With a measles outbreak sprouting in 14 states, and Georgia reporting its first case in almost three years, vaccine opponents are reviving claims about the dangers of vaccines against the disease in newsfeeds across the country.

An alert reader was skeptical of one Facebook post, reviving a 2012 story that purports to list "10 outrageous (but true) facts." It starts with the claim that helped spur the anti-vaccination movement, despite being repeatedly debunked: Childhood shots contain mercury, a powerful neurotoxin with no safe level of use.

"Doctors who inject children with vaccines are delusional," Mike Adams concludes from his list of claims. "They are practicing a medical holocaust against humanity."

How could PolitiFact Georgia resist jumping in to such subtlety?

Adams declined to speak with us about his conclusions regarding mercury. He referred us instead to studies on his website, most of which deal with effects of methyl mercury.

That is a very different chemical from ethyl mercury, a component in the vaccine preservative thimerosal.

We’ll get to that difference in a minute. But the story of thimerosal and vaccine safety bears repeating.

Early vaccinations carried the risk of dangerous and sometimes lethal bacterial infections, such as a staph infection that killed four children in 1916. Thimerosal was added to vaccines to keep multi-dose vials free from such bacteria starting in the 1930s.

In 1998, the British medical journal the Lancet published a paper that claimed there was a link between the MMR vaccine — for measles, mumps and rubella — and autism. The paper by Dr. Andrew Wakefield drew considerable publicity and is credited in pushing vaccination rates down in the United Kingdom and the United States.

In 1999, the Food and Drug Administration held meetings to discuss the idea that thimerosal — then used in three childhood vaccines such as the hepatitis B shot — was damaging immunized infants’ brains and contributing to the rise in autism diagnoses.

At the time, Dr. Neal Halsey was the chairman of the American Academy of Pediatrics committee on infectious diseases. The Johns Hopkins pediatrician had long pushed for expanding vaccinations as a way for children to live healthier lives.

But he was willing to entertain the idea that thimerosal was helping to impair infant brains, even though there were no data to support the theory.

Thimerosal was removed and has not been used as a preservative in any vaccine on the recommended schedule for children since 1999.

By 2004, Wakefield's study was retracted and would later be deemed an "elaborate fraud" by the British medical journal BMJ. Wakefield eventually lost his medical license over the ordeal.

At the same time, several studies, including ones in 2001 and 2004 from the Institute of Medicine, found no evidence linking vaccines and autism.

"There was no convincing evidence that there was any harm from thimerosal, but I felt we would have difficulties with public acceptance of vaccines if we continued its use," Halsey said. "We are not using it anymore. Now the science is definitive that the small amount of thimerosal did not cause autism or any other neurological damage. "

The damage may already have been done, though, regarding public suspicion of vaccines. Like Halsey, Dr. Paul Offit, a pediatrics professor at Children’s Hospital of Philadelphia, agreed that the science never supported the theory that thimerosal caused harm.

But Offit worried attention to the unproven fear of "mercury" was minimizing the proven fear of the diseases the vaccines were designed to keep at bay.

"Before there were vaccines, whooping cough killed 8,000 children a year, polio crippled 20,000 kids and killed 1,500 to 2,000," Offit said. "Measles killed 500 children a year and caused 48,000 to be hospitalized. The best way to address safety concerns is not to cater to false ones and focus on those real threats."

Adams also confuses the issue with the interchangeable use of the term "mercury" for both thimerosal’s ethyl mercury and the more problematic methyl mercury.

Think of ethyl mercury like ethyl alcohol — easily expelled from the human body but dangerous in large quantities. Thimerosal was used in trace amounts in vaccines.

Methyl mercury is the compound found in fish and water, which builds up because the body can’t easily expel it. While some amounts occur naturally in the environment, it’s more comparable to methyl alcohol, the methanol that fills your gas tank but isn’t fit for human consumption.

Repeated research studies agree that there is no evidence of harm from thimerosal’s mercury compound in vaccines.

But there is an even bigger problem with Adams’ claim.

The Centers for Disease Control and Prevention, aware of the controversy and confusion, lists all ingredients in childhood vaccines for review.

MMR, the vaccine making headlines now, never contained thimerosal. Neither did the shots for chickenpox, polio or others.

Thimerosal, remember, was designed to prevent bacterial growth with repeated entry into a vial.

Therefore, it wouldn’t be used in single-use vials most common in the United States. And there is no need for a preservative with the live virus vaccines such as measles, said Dr. Marie McCormick, a professor at the Harvard School of Public Health who oversaw the committee that conducted the 2004 IOM vaccine safety review.

In other words, our reader was right to be skeptical.

There has been debate in the scientific community about removing a mercury compound from some vaccines. But the science has always been clear that there is no evidence that compound causes harm.

Bluntly, vaccines help, not hurt.

And if you still don’t believe that? Consider that the recipe in question — that is, the series of shots recommended for childhood immunization — does not include the ingredient singled out as the problem.

Any claims to the contrary muddy what should be a simple issue.

We rate the claim Pants On Fire.