Measles cases have soared nationwide to 314, federal health officials revealed on Monday.
The total number of cases for 2019 is just 58 shy of last year's 372, the second-highest number of cases in the last 20 years.
So far, the Centers for Disease Control and Prevention (CDC) says there are confirmed cases in 15 US states since January 1.
The first measles vaccine was introduced in 1963 and, by 2000, it was considered to be eradicated in the US.
But the highly infectious disease has been spreading among people who are unvaccinated or live in states that allow non-medical exemptions for vaccines.
On Monday, new figures from the CDC revealed that 314 cases of measles have been confirmed in 15 states, just 58 shy of last year's number
Cases have been confirmed in Arizona, California, Colorado, Connecticut, Georgia, Illinois, Kentucky, Michigan, Missouri, New Hampshire, New Jersey, New York, Oregon, Texas and Washington.
Of those states, six - Arizona, Colorado, Michigan, Oregon, Texas and Washington - allow exemptions for philosophical and/or personal beliefs.
Washington and New York, particularly, have been struggling to contain the disease..
In January, Washington declared a public health emergency after a measles outbreak that has affected 73 people in Clark County and one in King County, where Seattle is.
Sixty-three of the cases are in residents who have not been vaccinated. Fifty-three cases are in children aged 10 and under.
STATES THAT ALLOW PARENTS TO OPT OUT OF VACCINES BASED ON PHILOSOPHICAL BELIEFS
- Louisiana (except no religious exemptions)
- Minnesota (except no religious exemptions)
- Missouri (only for daycare, not public school)
- North Dakota
STATES THAT RECENTLY REVOKED THIS ALLOWANCE:
- West Virginia
Meanwhile, in New York, there have been at least 332 confirmed cases in Brooklyn, Queens and Rockland County since October 2018 - all in Orthodox Jewish communities.
In fact, this month, health officials in New York City said a single yeshiva - an Orthodox Jewish school - is linked to 21 new cases.
The city's health department says Yeshiva Kehilath Yakov, and two other schools, disobeyed standing orders to keep out unvaccinated children.
Measles is a highly contagious infection caused by the measles virus.
When someone with measles coughs, sneezes or talks, infected droplets are sprayed into the air, where other people can inhale them and are then infected.
Symptoms present themselves between 10 to 14 days after infection and include fever, cough, runny nose and a total-body skin rash.
Once common, the disease is now rare due to the MMR (measles, mumps, rubella) vaccine.
The CDC recommends children receive the first dose at 12 to 15 months old and the second dose at four to six years old.
The vaccine is about 97 percent effective. But those who are unvaccinated have a 90 percent chance of catching measles if they breathe the virus in, according to the CDC.
Before the measles vaccine was available, more than 500,000 cases were diagnosed in the US every year, with about 500 annual deaths.
In 2018, 372 cases of measles were confirmed in 26 states and the District of Columbia, the CDC reported.
It is the second-greatest number since measles was considered eradicated in the US in 2000.
A report published by the World Health Organization in January said measles has seen a 30 percent increase in cases around the world.