Wednesday, October 24, 2007

The 1999 Thimerosal Joint Statement: A Case of Poor Risk Communication

The well-known vaccines-autism scare first started in the UK when Andrew Wakefield suggested that the MMR vaccine could be related to autism (this theory has been widely refuted elsewhere). But here in the US, the center of attention has been the mercury-compound thimerosal, which was used as a preservative in many (non-live) vaccines until a few years ago.

If you are not familiar with the history of this issue, you can read more about it here.

I will concentrate on the first official statement recognizing the potential health risk of thimerosal in vaccines: The Joint Statement of the American Academy of Pediatrics (AAP) and the United States Public Health Services (USPHS).

I have met several people involved with the writing of the joint statement and it is obvious that the final text was a compromise, a middle ground between opposite opinions. Some in the group thought that the lack of evidence of harm was sufficient reason to not do anything; others thought the potential risk of harm was enough to warrant the removal of thimerosal from vaccines. Apparently some went as far as suggesting stopping immunizations altogether until thimerosal-free vaccines became available. See also this reference.

To their credit, some things were done right (in terms of risk communication):
  • The announcement was made early
  • The scientific uncertainty was acknowledged
  • There was collaboration between agencies
Some of the words used, however, created confusion. These are a few of examples...

"The recognition that some children could be exposed to a cumulative level of mercury over the first 6 months of life that exceeds one of the federal guidelines on methyl mercury now requires a weighing of two different types of risks when vaccinating infants. On the one hand, there is the known serious risk of diseases and deaths caused by failure to immunize our infants against vaccine-preventable infectious diseases; on the other, there is the unknown and probably much smaller risk, if any, of neurodevelopmental effects posed by exposure to thimerosal. The
large risks of not vaccinating children far outweigh the unknown and probably much smaller risk, if any, of cumulative exposure to thimerosal-containing vaccines over the first 6 months of life."

Note the problem in the highlighted phrase above. If the risk was unknown, how did they know it was much smaller? A casual reader would fairly ask, 'Is there something you're not telling me?'

Without taking into consideration what we now know about the science of thimerosal/ethylmercury and neurodevelopmental disorders, the paragraph could be interpreted in one of two ways:
  1. The risk is really unknown and they're just saying it is "probably much smaller" to calm me down. In other words, they are insulting my intelligence with their overassurance.
  2. The risk is known and they think it is small but they don't want to tell me because I may think otherwise (Oh, my God, the risk is big!). In other words, they are lying.
Hint: the right interpretation is number 1. The problem, of course, is that overassurance is bad risk communication!

Let's look at the next paragraph in the joint statement:

"Nevertheless, because any potential risk is of concern, the US Public Health Service (USPHS), the American Academy of Pediatrics (AAP), and vaccine manufacturers agree that thimerosal-containing vaccines should be removed as soon as possible."

Whoa! That is alarming! If that didn't confuse readers after the previous over assuring statement, I don't know what would.

But don't take my word for it. According to David Kirby, the joint statement did cause confusion among parents. From Kirby's book, Evidence of Harm:

“In their far-flung corners of the country, parents like Liz [Birt], Lyn [Redwood], and Sallie [Bernard] read the statement and arrived at similar conclusions. The government and the AAP were posing an extraordinary contradiction. If thimerosal exposure had been so minimal, and if there was no evidence of harm, then why call for its removal as ‘soon as possible’?” (p.47)

Towards the end of the joint statement, there is another interesting remark:

"Although there is a margin of safety with existing vaccines containing thimerosal, there are steps that can be taken to increase that margin even further. Clinicians and parents can take advantage of the flexibility within the existing schedule for infants born to hepatitis B surface antigen (HBsAg)-negative women to postpone the first dose of hepatitis B vaccine from birth until 2 to 6 months of age when the infant is considerably larger."

Here's one reaction to the above from Kirby's book:

[Albert Enayati:] “Why would they postpone the birth dose if it is so ‘harmless’?”

“Because they know there’s something seriously wrong here,” Sallie said. “They wouldn’t just reschedule a vaccination unless they knew there was a real danger.”

Remember my two possible interpretations above? These people (Sallie Bernard, etc.) picked number 2. It was the wrong interpretation in the end, but still, I don't blame them, given the poor choice of words in the joint statement.

Among others, the authors of the joint statement did not apply these three principles of risk communication (I'm taking these from Peter Sandman):
  1. Don’t over assure—avoid saying, “everything is fine” because people may find it alarming
  2. Let the public know the reasons behind the policy decisions and the dilemmas faced
  3. Be transparent—communication should be candid, clear, and accurate.
Nowhere in the joint statement, the authors discussed the dilemmas faced. For instance, they did not explain the differences between ethylmercury and methylmercury and how there were no guidelines for exposure to ethylmercury. Perhaps they did not trust the public would understand the science. See Paul Offit's criticism of the joint statement here.

In the end, the overassurance and the contradictory statements gave the feeling of a non-transparent communication. If I were to rewrite two of the paragraphs above, this is what I would do:
  • The large risks of not vaccinating children need to be weighed against the potential but yet unknown risk of cumulative exposure to thimerosal-containing vaccines over the first 6 months of life.
  • Nevertheless, because any potential risk is of concern, … agree to err in the side of caution and remove thimerosal from vaccines while studies are conducted to assess the potential risk.
Here I acknowledge the uncertainty of the risk without over assuring and explain the policy decision (the removal of thimerosal from vaccines) by appealing to the precautionary principle (err in the side of caution).

Of course, it is easier to write this in retrospect and without a review committee approving or disapproving every word.

I'm not sure whether my version of the joint statement would have had a different effect in the public (risk communication principles do not always warrant success), but perhaps the postmortem review wouldn't have been as harsh as this one!

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