(CNN) In February this year, a holiday in paradise turned into a nightmare for one French family. Soon after their arrival in Costa Rica, their unvaccinated five-year-old son developed measles, the country's first case since 2014. The parents, also unvaccinated, tested positive for the virus too. They were believed to have brought it from France.

Measles is more contagious than Ebola, tuberculosis or flu, it has no specific treatment, and it can be picked up from the air or from surfaces for as long as two hours after an infected person has come and gone. Because it's so infectious, health experts see it as an early warning sign that there's a problem with vaccination coverage.

"Measles is like a canary in the mine," says Heidi Larson, director of the Vaccine Confidence Project and a professor of anthropology at the London School of Hygiene & Tropical Medicine.

To protect a population from measles, she says, at least 95% of people need to be vaccinated -- a higher threshold than for most other infections. This means that if vaccination rates start falling, "it's going to be the first to show its ugly head".

When last measured in 2017, Costa Rica had a vaccination rate of 96% for the disease -- a clear sign that its people shouldn't have been at risk from each other. That same year, France's measles coverage stood at only 90%.

This isn't surprising in light of the widespread concern about childhood vaccines in France. One in three French people think vaccines are unsafe -- the world's highest rate -- and nearly one in five believe they aren't effective -- second only to Liberia.

This is according to new data from the Wellcome Global Monitor, a worldwide poll of more than 140,000 people in 144 countries.

Global trend

France may be on the extreme end, but it's part of a global trend that has many health experts worried. Across the European Union, people are delaying or even refusing vaccines, contributing to a rise in disease outbreaks.

Between 2010 and 2017, over half a million French infants didn't receive a first dose of the measles vaccine. And last year, France was among the ten countries with the highest year-on-year increases in measles, with confirmed cases jumping from just over 500 in 2017 to nearly 3,000 in 2018.

According to the World Health Organization (WHO), reluctance or refusal to vaccinate is now one of the top ten major threats to global health. Even people in high-income countries, with good healthcare systems, are dying from easily preventable diseases. More than 70 people died of measles across Europe in 2018 -- three of them in France.

"It's a tragedy for Europe that a child or an adult has died because of a preventable disease," says Patrick O'Connor, a team lead in WHO Europe's Vaccine-Preventable Diseases and Immunization program. "We owe it to them to protect them."

A critical mass of unvaccinated people produces a wildfire effect, O'Connor adds. "Next year it could be diphtheria, or a variety of things."

In France, vaccine coverage rates exceed levels of trust -- people still vaccinate their children, even if they are skeptical. But the spike in measles shows that, for some diseases, coverage still desperately needs to improve.

Mandatory vaccination is one of the most successful ways of increasing immunization rates, says the World Health Organization.

Medical scares

Like many French parents, Priscille, a young mother who lives in Paris, says she is unsure about vaccinating her child. The 25-year-old doubts the effectiveness of injections, and is worried about damaging her baby's immune system.

"It's all about money for the pharmaceutical companies," she says

This lack of trust in vaccines, despite overwhelming evidence that they are safe and effective, can be traced at least in part to the French government's mishandling of a series of medical scares.

A blood transfusion scandal rocked the country in the 1990s. In the previous decade, thousands of people had been given blood contaminated with HIV -- most of them before the link between blood, HIV and AIDS was fully understood. Many of those who received contaminated blood died. Global headlines screamed "Transfusion of Death" and "Blood Scandal Ministers Walk Free" as the world's media tracked it for nearly a decade.

On the heels of this scandal came reports from medical professionals who claimed to have developed multiple sclerosis after receiving the hepatitis B vaccine. Although there was no evidence of a link, the government suspended its school vaccination program and promised to investigate in a bid to calm public outcry.

"But suspending it also raised suspicions," says Larson.

Then in 2009, France ordered 94 million doses of H1N1 vaccine at a cost of almost 1 billion euros. By the time the bulk of the vaccines had been delivered, it was realised that the pandemic was less dangerous than initially thought. At the end of the vaccination campaign in January 2010, the government was trying to cancel or sell off its surplus supplies. The process led to an "absolute, total breakdown of trust," Larson says.

"The public was angry at the WHO for what they thought was overinflation of the risk, angry at the government for having bought so much vaccine."

The percentage of people who were favorable towards vaccinations fell from 91% in 2000 to 61% in 2010, according to national health agency Santé publique France. By 2016 it had recovered somewhat, but only to 75%.

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Social media fuels scare stories

More recently, and as in many other countries, social media has enabled scare stories and misinformation about vaccines to spread fast. The Facebook group Info Vaccins France, for example, has a series of testimonies from parents saying their children either died or became seriously ill following routine inoculations.

Part of the problem is that if just one person in several million has an adverse reaction to a vaccine, their story will spread and inflate on social media with no wider context.

