The Americas have been declared free of endemic measles after a rigorous six-year review, the World Health Organization declared Tuesday, making it the first region in the world to achieve the feat.

“Measles is one of the most contagious diseases and affects primarily children. It is transmitted by airborne droplets or via direct contact with secretions from the nose, mouth and throat of infected individuals,” the WHO said.

The disease is characterized by symptoms like high fever, rashes and irritated eyes and can lead to severe complications like blindness, encephalitis, severe diarrhea, pneumonia and even death. Before the 1980s — mass vaccination began in the 1980s — the disease killed almost 2.6 million people a year, according to WHO. Last year, 244,704 measles cases were reported worldwide.

“This is a historic day for our region and indeed the world,” Carissa Etienne, director of the Pan American Health Organization — the Americas arm of WHO — told a meeting of the organization in Washington. “It is proof of the remarkable success that can be achieved when countries work together in solidarity towards a common goal. It is the result of a commitment made more than two decades ago, in 1994, when the countries of the Americas pledged to end measles circulation by the turn of the 21st century.”

While it usually takes three years without cases to declare that a disease has been eradicated from a region — endemic measles in the Americas took 14 years. According to experts, a number of factors contributed to this delay. A high number of mobile migrants, lack of access to certain areas because of violence, and a poor communications infrastructure are some of these.

Venezuela’s 2002 endemic measles outbreak was the last in the region. However, there have been numerous imported cases like the one in the U.S. in early 2015 linked to a Disney theme park in California. At least 147 people were infected, NBC News reported.

“I would like to emphasize that our work on this front is not yet done,” Etienne said, with a note of caution. “We cannot become complacent with this achievement but must rather protect it carefully. Measles still circulates widely in other parts of the world, and so we must be prepared to respond to imported cases.”

“It is critical that we continue to maintain high vaccination coverage rates, and it is crucial that any suspected measles cases be immediately reported to the authorities for rapid follow-up,” she added.