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82 IMMUNIZATION SAFETY RE VIE W the differing guidelines for mercury exposure. The differences in the exposure levels calculated by EPA, FDA, and ATSDR arise from use of different key studies and different uncertainty factors. In addition, concerns about risks related to thimerosal are difficult to convey, given the lack of direct evidence on the toxic effects of low-level exposure to ethylmercury. Partly because of the differ- ent exposure levels in the federal guidelines and the lack of evidence on ethyl- mercury, the messages about the risks of using thimerosal-containing vaccines have been complicated and not well understood by some professionals (Halsey and Goldman, 2001~. Second, finding information about thimerosal-containing vaccines on the websites of federal agencies is difficult. Federal agencies with responsibilities related to vaccines do not necessarily have pages on their websites devoted to concerns about thimerosal-containing vaccines. In order to design websites that respond more effectively to vaccine safety concerns, additional information is needed about how parents access such information. Similarly, additional infor- mation is needed about how clinicians (physicians, nurses, and others) access vaccine safety information and the most successful communication strategies for reaching them. Third, information about vaccine risks on government websites contains words that may be perceived as judgmental, such as "should." Using words that are less directive and prescriptive is important in effectively communicating vaccine risks (NRC, 1989~. For example, the message may be more effective if it is stated as "CDC is confident that ... ." rather than, 'parents should feel cony dent that...." Establishing trust is an important factor in communicating risk effectively, but the message loses its effectiveness when the source of the infor- mation is not considered credible and when the language is not understandable (Carpenter, 1995, NRC, 1989~. Communication strategies need to be specific to the intended audience and should ensure that language levels are appropriate for that audience. As the committee noted in its previous report (IOM, 2001), there are nu- merous broad and recurring communication concerns in various vaccine safety issues. The committee has not attempted to address these issues here, but will examine them in a separate, more general context. SUMMARY Mercury is a known neurotoxicant. Little is known about ethylmercury (the active component in thimerosal) compared to methylmercury. However, the committee believes that the effort to remove thimerosal from vaccines was a prudent measure in support of the public health goal to reduce the mercury ex- posure of infants and children as much as possible. The committee urges, in fact, that full consideration be given to removing thimerosal from any biological or pharmaceutical product to which infants, children, and pregnant women are ex-

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THIMEROSAL-CONTAINING VACCINES 83 posed. The committee concludes that although the hypothesis that exposure to thimerosal-containing vaccines could be associated with neurodevelop- mental disorders is not established and rests on indirect and incomplete information, primarily from analogies with methylmercury and levels of maximum mercury exposure from vaccines given in children, the hypothe- sis is biologically plausible. The committee also concludes that the evidence is inadequate to accept or reject a causal relationship between thimerosal exposures from childhood vaccines and the neurodevelopmental disorders of autism, ADHD, and speech or language delay. The limited and unpublished epidemiological data constitute weak and inconclusive evidence regarding causality. However, because thimerosal remains in vaccines in other countries and in biological and pharma- ceutical products in the United States, and because it is imperative to restore and maintain trust in vaccines, the committee believes that continued public health attention must be paid to this issue in the form of policy review and analysis, public health and biomedical research, and improved communication strategies. Box 5 summarizes the committee's conclusions and recommendations.

