The challenge for US health authorities is stopping the outbreak from spreading further.
A total of 555 cases of measles have been recorded in the USA since Jan 1, most of them in New York and Clark County in Washington state.
In New York, the most hard hit has been an Orthodox Jewish community in Brookyln.
They were infected by visitors from Israel, where a measles outbreak began a year ago.
This fall, it spread through Brooklyn schools and synagogues among children who had never been vaccinated or had not had the recommended followup shot between the ages of four and six years old.
In addition to Brooklyn, another Orthodox Jewish community in New York state, in Rockland County, has also been affected.
About 200 cases have been reported there, also infected by visitors from Israel.
An outbreak in the state of Michigan – where 39 cases have been reported – has been traced to a single person, according to The Washington Post.
The Orthodox Jewish man arrived in the United States from Israel, spent time in Brooklyn and then drove to Detroit, where he spread the disease through visits to synagogues and markets.
– Child from Ukraine –
In Clark County, Washington, the measles outbreak is concentrated among a Russian-speaking community.
A child brought the virus back from Ukraine in December and it spread to 74 other people, mostly children, through schools, supermarkets and a bowling alley.
“This kid was in a pocket of kids that weren’t immunized by choice,” said Scott Lindquist, state epidemiologist for the Washington state department of health.
“And that’s how it started out.”
“What we know about this cluster is they all match each other,” Lindquist said. “We did DNA fingerprinting, or genetic sequencing, essentially.
“And they all match each other identically. And they all match the Ukraine strain,” he said.
One theory links the outbreaks in Israel and Ukraine to an annual pilgrimage to the Ukrainian city of Uman by tens of thousands of Orthodox Jewish men to visit the grave of a revered rabbi.
Dr. Patrick O’Connor of the World Health Organization told The New York Times that the major Ukraine outbreak may have supercharged a modest one in Israel.
Dr. Nancy Messonnier, acting director of the Center for Preparedness and Response at the Centers for Disease Control and Prevention, said all the measles cases in the USA have come from other countries.
“In Washington state, the strain is traced through Ukraine,” Messonnier said. “The outbreaks in New York City and New York state are traced to a strain that was found in Israel.”
The number of cases in the United States are relatively few compared to other places but the seond most since the turn of the century, when the disease was declared eliminated.
“We certainly are concerned that it will be the largest number of cases since elimination in 2000,” Messonnier said
Tens of thousands of cases of measles have been reported in Africa and Europe. Ukraine alone has reported more than 30,000 cases and 11 deaths since January.
In the USA, the outbreaks have mostly been confined to tight-knit communities where vaccination rates are lower than the national average of more than 90%.
There have been outbreaks previously, for example, among the Amish community in Ohio in 2014 and Somali immigrants in Minnesota in 2017.
Health authorities have moved aggressively to prevent the disease from spreading.
In Clark County, 849 non-vaccinated students were barred from 15 schools where children had come down with measles.
“We were very aggressive about isolating any cases, quarantining anybody who was exposed to those cases, and excluding kids from school if they were unimmunized,” said Lindquist, the Washington state health official.
“And that is what’s slowed this outbreak down,” he said. “Currently, we have not had a case in almost 42 days.”
In New York, the mayor has ordered the vaccination of all residents of Williamsburg, a Brooklyn neighborhood, and threatened to impose fines of US$1,000.