EVERY day seems to bring a new story of a politician saying something stupid or evasive about vaccines. Rand Paul frets that they might cause mental disorders. Chris Christie said that his own children had taken their shots but that "parents need to have some measure of choice". Barack Obama, who once waffled on this subject, has declared his strong support for vaccinating children against measles, as has Hillary Clinton. The airwaves and the internet are filled with discussions about whether or not vaccines are safe. Health officials are worried that the discussion itself could scare more parents into shunning them. This actually happened in Britain in 2002, after the idea of a link between the measles, mumps and rubella (MMR) vaccine and autism was first publicised. Despite plentiful scientific evidence that the MMR vaccine was safe, and that it was used in over 90 countries, what people heard was that there were two sides to the debate. Vaccination rates plummeted (though they have since rebounded).

An investigation into the decline of vaccinations in Britain found that the problem, in part, was the way the media covered the matter. In an effort to offer balance—and to entertain viewers with a lively verbal jousting match—news outlets lined up people with opposing views. In one corner were health experts who supported the vaccine. In the other were charismatic quacks or parents who were utterly convinced that the vaccine had made their children autistic (and whose genuine grief swayed many viewers). In America minor celebrities have joined this mix.

As government scientists in Britain spoke up to defend vaccinations, the issue became political. Some right-wing newspapers promoted alarmism about vaccines as a way to attack the left-wing government that administered them; others did so simply because scare stories sell papers. Suddenly there was a great deal of pressure on the prime minister to disclose whether or not he had vaccinated his son.

When scientific disputes are politicised, the truth suffers. For example, in 2002 the Sunday Times ran the headline, “The government has mounted campaigns to persuade parents the MMR jab is safe after some research linked it to autism and bowel disorders in children”, which invites casual readers to question whether they trust the government more than the researchers. Similarly, American headlines now say "Vaccination debate intensifies as measles outbreak spreads", which can be read to mean that there is a legitimate debate about the safety of measles vaccinations.

Since 2006 big measles outbreaks have occurred in Bulgaria, France, Ukraine, Georgia and Turkey. France went from 40 cases in 2007 to 15,000 in 2011, an epidemic caused by parents failing to have their children jabbed. Trust in vaccines appears to be tied to trust in government. A new study of swine-flu vaccinations in 2009 found that Republicans and independents, who were less likely than Democrats to trust the government, were also less likely to say they would get their children vaccinated. Kent Schwirian of Ohio State University says that people who trusted the government were almost three times as likely to vaccinate.

Health officials have not yet found a way to overcome the public's doubts. A study published in the Journal of Pediatrics found that none of the pro-vaccine messages disseminated by public-health authorities made people more likely to have their kids inoculated. Myths are hard to scotch, especially if believing them causes the believer no immediate harm, which is why plenty of people still think the world was created in a week.

There is a lot at stake here. The Centres for Disease Control and Prevention says that among children born during 1994-2013, vaccinations prevented 322m illnesses, 21m hospitalisations and 730,000 deaths. The lesson for politicians and the media is clear. Primum non nocere. First do no harm.