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84 IMMUNIZATION SAFETY RE VIE W B::::::::::::::::::::::::::::::: --: :::_:::::: :::::::::::::::::: 'at ~ :::::::::::::::::::::::::::::::::::::::: ~ : : - : - ~ - : : : : : ~ : : : : : I : : : - : ^ ~ : ~ : I T T ~ ~ : : : ~ ~—^ m - m - ~ - n If - = T ~ - ^ n - = - - - = n - ~ - - - ~ ~ - n ~ - I l ~ l ~ n ~ :~ ... - ~ ~ ~~ :~:V! :1: :! ~ :I: :! ~ L:L~. A::: -:-:~-VVl: :!: l:l: :!: l:~:!: ~ ~ :~:~I:V~-:-l:~ --:-::-:-: ~V:!: :! V:—:~1:~:!: :!—::::::: ~ - - ~ in- - -~n-m- m l- ~ Hi- - -~n-n-^ I ~ r -ma- - - ~ ~ - -= . ~ m~ l ~ A- - - . An- - - ~~ / n ~ ~-~-~ ~ ~-~- - - ~Y-nn-~-l - l-r~- - - n ~ :~ : :-:- 1 A:::—V:!: l: l: l: l: :! l: ·~ A::: .~. :~:! :1:~ -~:% :: : :! 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' V'~~'I I' 'I ==' ' ' 'I'I ' I' ' ' ' tI 't=' ' ' ' ~'I' I'I'L=~ '-' ' '~'i~ L~ ~ ~ ' ' 'U'=O'~ l'i~ ' ' ' 'Ll ' l'= '-' ' 'l =~i ' ' ' [l 't='t ' ' ' 'Ll ' l'=t' =' ' ' ' ' ' ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::: :::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::::::::::::::~:::::::: :::::::~::: ~ l---ne~ co-m-m-l-llee~ reco-m-me-nas~ lnal~ Tu-l-l~ cons-l-a-e-mll-on~ De~ g-lve-n~ Dy~ ap-p-~-p-rl~ :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::::::::: :::::::::::::::::::_ :::::::::::,:::::::::::::.:::::::::::::::,:::::,:i::::::::::::::::::::::~:::::::::::::::::::::::: ........ ~ ............. .... .................... .... ... = ... ............................................................................................................................ . . . ........... t.rom~ vaccl-n-es~ aC-m-l-n-l-s.t e-~ .t. ~ l-n~-n-Is~ cn-l-l-~-mn~ or~ p-eg-n-a-nt~ wome-n~ l-n~ Ine~ ~ T-h-e~ Co-m-m-ltte-e~ recom-m-e-n-d-s~ th-at~ a-n-n-~-nnate~ n-ro~ss-l-o-nal~ s-ocl-etl-es~ a-n-d~ ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::: :::::::::::::::::::::: :::::::::::::: ::::::: ::::::::: t ' '' ' " '1'' ' "'"'' ' '1' ' ""'th" '1" """' ' '1'1" 1'' ' """' b~ t'~ t'h'' """' '' " ' I b! 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THIMEROSAL-CONTAINING VACCINES 85 ~-e~ com-m--lt Beet reco-m-m-e-~-c -so ms-e- conlro ~ sl-u-c -lest ~-m-l-~-l-~-g~ ; Noel po Cent ............................................................................................................................ ............................................................................................................................. :::::: ::::::::: :::: : ::::::::: ::::::::: _: ::::::: : -:: -:::: -::: -:::: - - - -: ::::::-:::::::-:::-:::-:::::-:::::::-:- -:::::::::::':::'::::::: :-:::::::-:::::::-:::::::::::::::::::::::::::::1:::: ::: la -l-n ~ Ye wee-n~ n-~-u-roueve- -o-p-me-n-~a- ~ u--lso-~-e-~ a-n-u~ ~-l-m-emsa- --coin- a-l-n-l-n-g ......................................... ..................................................... ...... .~ lintel co-m-m-itie-e~ recOm-m-enus~ Tu-rI-~-er---a.-n-al-y.s-ls~ ~~ n-eumueve-lop-m-enta-l~ ............................. ............................................................................................................................ ( ..lso-rc -era lint co- 1-or ;s~ o ~ cat 1"1' C -- en won- ala nook reseller m-!-m-e osa-!--~-n-la-l-n-l-n-g~ ::::::::::: :1 !::::: i:::::::: :1:::::: i:::: :1::::::: :: A:::: :::::::::::::::: ~ :-:-:-:-:- ~-~-:~ ~-T:-:-:~:-:-:~:~-I:-~-I:~:~:~ Too: :^T.~ ~~ ~-~-~-I:~:~-:-::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::::"V=~::~:.:~:I :~:.:~:1 ::~::: - li:l::ll:~:l:::Ll:l:=l::.:V l::.: i ::I::"i ::::::V "~=I:I:i:~ ::::::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::::~::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: nor tumult ~~ r~^mm=-n- ~=~ man E:l:l: TE:n^:::::=^l^~:m:l^:l~ E:l~=l:::::~:TE::l~l:~:::::Tn~r::::: i I l ~ ~~t ! l ! l l I ~~ ~ - REV l l ! l ! ! At! l ~~ TV ~ t~ ~ ~~t 1 My ~ lo. -Id ~ I I ! 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