The U.S. is dealing with the largest outbreak of measles cases since the disease was declared eliminated in 2000, the Centers for Disease Control and Prevention said.

Health authorities as of Wednesday had logged 695 measles cases this year across 22 states, the CDC said in a statement. The announcement confirmed similar findings by CNN that cited data from state and local health departments.

The CDC attributed the 2019 measles surge primarily to the disease being imported into communities with under-vaccinated populations, sparking significantly large outbreaks in Washington state, New York City and New York state.

The outbreaks in New York City and New York State are among the largest and longest lasting since measles elimination in 2000. The longer these outbreaks continue, the greater the chance measles will again get a sustained foothold in the United States, the CDC said.

Importation involves an unvaccinated traveler visiting a location where there is a measles outbreak and contracting the disease, then returning home and exposing others, the CDC said.

When measles is imported into a community with a highly vaccinated population, outbreaks either dont happen or are usually small. However, once measles is in an under-vaccinated community, it becomes difficult to control the spread of the disease, the CDC said.

Health and Human Services Secretary Alex Azar, in a statement responding to the CDCs figures, cautioned that measles is not a harmless childhood illness, but a highly contagious, potentially life-threatening disease.

Vaccines are a safe, highly effective public health solution that can prevent this disease, Azar said. The measles vaccines are among the most extensively studied medical products we have, and their safety has been firmly established over many years in some of the largest vaccine studies ever undertaken.