Recently, I logged onto Facebook and found the following article in my feed:
I was in a bit of a sour mood at that moment, so when I hit the share button I wrote a vitriol-filled post to accompany it. To my surprise, it got shared quite a bit—meaning 15 or 20 reshares compared to my typical two. Honestly, I was a little surprised. And then, I got my very first screed from anti-vaxxer troll, a self-described “old school hippie, loving mother and proud grandmother, happy and joyous and free” who “studied at the School of Hard Knocks”:
richard, please let me inject you and your babies with mercury ok?
I replied, of course. But the response got me thinking, and it got me wondering, and it got me asking questions.
Thinking? Isn’t that dangerous?
It sure seems like some people believe that, yes.
Now, if you’re reading a blog like Skeptoid, you’re probably no stranger to the whole antivaxxer “mercury in your vaccine” thing. It seems to be one of the common tropes they pull out, waving the word around as if it were a crucifix capable of banishing the vampires of reason and science. But then, it occurred to me, that I don’t actually know much about mercury in vaccines. Is it still there? Some of the people who dogpiled on Old School Hippie said that it can no longer be found in vaccines. Full stop.
I believed them at first. But, hey. I’m a skeptic, and just believing someone I don’t know because they happened to make a Facebook post I agree with is a terrible example of skepticism. So, I decided to look it up. And it turns out that the CDC has a wonderful two-page fact sheet titled Understanding Thimerosal, Mercury, and Vaccine Safety.
You can probably guess what it’s about from the title, right?
Seriously? You’re going to talk about mercury in vaccines?
Yes. I am. Because I don’t know much about it, and I suspect that I’m not alone in that. We’ve got a whole lot of other really good articles on this site if you aren’t interested.
Mercury in vaccines takes the form of thiomersal, aka thimerosal, aka Ethyl(2-mercaptobenzoato-(2-)-O,S) mercurate(1-) sodium, aka Mercury((o-carboxyphenyl)thio)ethyl sodium salt, aka C9H9HgNaO2S, and the Material Safety Data Sheet does make it look a little spooky. It is an ethylmercury, which the CDC says is “broken down by the body differently and clears out of the blood more quickly” than the methylmercury that can damage the nervous system in large doses.
Still, you don’t want to play around with it. Thimerosal is “hazardous in case of skin contact (irritant), of ingestion, or inhalation” and “may be toxic in kidneys, liver, spleen, bone marrow, central nervous system”. The LD50 is 75 mg/kg for rats, and 91 mg/kg for mice.
Keep that number in mind. I’ll be back to it.
Thimerosal is used as a preservative in multi-dose flue vaccine vials, to prevent the growth of bacteria and fungus. It is also used to prevent the growth of microbes during the manufacturing process of DTaP (diphtheria, tetanus, and acellular pertussis) and DTaP-Hib (diphtheria, tetanus, and acellular pertussis-haemophilus influenzae type b) combination vaccines, the only childhood vaccines that contain any traces of the chemical, and the thimerosal is removed from the vaccine as part of the finishing process when it is manufactured. Manufacturers stopped using it in all other childhood vaccines following recommendation by the US Public Health Service, who wanted to “reduce mercury exposure among infants as much as possible”.
What’s this “LD50” thing you mentioned?
I’m glad you asked!
LD50 is short for “Lethal dose, 50%” and is also known as the median lethal dose. It is, simply put, the quantity of a substance required to kill half the members of a tested population after a specified test duration. Thimerosal has a LD50 of 75 mg/kg to 91 mg/kg, depending on the type of rodent you are trying to kill with it. Extrapolating outwards to humans from the lower number – always a risk, since the Material Safety Data Sheet didn’t have the information for humans – let’s do a little math. My son weighed right around 4 kg when he got his vaccine. So, to hit the LD50 dosage, he’d have had to get a 300 mg dose of pure thimerosal.
(Incidentally, the LD50 of water is “>90 ml/kg” for rats. Just so you know.)
I don’t remember how much he weighed at two months, when he got his first DTaP vaccination, but let’s assume his LD50 level was still right at 300 mg at that point. Now, the total volume of fluid in the DTaP injection is which is 0.5 mL (which is 0.5 grams or 500 mg in weight), a quantity that is 1.66 times larger than the LD50 we calculated. Which sounds frightening, right?
Maybe not. After all, on how much of that 0.5 mL is actually thimerosal. And it turns out that the answer is “most likely none whatsoever”. The FDA, lists three different DTaP vaccines, one DTap-HepB-IPV vaccine, one DTaP-IPV/Hib vaccine, and one DTaP-IPV vaccine from two different manufacturers. Only one of them has any thimerosal in it whatsoever, and that measures at less than 0.3 µg in a 0.5 mL dose. A “µg” is a microgram, if you aren’t familiar with that notation, and it is a one-thousandth of a milligram or a one-millionth of a gram. Which means that there is less than 0.0003 grams of thimerosal in the vaccine in the worst case scenario. So, assuming my son was exposed to any at all, it was less than million times smaller than the LD50 dose.
But let’s dig deeper!
The FDA also discusses some other dosage levels for thimerosal. A 1931 study determined that the “maximum tolerated doses not associated with death” was 20 mg/kg for rabbits and 45 mg/kg in rats, with “no histopathological changes in the brain or kidney” after 190 days of daily intranasal doses at that level. Some cases of acute mercury poisoning from thimerosal-containing products have been observed in the medical literature, however, at concentrations as low as 3 mg/kg. Still, even at that extreme level, my son would have needed to be exposed to approximately 12 mg (12,000 µg) of thimerosal to risk even acute mercury poisoning – more than 40 times more than he might have been exposed to. Because, remember, he only had a 1-in-6 chance of even getting a DTaP vaccine that contained any thimerosal at all.
This is just one of the factors that led me to write what I did when I responded to “Old School Hippie”. Here’s what I said:
My son has been vaccinated. Why? Because I trust thousands of peer reviewed papers over multiple decades of research that demonstrate the safety and efficacy of vaccines over proven fraudsters and quacks like Andrew Wakefield and his ilk. I trust the science and medical expertise that has wiped out smallpox, eradicated measels from the Americas, nearly wiped out polio, and transformed chickenpox from a fact of life into the vague memories of older folks like me.
No, I owuldn’t let you inject mercury into my son. I also wouldn’t let you put him under a general anaesthetic or implant tubes in his ears, or prescribe him antibiotics. But I would allow a trained medical doctor, making use of the best medical knowledge available and acting under accepted standards of medical care within his or her own area of expertise, to do any or all of these things. Because they, unlike you, know what they’re doing.
It’s nothing personal. I also wouldn’t let you rewire my house or fix my plumbing unless you happen to be an electrician or a plumber.
Always remember: the School of Hard Knocks is full of lessons taught by Professor Dunning-Krueger.
(Note: The article was corrected after publication to correct and remove a math error. Thanks to MBDK for letting me know!)