A measles outbreak has killed 35 people across Europe in the past year, with the World Health Organization (WHO) calling the deaths an “unacceptable tragedy.”

“We are very concerned that although a safe, effective and affordable vaccine is available, measles remains a leading cause of death among children worldwide, and unfortunately Europe is not spared,” Dr. Zsuzsanna Jakab, the WHO regional director for Europe, said, in part, according to BBC News. “I urge all endemic countries to take urgent measures to stop transmission of measles within their borders, and all countries that have already achieved this to keep up their guard and sustain high immunization coverage.”

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While 31 deaths have been recorded in Romania, more than 3,300 illnesses were recorded in Italy, BBC News reported. Deaths were also recorded in Portugal and Germany, where the government has launched an initiative to fine parents who fail to vaccinate their children, or consult with a doctor about vaccinations.

In Italy, which counts a 6-year-old child as its latest victim, officials have also moved to tighten rules on vaccinations. In May, the country ruled that children must be vaccinated against common illnesses in order to be eligible for enrollment in kindergarten.

According to Reuters, lack of public trust in vaccines has caused a spike in many preventable diseases. Experts said negative attitudes may be due to fears over suspected side-effects and hesitancy among some doctors.

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“Nobody can be indifferent to the fact that people are still dying of measles,” German Health Minister Hermann Groehe told Bild newspaper, in May. “That’s why we’re tightening up regulations on vaccination.”

The highly contagious measles virus typically start with fever, runny nose, cough, red eyes and sore throat, which progresses to a rash, according to The Centers for Disease Control and Prevention (CDC). Children younger than 5 and adults over the age of 20 are more likely to suffer from complications which could include ear infections, diarrhea, pneumonia, encephalitis or even death.