The Centers for Disease Control and Prevention says more than 1,500 cases of measles and three deaths in three states were reported.
That is about 10 times the number of confirmed cases and more than double the number that have been confirmed in the past two weeks.
The measles outbreak began with an outbreak in Texas in March, and has since spread to more than 30 states and the District of Columbia.
The first cases were reported on March 15 in California, but those were isolated and not linked to a new strain.
On Monday, CDC Director Dr. Tom Frieden said the agency has been “trying to figure out why this is happening and what the underlying conditions are.”
The agency said it has been analyzing records from state health departments and hospitals and is analyzing data from coronavirus testing.
Officials said a third state, New Jersey, has confirmed four new cases and two deaths, but there are no confirmed cases in New York City.
“The outbreak is continuing in the New York metropolitan area, and New Jersey has confirmed a new case,” the agency said in a statement.
“We continue to monitor the situation and will continue to update this website as more information becomes available.”
The outbreak in New Jersey is the most recent of its kind.
There were more than 50 confirmed cases reported in that state, the highest number in any state.
A spokeswoman for the New Jersey Department of Health and Mental Hygiene said in an email Monday that the department was working to contain the outbreak and is taking steps to increase surveillance, vaccinate people and vaccinate children.
“We are doing our part to limit the spread of the disease,” the spokeswoman said.
“It is extremely important that people stay away from all areas that may be susceptible to spreading this disease, including schools, hospitals and places of employment.”
The New Jersey health department has also been working to prevent measles in the community and at home.
It said that about 3,500 people who are eligible for the state’s health insurance program were eligible to receive vaccinations for the virus.
People who are not eligible for coverage could be forced to pay out of pocket.
New Jersey Gov.
Chris Christie said on Monday he is ordering a statewide recall of all adult vaccines, including the measles vaccine, for everyone 18 and older.
The Associated Press contributed to this report.