(CNN) Four European countries have lost their measles -free status as the region suffers a significant setback in eliminating the disease, according to the World Health Organization (WHO).

With measles cases across Europe on course to double this year, the disease has made a comeback in Albania, Czech Republic, Greece and the United Kingdom, where it had previously been eradicated, a WHO report said.

Recent outbreaks of the potentially fatal viral disease in various countries have been blamed on the growth of the anti-vaccination movement, which has spread via social media and discourages parents from immunizing their children against measles and other diseases.

Right-wing populist politicians including Italy's Matteo Salvini, who has promoted a bill removing mandatory vaccination for children, have also been influential in pulling the public away from scientific orthodoxy.

At the end of 2018, 33 of 53 countries in the European region retained their measles elimination status. Two -- Austria and Switzerland -- achieved measles-free status, according to the WHO, which means they have not had any new cases of the disease for 36 months.

Two countries have interrupted measles transmission, having had no new cases for 12-35 months. Twelve countries remain endemic for measles, and the four countries mentioned previously have re-entered measles transmission.

"Re-establishment of measles transmission is concerning," said Dr Günter Pfaff, Chair of the European Regional Verification Commission for Measles and Rubella Elimination (RVC).

"If high immunization coverage is not achieved and sustained in every community, both children and adults will suffer unnecessarily and some will tragically die."

There have been approximately 90,000 cases of measles reported in the first half of 2019, continuing a surge in the number of cases that began in 2018, when 84,462 cases were reported over the course of the year.

The WHO has declared a Grade 2 emergency and intensified its efforts against measles, according to a statement.

"Great efforts to control this highly contagious disease have brought us a long way towards regional elimination," said Zsuzsanna Jakab, WHO regional director for Europe.

"This is the time and opportunity to address any underlying health system, social determinants and societal challenges that may have allowed this deadly virus to persist in this region."

Online misinformation a key culprit

Social media is proving to be a key battleground for health information.

Professor Martin Marshall, vice-chair of Britain's Royal College of GPs, said the findings were "disheartening" at a time when Europe had been making good press in eliminating an entirely preventable disease. He drew a link between declining vaccination rates and the spread of "anti-vaxer propaganda" on social media.

"It is clear that we are still suffering from entirely debunked claims around MMR (the measles, mumps and rubella vaccine) that were perpetuated in the nineties -- and are now resurgent on social media and other online platforms," said Marshall in a statement.

However some social media platforms are taking action.

From August 28, Pinterest searches for vaccine-related terms will now only show results from institutions such as the World Health Organization, the US Centers for Disease Control and Prevention, the American Academy of Pediatrics and the WHO-established Vaccine Safety Net, in an attempt to stop the spread of anti-vax content.

"Misinformation about vaccines is as contagious and dangerous as the diseases it helps to spread," said the WHO in a statement.

"The World Health Organization welcomes Pinterest's leadership in protecting public health by only providing evidence-based information about vaccines to its users."

A global issue

The issue has also reared its head in the US, which may soon lose its measles-free status due to outbreaks that have caused more than 900 cases of measles.

Those outbreaks have largely been among children in the ultra-Orthodox Jewish community whose parents have refused to vaccinate them.

When the WHO declared in 2000 that the United States had eliminated measles, it was hailed as one of the biggest public health achievements in the nation's history.

Now there is a " reasonable chance " the country could lose its status in October, according to Dr. Nancy Messonnier, director of the US Centers for Disease Control and Prevention's National Center for Immunization and Respiratory Diseases.

Health professionals are looking into a number of solutions, including making vaccinations mandatory and imposing fines on parents who refuse.

One example is Germany, where the government has backed a bill that would see parents fined up to €2,500 ($2,800) if they fail to vaccinate their children before entering school or kindergarten.

Heidi Larson, director of the Vaccine Confidence Project at the London School of Hygiene & Tropical Medicine, told CNN that health authorities should be in emergency mode due to the seriousness of the situation.

"Ninety-thousand cases in the first half of the year is a massive amount of measles," she said, adding that nations should be ramping up routine vaccination programs to proactively identify under-vaccinated groups.

Larson also recommends that health authorities get involved in social media to spread positive messages about vaccines.

"That's where people are," said Larson, who believes that efforts need to be made to crowd out anti-vax information.

"We've created a world that's absolutely dependent on vaccination," she added.

"This is a really big deal."