Aug 24 2015

Antivaxxers Still Flogging Thimerosal

I gave a talk on the vaccine controversy over the weekend. I was not surprised that a couple of audience members had a lot of questions taken directly from anti-vaccine propaganda sites. What was interesting was that they were still pushing the idea that thimerosal, a mercury-based vaccine preservative, is linked to autism.

The reason this is interesting and illuminating is that the thimerosal hypothesis is not just mostly dead, it is most sincerely dead. It is pushing up the proverbial daisies.

A Brief History of Thimerosal

Thimerosal was developed as an organomercurial anti-microbial agent shortly after World War I. It was soon discovered that it has great anti-microbial properties and was well tolerated when injected into rabbits or rats even at high doses. This made it superior to anything else available at the time.

Bacterial contamination was a serious problem for vaccines in the first half of the 20th century. Thimerosal in tiny doses, well below safety limits, proved to be an effective agent for preventing contamination. By the 1940s thimerosal was being added to several vaccines for this purpose.

It should be noted that thimerosal can only be used in inactivated vaccines or those that contain just proteins, not in live attenuated virus or bacterial vaccines. Thimerosal would inactivate any live vaccines.

Safety concerns about thimerosal were first raised in the 1970s with increased awareness of the neurotoxicity of mercury. However the dose of mercury in vaccines was orders of magnitude below the levels showing any clinical effect. No safety concerns were raised with thimerosal and vaccines until the late 1990s. At this time Wakefield was raising alarms over the MMR vaccine (which never contained thimerosal) and autism. His research was later revealed to be flawed and even fraudulent, and independent studies showed convincingly that there was no connection between MMR and autism. So anti-vaxxers moved on to their next target, thimerosal.

Anti-vaxxer focus on thimerosal was given a huge boost by David Kirby and his book, Evidence of Harm. Robert Kennedy also jumped on the anti-thimerosal bandwagon, mainly coming from an environmental perspective.

A History of Dose and Thimerosal

The core of the anti-vaccine claim against thimerosal was that as the cumulative dose of thimerosal increased so did the incidence of autism. This claim was never scientifically validated. Correlation by itself is weak evidence for caustion, and the correlation itself really didn’t hold up. But the vague correlation of – both thimerosal and autism were increasing in the 1990s – was enough for the anti-vaxxers. Of course I have to show this graphic indicating that the correlation between autism and organic food is even better.

Despite the lack of any convincing scientific evidence, the CDC decided “out of an abundance of caution” to remove thimerosal from the routine vaccine schedule by 2002. This provided an opportunity for a mega-experiment. If the increasing dose of thimerosal caused increasing autism diagnoses, then a decreasing overall dose of thimerosal should cause the incidence of autism diagnoses to fall to pre-1990s levels.

David Kirby directly predicted a drop in autism incidence and correctly noted that if the rates do not drop that would call into question the role of thimerosal. In 2007 he was writing that the rates were starting to drop – extrapolating wrongly from short term fluctuations in the data.

The other game that anti-vaxxers were playing was that thimerosal was not really removed by 2002. There were still some vaccines with thimerosal. As autism rates continued to rise, they kept pushing back the date of when thimerosal was removed. Apparently doctors were holding onto their vaccines with thimerosal to the bitter end.

Vaccine manufacturers stopped making vaccines with thimerosal (except for some flu vaccines, more on that later) between 1999 and 2001. Vaccines have a two-year shelf life. Even if you take the latest estimates, the thimerosal-containing vaccines were gone by 2004. However, anti-vaxxers take the 2004 date then tack on two more years and claim that vaccines contained thimerosal as late as 2006. This is nonsense, however; 2004 is really the latest date that some doctor in the US might have given the last vaccine with thimerosal right before its expiration date. Even then the total dose of thimerosal given to the pediatric population was steadily declining between 1999 and 2004.

I can see why in 2007 anti-vaxxers were dickering about the exact date of thimerosal removal. But now we are in 2015 – 8 years later. We are a full 11 years after the latest realistic date of any lingering thimerosal in the vaccine schedule. Certainly anti-vaxxers cannot still be holding onto their thimerosal delusions. Well, of course they are.

Let’s review the historical dose of thimerosal in vaccines. In the 1970s, one vaccine contained thimerosal,¬†diphtheria-tetanus-pertussis combination. One vaccine dose typically contains 25 micrograms of thimerosal. By the early 1980s, before the autism diagnosis increase, the total dose of thimerosal in the routine vaccine schedule was up to 100 micrograms. Total thimerosal dose to infants and young children peaked at 187.5 micrograms of thimerosal. This dose was by 6 months of age, and did not include any additional thimerosal from the flu vaccine, which is recommended but optional and not considered part of the routine schedule.

After around 2002 the 187.5 micrograms of thimerosal was completely removed. However, anti-vaxxers are now claiming that increased use of the flu vaccine has replaced this missing thimerosal. Simple math shows that they are wrong.

Multi-dose flu vaccines contain 25 micrograms of thimerosal. Even if a child gets the flu vaccine at six months, one year, and then every year after that they are still not getting anywhere near the total dose of thimerosal in the schedule prior to removal. Even if you include the shot their pregnant mother received (which the anti-vaxxers do) you are still not near the total dose.

Further still – in 2012 the CDC reported that only 1/3 of the flu vaccines produced in the US were mutli-dose vials. The single dose vials and the live virus vaccines do not contain thimerosal. So only one third of those flu vaccine doses in 2012 contained thimerosal, with similar numbers projected for this year.

So if we are considering the population dose of thimerosal, we have to drop the average dose from the flu vaccine to one third. That means that children who get all their flu vaccines by age six are getting about 50 micrograms of thimerosal total on average over six years, compared to at least 187.5 micrograms plus flu vaccines in 1999.

In addition to all this, some states, like California, have banned flu vaccines with thimerosal. So children in California get a total thimerosal dose of zero.

Autism Incidence

The CDC has statistics on autism incidence through 2010, which includes the birth cohort for 2002 – which is clearly after the total thimerosal dose had plummeted.

The CDC has also published data for 2011-2012, showing the autism rate has continued to increase, from 1 in 68 to 1 in 50 – that’s the 2003-2004 birth cohort. The rates in California are no different.

I should note that the evidence shows that the increasing rate of autism diagnoses is largely an artifact of expanding diagnosis, diagnostic substitution, and increased surveillance. There may not be any real increase in autism itself. A small real increase is possible, but unproven.


There is only one reasonable interpretation of this data – the removal of thimerosal from the vaccine schedule resulted in a dramatic decrease in the total thimerosal dose given to the pediatric population in the US, starting in 1999, completed by any reasonable estimate by 2004. Only one third of flu vaccines still contain a small dose of thimerosal, and some states, like California, have banned even those.

Despite this dramatic overall reduction in thimerosal, there has been no change in the rate of increase of autism diagnoses. The plummeting of autism diagnoses predicted by David Kirby and others never materialized.

Attempts to keep their thimerosal delusions alive are getting increasingly desperate and ridiculous. Antivaxxers are now at the point where they have to deny simple math.

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