The World Health Organization on called on European nations Wednesday to step up vaccinations against the highly contagious measles virus after an outbreak of over 22,000 cases across the continent since 2014.

"We must collectively respond, without further delay, to close immunisation gaps," said Zsuzsanna Jakab, WHO's Europe director. "It is unacceptable that, after the last 50 years' efforts to make safe and effective vaccines available, measles continues to cost lives, money and time."

According to the UN health agency, 22,149 cases of measles have been reported in seven countries across the region since the start of 2014, with Kyrgyzstan, Bosnia and Herzegovina as well as Russia hit hardest.

However, the outbreak has also struck Georgia, Kazakhstan, Italy and Germany, where an 18-month-old boy died February 18 after coming down with the illness.

The resurgence of the preventable disease in some European countries, as well as parts of the United States, coincides with a movement among some parents to refuse to vaccinate their children.

Measles causes fever and rash and in severe cases can lead to pneumonia or brain swelling, sometimes fatal. The disease is highly contagious because it is transmitted through the air.

Even if the number of measles cases dropped by 50 percent from 2013-2014, the current epidemic has put into serious doubt the objective of eradicating the disease in Europe by the end of the year.

"The priority is now to control current outbreaks in all affected countries through immunisation," Nedret Emiroglu, a deputy director in WHO's Europe office.

Many people who do not vaccinate their children say they fear a triple vaccine for measles, mumps and rubella is responsible for increasing cases of autism -- a theory repeatedly disproven by various studies.

The controversy dates back to the publication of a now debunked article in the Lancet medical journal in 1998.