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Amy Martin, a nurse, wears a Mississippi Parents for Vaccine Rights button before a legislative meeting about a bill that would loosen the state's strict vaccination laws, at the Capitol in Jackson, Miss., Feb. 3, 2015. Mississippi is one of two states -- West Virginia is the other -- that permit neither religious nor philosophical exemptions to its vaccination program. "All medical treatments - medication, procedures - they all come with risk, " says Martin. (William Widmer/The New York Times

J.B. Handley believes vaccines can cause autism – and he doesn't much care if the vast majority of doctors and medical researchers disagree.

"I had a normal developing child who had a stair-step decline that matched his vaccine appointments," says Mr. Handley, an asset manager based in Portland, Ore., who founded Generation Rescue, an autism organization perhaps most widely known because of its affiliation with actress Jenny McCarthy.

"I watched my son descend into autism over about a five- or six-month period and it was a perfect match with the 13-month vaccine where you get [the measles, mumps and rubella vaccine] along with many other vaccines. That was about September. Then there was the flu shot, and December seemed to be the final blow that pushed him over the edge. By March, we had a completely different child."

The opinion Mr. Handley espouses has, over the past decade and a half, become an increasingly prevalent one among a growing number of parents around the United States. The result is a potential public health time bomb.

An ongoing measles outbreak across more than a dozen states has once again cast a spotlight on declining vaccination rates across much of the United States. For more than a decade, and for a variety of reasons, more and more parents have opted not to vaccinate their children – so much so that some parts of the country are experiencing outbreaks of diseases once deemed eradicated.

But beyond the immediate public-health implications, the declining vaccination rates have also suddenly become a matter of politics – pitting issues of public safety against the traditional Republican talking points of personal freedom and limited government intervention.

This week, two likely Republican presidential candidates weighed in on the vaccine issue. New Jersey Governor Chris Christie said there should be a "balance" between public health – all states have some form of mandatory vaccination schedule – and parental choice. After his comments were widely criticized, he backtracked, saying there is "no question" children should be vaccinated against a disease such as measles.

However, Kentucky Senator Rand Paul – also a likely 2016 presidential contender – took a much less tepid stance, saying in media interviews that he believes most vaccines should be voluntary, and claiming that he has heard cases of children suffering "profound mental disorders" after vaccinations.

Both men – in addition to President Barack Obama, who supported vaccination unequivocally in an interview this week – were prompted to speak out on the issue as a result of a relatively large measles outbreak that is believed to have started in the Disneyland amusement park in California. According to the Centers for Disease Control and Prevention, there were 102 measles cases in 14 states in January, 92 of them in California. Measles was declared eliminated in the United States 15 years ago, meaning that it is no longer native to the country.

But while foreign travellers may still bring the disease with them, the spread of measles and other conditions for which there exist vaccinations is largely a function of how many people choose to accept that vaccination. And in the past decade and a half – spurred by everything from government mistrust to since-debunked research studies to a desire for a "toxin-free" lifestyle – vaccination rates have dropped significantly in some pockets of the country.

Oregon is one such example. According to Paul Cieslak, the medical director of communicable diseases and immunizations at the state's health authority, the rate of children entering kindergarten with a "non-medical" vaccine exemption was about 1 per cent in 2000. Since then, that rate has risen every year. For the 2013-14 school year, it stood at 7 per cent.

Among those parents who have chosen to refuse vaccination for their children, a number of similarities exit. Perhaps most significantly, a large number belong to a demographic that is rich, college-educated and very well-read on the issue – even if the credibility of the reading material is questionable.

"I found that for [low-income] families, this was not their issue. ... They have jobs, they don't want to risk having to miss work because they've got a sick kid," says Jennifer Reich, an associate sociology professor at the University of Colorado Denver, who conducted extensive interviews with parents who refuse to vaccinate their children.

"Overwhelmingly [those refusing vaccination] were college-educated families with more secure income. They really prioritized attention to their children because they can."

One of the primary reasons vaccine-refusal rates have spiked in the past 15 years is the work of a British doctor and researcher named Andrew Wakefield, who in 1998 wrote a paper alleging a link between the measles, mumps and rubella vaccine and autism. The paper has since been found to be fraudulent, and its author barred from practising medicine in Britain, but its impact can still be felt.

More recently, refusing vaccination has become a trend among myriad affluent communities throughout the United States, where it is seen as part of a wider movement to limit the amount of "toxins" to which a child is exposed.

