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Vaccine injury is a tricky thing to prove — medically and legally. So it was inevitably controversial when the Supreme Court ruled on Tuesday against the parents of Hannah Bruesewitz, 18, who suffered seizures and permanent brain damage after receiving a diptheria-tetanus-pertussis (DTP) vaccine in 1993.

The Bruesewitz family sued Wyeth, then the parent company of the maker of the DTP vaccine that Hannah received as an infant, arguing that Wyeth was responsible for Hannah’s condition and should be held accountable. The high court disagreed.

Rather, the Court said, vaccine makers cannot be held liable for negative side effects that parents believe are the result of their products. To be compensated, parents must go before special no-fault tribunals established by the National Childhood Vaccine Injury Act of 1986. Congress created these “Vaccine Courts” with the participation of pharmaceutical companies as a sort of “societal bargain” — as Justice Antonin Scalia noted in the majority decision in Bruesewitz v. Wyeth — to ensure the future of vaccine availability in the U.S. At the time of the Act, hundreds of injury lawsuits were piling up against vaccine manufacturers, causing them to abandon manufacture of major vaccines and threatening immunizations for the entire country. (More on Time.com: Q&A: The Dangers of the Anti-Vaccine Movement)

The terms? Injured parties may file for compensation without having to prove cause — they simply have to demonstrate that the injury occurred immediately after vaccination. Further, the claimed injury does not have to be among the listed contraindications on the vaccine package, but must be included in a list of side effects called a Vaccine Table, kept by the Vaccine Court. In exchange, pharmaceutical companies cannot be held liable in civil court for adverse effects, and there is a cap on how much claimants may be awarded. (The fees come from taxes on immunizations.)

The National Childhood Vaccine Injury Act “reflects a sensible choice to leave complex epidemiological judgments about vaccine design to the FDA and the National Vaccine Program rather than juries,” Justice Scalia wrote on behalf of the Court.

This kind of agreement illustrates the complicated relationship the public has with a product that is designed to keep it safe (but, like any therapeutic drug, holds the risk of harm), and shows how far outside the realm of scientific evidence public fears extend. The Bruesewitzes’ case also, once again, brings to the forefront the ever-stronger antivaccine movement in the United States. In some regions of the country parents are increasingly refusing to vaccinate their children, influenced not only by the remote possibility of adverse reactions, but also by fringe activists who believe vaccines cause autism or other unrelated ailments. (More on Time.com: The Link Between an H1N1 Vaccine and Narcolepsy)

To be clear, autism was not Russell and Robalee Bruesewitz’s official complaint. Their daughter Hannah suffered seizures and subsequent brain damage after she received the DTP vaccine. Hannah began having seizures within hours of receiving the third of a scheduled five doses of Wyeth’s Tri-Immunol DTP vaccine, at 6 months old. Within 16 days, she had an additional 125 seizures. Her medical records included speech delays and “autistic-like features” among her symptoms. She now suffers from residual seizure disorder and remains developmentally impaired.

The DTP vaccine that Hannah received came from a batch associated with an unusually high number of adverse events, including one death and eight children who had convulsions. The Bruesewitzes took their case to a Vaccine Court in 1995, but were denied a settlement because residual seizure disorder wasn’t one of the adverse events appearing on the Vaccine Table for DTP. The petition was dismissed in 2003.

So the family sued under Pennsylvania tort law, this time arguing that Wyeth was negligent in maintaining its Tri-Immunol DTP vaccine. The Bruesewitzes argued that the vaccine, which was developed in the 1940s, hadn’t been updated since, even while other companies had updated their DTP vaccines, and thus resulted in design defects that caused “avoidable” harm.

Federal law prohibits parents from suing vaccine makers for design defects, saying that no vaccine maker can be held liable for death or injuries arising from “side effects that were unavoidable even though the vaccine was properly prepared and was accompanied by proper directions and warnings.” But in this case, the Bruesewitzes argued, since Wyeth ultimately withdrew this version of Tri-Immunol from the U.S. market in 1998, Hannah’s injuries should be considered avoidable — the company could have made an earlier update and didn’t for financial reasons, they claimed. (More on Time.com: Gardasil Protects Boys and Men From HPV Too)

Ultimately, the case landed in the Supreme Court, which voted 6 to 2 in favor of vaccine makers on Tuesday. Writing for the majority, Justice Scalia said that to allow civil litigation for vaccine injury claims that had failed in Vaccine Court would set a precedent that would destroy the market for vaccines. Juries would be sympathetic to the plight of parents, regardless of medical evidence, and litigation would go through the roof.

Despite the overwhelming scientific evidence that vaccines do not cause permanent disorders like autism and epilepsy, many parents cling to the myth. Why? Much of the confusion stems from timing: the first signs of epilepsy, other seizure disorders or autism often appear right about the time children receive many their routine vaccines. The co-occurrence is coincidental.

“The thing about epilepsy is that it’s often a seemingly random event. The human response is to try to find a reason. I’ve literally had people come in and say that their epilepsy has to do with the phases of the moon,” said Dr. Carl W. Bazil, neurology professor at Columbia University. “Can I prove that epilepsy is never caused by vaccine? No. But as a neurologist I can say that it’s extremely unlikely.”

To get a better understanding of the antivaccine mindset, I spoke with Dr. Paul Offit a foremost vaccine expert and author of Deadly Choices: How the Anti-Vaccine Movement Threatens Us All. Offit has seen the most unusual medical reactions to vaccines and the most dire consequences of vaccine distrust: preventable childhood deaths, community outbreaks of outdated diseases and misinformed, angry parents. You can read our conversation here.

Related Links:

Why People Reject Things That Keep Them Safe

Autism, Vaccines and Fraud: Q&A With Author Seth Mnookin