US health officials on Thursday reported 971 measles cases so far this year, the highest tally in 27 years, and experts say it's not clear when the wave of illnesses will stop.
Measles, once common in the US, became rare after vaccination campaigns that started in the 1960s. A decade ago, there were fewer than 100 cases a year.
The new numbers from the Centers for Disease control and Prevention (CDC) pushed the US tally higher than the 963 illnesses reported for all of 1994. The nation last saw this many cases in 1992, when more than 2,200 were reported.
Overall vaccination rates have remained fairly high, but outbreaks have been happening in communities where parents have refused recommended shots, US health officials say.
"What's causing these outbreaks is lack of vaccination," said Dr. Mark Roberts, chair of health policy and management at the University of Pittsburgh Graduate School of Public Health.
Illnesses have been reported in 26 states, but the vast majority are in New York City.
The city's outbreak, which began last October, is already the largest local measles outbreak in the US in nearly 30 years. It started when some unvaccinated children visited Israel, where a measles outbreak is occurring, and came back to New York.
More than 500 cases have been diagnosed in two Brooklyn neighborhoods — Williamsburg and Borough Park — and mainly among unvaccinated children in Orthodox Jewish communities. Forty-two have been hospitalized, including 12 treated in intensive care units.
US authorities declared measles eliminated in the US in 2000, a goal set in 1963 with the introduction of the vaccine.
Measles is considered eliminated when there is an absence of continuous disease transmission for 12 months or more in a specific geographic area, according to the CDC.
The ongoing outbreak in and around New York City that started last fall is threatening the US's "elimination status" -- if it continues for four more months, the country will no longer be able to say it has eliminated measles.
The US has never counted zero measles cases. Since 2000, the number has fluctuated between a few dozen and a few hundred cases per year, with 667 cases recorded during a 2014 outbreak in Ohio, especially in Amish communities.
(Source: Agencies)