The notion that vaccines might cause autism was refuted nine years ago, when a British medical panel concluded in 2010 that Andrew Wakefield, the doctor with undisclosed financial interests in making such claims, had acted with "callous disregard" in conducting his research.

But in 2019, professional epidemiologists are still devoting time and resources to discrediting Wakefield's work, which set off a steep decline in vaccinations, including in the United States, where Wakefield moved in 2004. An increasing number of parents are exempting their children from immunization for religious, personal and medical reasons, in a trend that public health experts warn is threatening to reverse the progress that allowed officials to declare measles eliminated in the U.S. in 2000. In January and February of this year, 206 individual cases were confirmed in 11 states - more than the number of cases in all of 2017.

Video: Despite the massive evidence, the anti-vaccination movement is gaining strength. Scientists are concerned measles could return even though it was 'eliminated' in the U.S. 20 years ago. (Luis Velarde/The Washington Post)

The latest evidence unequivocally denying any link between autism and the vaccine for measles, mumps and rubella - a two-dose course that the Centers for Disease Control and Prevention says is 97 percent effective - came Monday in a paper published in the Annals of Internal Medicine.

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Researchers at Copenhagen's Statens Serum Institut examined data for Danish children born from 1999 through the end of 2010, more than half a million people altogether. The epidemiologists and statisticians then used population registries to link information on vaccination status to autism diagnoses, as well as to sibling history of autism and other risk factors.

The findings show that the vaccine does not increase the risk of autism, lending new statistical certainty to what was already medical consensus. The researchers further concluded that vaccination is not likely to trigger the developmental disorder in susceptible populations and is not associated with a clustering of cases appearing after immunization.

"The appropriate interpretation is that there's no association whatsoever," Saad Omer, a professor of global health, epidemiology and pediatrics at Emory University, said in an interview with The Washington Post.

In an editorial accompanying the study, however, Omer and a colleague asked whether vaccine research was best conducted as a "response to the conspiracy du jour." Limited resources, they suggested, might be better spent on promising leads in autism research than on continuing to engage with "vaccine skeptics."

Omer nevertheless hailed the Danish paper as "the largest, or one of the largest, studies on the subject." Its only limitation, he said, was one basic to all observational studies, that "you can't intentionally vaccinate people or prevent them from vaccinating to study the effects, which would be unethical."

He offered that assessment on the eve of a Senate hearing on vaccines and the outbreak of preventable diseases, where he is scheduled to give expert testimony Tuesday alongside public health officials and other researchers, as well as a teenager, Ethan Lindenberger, who got vaccinated against the wishes of his parents.

Though the study was not intended to coincide with the congressional inquiry, the results did come at a critical juncture. Measles cases are multiplying, causing experts to warn that a nationwide outbreak is possible.

Six outbreaks are currently ongoing in the U.S., according to the CDC. Seventy-one people have been infected in Washington state, where an outbreak took hold this year in Clark County, an area across the Columbia River from Portland, Ore., that researchers call an anti-vaccination "hotspot" because of the high rate of nonmedical exemption from required vaccines.

The virus has similarly been spreading globally. An unvaccinated French 5-year-old recently reintroduced the disease to Costa Rica, which had been free of measles for five years. An outbreak in an Orthodox Jewish community in New York began when an unvaccinated child returned home after acquiring the disease in Israel, where a major flare-up is occurring.

The World Health Organization reported 72 deaths from measles in Europe last year. In 2015, a woman in Washington state died of pneumonia after contracting measles. It was the first U.S. death from the disease since 2003.

Measles is highly contagious, remaining for as long as two hours in the air of a room where an infected person has been. While the illness often begins with cold-like symptoms and a rash, infected people may fall victim to additional complications, including pneumonia and, in more severe cases, inflammation of the brain known as encephalitis and even seizures.

It is precisely the success of the measles vaccine - rendering these conditions rare, at least in the U.S. - that has enabled a small but fervent opposition movement to take root, Omer said.

"It is in some sense a victim of its own success," the Emory professor said of the vaccine, which became available in the U.S. in 1963. "It's hard to see the benefit if you don't see the disease."

Suzinne Pak-Gorstein, a pediatrician in Seattle and a professor at the University of Washington, said public awareness had grown since Washington Gov. Jay Inslee, a Democrat who recently launched a bid for the presidency, declared a state of emergency in January. Still, she lamented that a crisis was necessary to jar residents into protecting themselves.

"People forget that they should be worried, and then, here we go again," she said. "I do blame our non-vaccinated population, which has been scared by unfounded links to autism, which have been shown to be false."

But almost a decade after the journal in which Wakefield published his findings, The Lancet, fully retracted his 1998 paper, following the conclusions of the medical panel, concerns about the safety of the two-dose course for measles have not disappeared.

The public rebuke from the gatekeepers of his field did not silence him. To the contrary, it expanded Wakefield's platform, as he turned the criticism into a catchphrase for his 2010 book, "Callous Disregard: Autism and Vaccines - The Truth Behind a Tragedy," which is a bible of the anti-vaccination movement. A paperback reprint came out in 2017, and is No. 12 on Amazon in the category "Preventive Medicine" as of early Tuesday. Critics were outraged when Wakefield appeared at one of President Trump's inaugural balls, shooting live video in which he mused about a "huge shake-up" at the CDC.

The discredited link between the measles vaccine and autism "continues to cause concern and challenge vaccine acceptance," the authors noted in their new paper, which is a follow-up to a similar study they conducted in 2002. Theirs are among numerous attempts to stamp out misinformation about the vaccine.

That these efforts haven't been successful suggests new methods are required, Omer said.

"The question to my mind is should we continue to do more studies on this topic or is the uncertainty that is needed for having a researchable question gone at this point," he said. "This new study isn't going to change anyone's mind."

A more promising approach, he said, lay in developing communication strategies and behavioral science interventions that could be deployed by clinicians. Requiring patients to opt out of vaccines, rather than expecting them to elect to get immunized, would increase protection. So, too, would what he called "presumptive communication," in which clinicians frame immunization as an expectation, rather than an option.

"So it would be, 'Time for little Johnny to get vaccinated,' instead of, 'Should little Johnny get vaccinated?'" Omer said. "That sort of framing has an effect."

This article was written by Isaac Stanley-Becker, a reporter for The Washington Post.