There were 695 reported cases of measles across 22 states as of Wednesday afternoon, the highest since the disease was domestically eradicated in 2000, according to the Centers of Disease Control and Prevention.
The CDC attributed the "high number of cases" to large outbreaks in Washington state and New York, which began late last year.
"The outbreaks in New York City and New York State are among the largest and longest lasting since measles elimination in 2000," CDC said in a statement. "The longer these outbreaks continue, the greater the chance measles will again get a sustained foothold in the United States."
The World Health Organization noted a 300% increase in measles cases worldwide earlier this month in comparison to the first three months of 2018.
Health and Human Services Secretary Alex Azar said vaccinations are the only way to keep the country safe.
"Measles is not a harmless childhood illness, but a highly contagious, potentially life-threatening disease," Azar said late Wednesday. "We have the ability to safely protect our children and our communities. Vaccines are a safe, highly effective public health solution that can prevent this disease."
"All Americans would be safer and healthier if we received measles vaccines on the recommended schedule," Azar added.
Measles are highly contagious and can be transmitted through the air by coughing or sneezing.
The spike in cases stems, in part, from the spread of misinformation about vaccines online. Anti-vaccination activists have gained more traction on social media amid false claims linking vaccinations to autism.
The measles vaccine, now administered along with immunization for mumps and rubella, is regarded by public health practitioners as safe and highly effective. The CDC recommends for all Americans above age 1 to get vaccinated.
"Stopping these measles outbreaks is a priority for CDC and we are working 24/7 to protect Americans from this contagious disease. Vaccination is the best way to protect against measles," the CDC said. "Today, the overwhelming majority of parents choose to protect their children with vaccines, and we’ve seen high and stable immunization rates in the U.S. for several years."