Nobody wants to hurt a child. The howl of pain, the tears, the look of betrayed shock all make it very hard to deliver a cherished small person to the needle-wielding nurse. But that’s part of being a responsible parent. It becomes a lot tougher if mum or dad is being fed mixed and erroneous messages about the benefits and harms of vaccination.

Parents have to make a lot of decisions about what’s right for their child, from the school they should go to and the food they should eat to the hours they should spend on the phone or watching TV. In the US, Donald Trump wants them to add another: do they choose to give their children separate jabs against measles, mumps and rubella, involving six visits to the clinic, or should they opt for the combined MMR vaccine, which does the job in two? For decades, the vast majority of families have been taking the advice of doctors, accepting their........

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