Henri Joyeux, a former surgeon and an honorary professor of oncology at the University of Montpellier, has been a leading campaigner against injections containing aluminum. It's used in some vaccines to boost the immune response and make a vaccine more effective and long-lasting.

Joyeux, who was nearly disbarred from the medical profession for his views against vaccination, launched a campaign against Infanrix hexa -- the six-in-one children's vaccine -- in 2016.

Vaccinations against some diseases -- like polio, diphtheria and tetanus -- are important, he says. But his complaint against Infanrix hexa is that it contains aluminium and formaldehyde, "dangerous substances for humans and especially infants". Infanrix hexa, he says, also contains "the hepatitis B vaccine suspected of being linked to multiple sclerosis".

The scientific consensus -- across the European Medicines Agency, the Vaccine Knowledge Project, the UK NHS and others -- is that Infanrix hexa is safe.

"There's a lot of pretty negative, discouraging and sometimes quite frightening material about immunization on websites," says Larson.

In response, French health bodies have put out information on social media platforms including Twitter, Facebook and LinkedIn. They have funded YouTube videos like 'Nine bullshits on vaccines', which has had over 350,000 views in 18 months, and sometimes they engage directly in online chats.

"It is very important that communication around vaccination continues and is always present vis-à-vis fake news," says Sylvie Quelet, director of health promotion and prevention at Santé publique France. "The battle is not won."

Making vaccines mandatory for attending school

Véronique Dufour is the deputy chief physician of the Paris maternal and infant protection center. She says her job has become much easier since the government increased the number of mandatory vaccines in January 2018.

Previously, the three vaccines that were mandatory -- diphtheria, tetanus and polio -- had good immunisation rates of 95% or higher.

But eight other vaccines, including measles, meningococcal C and hepatitis B, had only been recommended -- and coverage was lower. Now, unless they have a medical exemption, children need to have had all 11 of these vaccines to attend a state-run school.

"People are asking fewer questions. Very, very few are not vaccinated," Dufour says.

Early signs are that confidence in vaccines is rising since the law change. A small study of parents with young children carried out in February this year found that 91% think vaccination is important for their children's health, and 87% think it is important for protection of the wider population. Both figures show a 5% increase since a similar study was carried out in June 2018.

Vaccination rates have also increased. The percentage of babies receiving the first dose of meningococcal C vaccine rose from 39 % in 2017 to 76 % in 2018. In the same period, vaccination rates for hepatitis B, human papillomavirus (HPV) and measles also modestly increased.

Mandatory vaccination is one of the most successful ways of increasing immunization rates, according to the WHO. Indeed, France is not the only EU member to mandate large numbers of childhood vaccines. Others include Italy, Hungary, Bulgaria, Croatia, Czech Republic, Latvia, Poland, Slovakia and Slovenia.

But the WHO warns that this approach can also provoke vaccine hesitancy -- "not necessarily because of safety or other concerns, but due to resistance to the notion of forced vaccination".

"Taking away freedom of choice can easily appear authoritarian," says Jeremy Ward at the VITROME laboratory. In France's highly politicized environment, he says, this could prompt a minority who were previously hesitant to become rampantly anti-vaccine.

There might also be an increase in the number of children being given medical exemptions, Ward adds, which are supposed to apply if a child is allergic to eggs or if their immune system is too fragile. "There are doctors in France -- as in all other countries -- who provide fake medical exemptions."

Parents like Priscille plan to keep their children away from public nurseries and to send them to alternative schools that are outside the state education system.

"I'm lucky because I don't work, so I don't have to put them in the crèche, so I have the choice over vaccines," she says.

Cautious optimism

Laws are relatively quick to change. Attitudes and behavior may take longer.

Sitting in a café near Notre-Dame cathedral, the day after it was devastated by fire, pediatrician Sydney Sebban says doctors have to take the time to build trust with parents.

He starts to do this when they bring their babies in for the first time at 15 days old, and he asks them about vaccinations, alongside feeding and sleeping. It's a matter of listening to their questions, taking time and giving them quality information, he says.

"I say to them there is no product which has zero risk. It doesn't exist. People must think about side-effects of ibuprofen or antibiotics."

He encourages the parents to talk to their parents and grandparents about illnesses that have been forgotten since widespread immunization began.

"The most devastating consequence of vaccine hesitancy, or inadequate vaccine uptake, would be a resurgence of fatal or life-changing vaccine-preventable diseases which are currently well-controlled, or are targeted for elimination," says the European Centre for Disease Prevention and Control's chief scientist, Mike Catchpole.

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All in all, most experts say the public need to be reminded that vaccines protect lives from killer diseases that few people in Western countries have any experience of.

"Vaccines are one of the best public health interventions that we have, and they work. We just have to make sure people understand that," says O'Connor.

Among officials, doctors and academics, there is a cautious optimism that France will continue to see a rise in immunization rates.

"France can bring down the levels of skepticism with some extraordinary efforts," says Larson. "But I think it's never going to fully go away."