of unvaccinated children out there.

CURTIS ALLEN: Next question, please.

OPERATOR: Thank you. Our next question comes from David Kirby with the Huffington Post.

DAVID KIRBY, HUFFINGTON POST: Yes, high everyone. Thank you very much. My question is kind of three part but I′ll try to keep it as simple as possible. Thank you very much for clearing up the question about the relative risk for TICs. I was a little confused about that. And I just want to make sure I′m reading this correctly that by “the rating by the evaluators, the relative risk for phonic TICs was 2.44. That′s comparing highest exposure to lowest exposure groups. And motor TICs was 2.19”, is that correct? Is that an accurate statement?


DR. ANNE SCHUCHAT: Well I think, remembering it′s in boys.

DAVID KIRBY: In boys yes, vaccinated zero to seven months. OK. Well that seems very high to me, and it′s certainly much, much higher than any other of the findings in the study. And as you say, in the study itself, it does replicate the Verstraten study in the final published form that also found six. I don′t believe they were nearly this elevated. And so it seems to me that this is news. That this study is finding, not only confirming what Verstraten found in terms of TICs but actually finding them at an even higher rate. And because you don′t look at simple or – hello?

CURTIS ALLEN: Yes, do you have a question?

DAVID KIRBY: My question is – OK, I guess my question then is it does seem to me like a significant finding. You also found elevated risk for certain speech and articulation problems and in the study noted that in one HMO in the original Verstraten study speech delay was also – there as an association.

So I guess my question is you′re telling us that this is all very reassuring and this is all very good news. But how do you reconcile the conflicts in two different statements in your study? One, and I′m going to read the quote, and then just tell me your answer, “this suggest” – the first statement is “this suggests the possible adverse association between neonatal exposure to mercury and language development coming on top of two-and-a-half times risk for phonic TICs. And when I looked up phonic TICs they are more serious than they might sound.

But anyway, that conflicts with your conclusion which reads “our study does not support a causal association between early exposure to mercury from thimerosal containing vaccines and deficits in neuropsychological functioning in the age of seven to 10 years.” In other words, you are suggesting a possible adverse association but then conclusion your studies of not support a causal association. And I′m just wondering how you reconcile what appears to me, and I could be wrong, seems to be conflicting statements.

DR. ANNE SCHUCHAT: Thank you for that question. I think as you noticed and others, there were an awful lot of tests that were carried out in this study.

One of the things that we′ve tried to do to assure access and availability of this – the information from this study to others who have interest is we′re making the data publicly available so that researchers or others can request access to the data so that they can look at the information themselves, and get at some of these additional questions that they may have. At CDC we′ve been very committed to a transparent process to a scientifically credible process to identify questions and concerns that people have and make the best case we can in addressing them.

And I think there will be lots of people, perhaps, such as you who want to do additional work on these data. We tried to take the statistical scientific expertise we have and make this translatable. Our assessment is that these are very reassuring findings. That the motor and phonic TIC did show up in this study in boys but not girls, in the evaluators but not the parents report and we are having experts in that discipline look further into the information from this study. So that′s the approach that we′re taking on that. So thanks.

DR. JOHN ISKANDER: I just – this is John Iskander, again. I just wanted to make sure that folks were aware that CDC is supporting a portfolio of studies in this area and that there are two additional studies that are likely to come out within the next approximately 12 months or so. One is a case control study which is going to involve enrolling about 250 autistic children and again looking at the issue of their exposure to thimerosal compared to non autistic children. And there is also a study that is being conducted in Italy which is, in some ways, very similar to Dr. Thompson′s study, again, looking at objective evaluations of psychological and developmental performance in children exposed to different amounts of thimerosal.

DR. ANNE SCHUCHAT: I think that Dr. Iskander may have said they are coming out in a month but it′s within a year. So just in case I misheard that. I′m not sure.

CURTIS ALLEN: Yes I think, we have time for one more question. Any more questions?

OPERATOR: Thank you. Our final question comes from John Donvan with ABC Nightline.

JOHN DONVAN ABC NIGHTLINE: Hi. Thanks for doing this. I just want to follow up on David Kirby′s question, because I think I heard him ask a specific question which was how do those two statements not contradict? And your answer went to the issue of the data is transparent and people can do more studies. But you really didn′t – I really didn′t hear an answer to the question about how your conclusion does not contradict – or how the statements earlier in the report do not contradict your conclusion? And I wanted to ask you to take one more crack at that.

DR. ANNE SCHUCHAT: Yes, thanks for that chance. I think to clarify why those statements aren′t contradictory you might want to the thought experiment of saying whether the headline for this should be high thimerosal exposure associated with better performance? Because the reality is 12 of the statistical findings showed better performance in children who have high thimerosal exposure. And seven of the findings had lower performance in children who had high thimerosal exposure. There were a variety of tests that looked at speech in different ways, a variety of tests that looked, you know, the intelligence functions, the language motor et cetera. And so our look at the statistical findings is informed by the idea that chance alone probably explains these. But we wanted to make special emphasis that the motor TIC finding has been shown in two earlier studies. And was in boys but not girls but the TIC variable was observation and couldn′t separate transient not medically important TIC for more serious ones. And so that particular finding, we saw it might merit more consideration.

So the statements aren′t contradictory because we′re talking about looking at the results of the whole, the very strong possibility that every single statistical association in this study is a random chance one and the possible extra concern about the motor TICs. So the transparency issue was we understand scientists and others may disagree and may want to look at these data and make sure that they understand them. So I didn′t mean to dismiss the question.

DR. JOHN ISKANDER: I just – if you look at all of the comparisons that were done, so depending on how you look at the statistics between 98 and 99 percent of the comparisons showed really no association between thimerosal exposure and a positive or negative outcome. So, I think, that is really very consistent with the statement that there is not – that′s pretty strong evidence that there isn′t a causal association here.

CURTIS ALLEN: OK. Thank you very much for joining us today. The transcript of this press briefing will be on the Web site at probably later this afternoon. Thank you very much.

OPERATOR: Thank you. This does conclude today′s conference call. We thank you for your participation. And you may now disconnect.




  • Historical Document: September 26, 2007
  • Content source: Office of the Associate Director for Communication
  • Notice: Linking to a non-federal site does not constitute an endorsement by HHS, CDC or any of its employees of the sponsors or the information and products presented on the site.
CDC 24/7 – Saving Lives. Protecting People. Saving Money Through Prevention. Learn More About How CDC Works For You…
CDC 24/7 – Saving Lives. Protecting People. Saving Money Through Prevention. Learn More About How CDC Works For You…
Contact Us:
  • Centers for Disease Control and Prevention
    1600 Clifton Rd
    Atlanta, GA 30333
  • 800-CDC-INFO
    TTY: (888) 232-6348
  • Contact CDC-INFO
 The U.S. Government's Official Web PortalDepartment of Health and Human Services
Centers for Disease Control and Prevention   1600 Clifton Rd. Atlanta, GA 30329-4027, USA
800-CDC-INFO (800-232-4636) TTY: (888) 232-6348 - Contact CDC–INFO

A-Z Index

  1. A
  2. B
  3. C
  4. D
  5. E
  6. F
  7. G
  8. H
  9. I
  10. J
  11. K
  12. L
  13. M
  14. N
  15. O
  16. P
  17. Q
  18. R
  19. S
  20. T
  21. U
  22. V
  23. W
  24. X
  25. Y
  26. Z
  27. #