Many parents who refuse to vaccinate their children do not want the practice eliminated entirely. Instead, they argue that the schedule of recommended shots has ballooned over the past 30 years, and they propose a much smaller list of recommended vaccines, administered over a longer period of time.

However, medical experts argue that there is a reason the list of recommended vaccines has grown.

"It's a function of the fact that they work," Dr. Cieslak says. "I don't think it's too many at all. If you look at the composition of the vaccines themselves, the number of things [the recipient is] exposed to has gone down."

Nonetheless, vaccination rates continue to decline. And perhaps most worryingly, while the overall rate remains relatively high, some communities and schools where the vaccine-skeptical mindset is prevalent tend to have very low vaccination rates, making them much more prone to outbreaks.

Health officials around the country have tried to counter the trend – a Washington state law requires guardians who refuse vaccinations on certain grounds to sign a form indicating that they have been informed of and understand the risks of doing so. The law has helped to bring the state's overall refusal rate down, but the countrywide trend continues.

"We've achieved a great public-health victory and I'd hate to see us casually toss that victory aside," Dr. Cieslak says.

"I think we're on the verge of that."

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Vaccination rates

Measles is highly contagious and in unvaccinated populations each person with the virus will infect, on average, 16 to 18 others. So high vaccination rates – 95 per cent and up – are needed to block measles transmission and achieve herd immunity. Canada's coverage is probably close to that figure, says Dr. John Spika of the Public Health Agency of Canada. People who don't vaccinate for religious reasons tend to live in clusters around churches or faith-based schools. Within those pockets, vaccination coverage might be very low. Experts suggest this clustering is driving outbreaks.

The Toronto outbreak

Doctors in Toronto are responding to a small measles outbreak in which four people – three of whom had not received their measles vaccine – were diagnosed with the infectious disease in less than a week. Health officials said on Monday that tests confirmed infections in two adults and two children under the age of 2. One case was confirmed on Jan. 29, two on Jan. 30 and one on Feb. 1. Public-health officials have not been able to find any connections between the four patients, nor have they been able to determine where they contracted the virus.

The U.S. outbreak

Considered eradicated from the United States in 2000, measles re-emerged in December in an outbreak clustered around the Disneyland in California. Since then, 102 cases of measles have been reported in 14 states according to the Centers for Disease Control and Prevention. In 2014, the parents of 79 per cent of unvaccinated children asked authorities of their state to be exempt from immunization on the basis of their beliefs, the CDC reports. The United States had 644 cases of measles in 2014, a record number since 2000. There were 173 cases in 2013.

Measles by air travel

Before the measles vaccine was introduced around 1970, outbreaks generally occurred in late winter and spring. Now, Dr. John Spika of the Public Health Agency of Canada says, importations often happen in periods when families travel, such as over the Christmas holidays or during the March break. Cases come to Canada from places where the virus is spreading, which in recent years has included the Netherlands, France, India, China and the Philippines.

Roald Dahl

Children's author Roald Dahl, who lost his daughter, Olivia, to measles in 1962, wrote a plea to an English regional health authority in 1988 urging everyone to get their chilrden vaccinated. "As the illness took its usual course I can remember reading to her often in bed and not feeling particularly alarmed. … 'Are you feeling all right?' I asked her. 'I feel all sleepy,' she said. In an hour, she was unconscious. In 12 hours she was dead. … Today a good and safe vaccine is available to every family and all you have to do is to ask your doctor to administer it."

Wire services

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By the numbers

50-75: Approximate number of deaths from measles annually in Canada in the prevaccine era, compared with about 400 to 500 in the U.S.

99%: The decline in annual measles cases in Canada as a result of the introduction of the vaccine, from about 350,000 a year before 1963 to less than 2,000 a year in 1995

2002: The year the virus was eliminated in the Americas, according to the World Health Organization. The last endemic measles case was reported on Nov. 16, 2002

102: Number of measles cases reported in the U.S. in January, 2015, most of which have been linked to visits to Disneyland in California. Infections have been reported in 14 states

84%: Share of the world's children who received one dose of the measles vaccine by their first birthday through routine health services in 2013 – up from 73 per cent in 2000

145,700: Number of measles deaths globally in 2013 – about 400 deaths every day or 16 deaths every hour

15.6 million: Estimated number of deaths the measles vaccination has prevented globally between 2000 and 2013, according to the World Health Organization

Sources: World Health Organization, Health Canada, Immunization Canada, U.S. Centers for Disease Control

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