The Republican Party was on the defensive yesterday after two of its leading presidential hopefuls indicated that they had sympathy for parents unwilling to vaccinate their children against infectious diseases, even in the midst of a dangerous outbreak of measles centred in California.

One day after both Senator Rand Paul and Governor Chris Christie publicly questioned the requirement for parents to vaccinate their offspring, the Speaker of the House in Congress, John Boehner, said all children in the United States should be vaccinated. So far the outbreak, which has been traced back to the Disneyland theme park in California, has infected more than 100 people in the US.

Christie, the Governor of New Jersey, made his comments on Tuesday while touring MedImmune, a medical vaccine company in Cambridge, during a three-day trade mission to Britain that ended yesterday. He told reporters accompanying him that parents "need some measure of choice" on the matter.

In an interview with CNBC, Paul was more forthright. "The state doesn't own your children," he said, adding that he had deliberately spread out the vaccinations of his own children so they didn't receive too many at one time. He said he had "heard of many tragic cases of walking, talking normal children who wound up with profound mental disorders after vaccines". But he offered no details.

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A so-called "anti-vaxxers" movement has grown up in the US in recent years, thanks in part to a widely debunked 1998 report in the Lancet, the British medical journal, suggesting a link between MMR vaccinations against measles and the onset of autism in children.

On Monday, President Barack Obama told NBC every child should be protected by vaccinations. And on Tuesday, Hillary Clinton, widely expected to be the Democrat nominee in 2016, added her voice in a mocking Twitter post. She said. "The science is clear: The earth is round, the sky is blue, and #vaccineswork. Let's protect all our kids. #GrandmothersKnowBest."

Christie attempted to row back on his Cambridge remarks with a statement issued soon afterwards by his office in New Jersey. It said he believed "there is no question kids should be vaccinated when it comes to diseases like the measles".

But by then his earlier remarks had already been seized on by the Democratic Party. "Chris Christie isn't a scientist. He isn't a doctor. And he sure as heck isn't a leader," Mo Elleithee, a spokesman for the party, said. "If his campaign is going to be about kissing up to the radical, conspiracy theory base that's wagging the dog of today's Republican Party, that's up to him and his cracker-jack team."

Spotlight on measles

Who gets the measles? What are the symptoms?

Mostly children, but unvaccinated adults who have never had the disease are at risk too. It starts with a fever, runny nose, sore throat and red eyes and is followed by a rash of red dots that can cover your entire body, according to the Centres for Disease Control and Prevention.

But I've heard measles can be more dangerous than that.

Yes. Deaths are rare in the United States, but one or two of every 1000 children who contract measles will die from the disease, says the CDC. More common complications include pneumonia, encephalitis (swelling of the brain) and deafness. Pregnant sufferers may give birth prematurely or deliver low-birth-weight babies.

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How is it spread?

Mostly by coughing and sneezing. The virus can live on a surface or hang in the air for as long as two hours after an infected person has coughed or sneezed. It is so contagious that 90 per cent of people who are not immune can get it from being near a single sufferer. People are contagious from four days before the rash appears until four days after.

When did the vaccine become available?

The vaccine, which uses a live virus to stimulate production of antibodies to the measles virus, became available in 1963.

If I've had the measles, am I immune? What if I'm not sure?

Yes, you only get the measles once. If you're not sure, your doctor can perform a simple blood test.

How effective is the vaccine?

The first dose, given at 12 months, is about 95 per cent effective. The second, at 4 to 6 years, improves those odds to 98 or 99 per cent.

Why is measles spreading in the US?

The CDC says last year's outbreak - and most likely this year's too - was caused by unvaccinated travellers becoming infected abroad and returning to the US. In areas where many are not immunised, the disease is more easily spread.

- Washington Post-Bloomberg