NEW YORK (NYTIMES) - The measles outbreak continues to spread in the United States, surpassing 700 cases this year, federal health officials said on Monday (April 29). The virus has now been found in 22 states.

More than 500 of the 704 cases recorded as of Friday were in people who had not been vaccinated, the Centers for Disease Control and Prevention reported. Sixty-six people have been hospitalised.

About 400 of the cases have been found in New York City and its suburbs, mostly in Orthodox Jewish communities. That outbreak has spread to Detroit.

Los Angeles is now experiencing a fast-growing outbreak, and hundreds of university students who are thought to have been exposed and cannot prove that they have had their shots have been asked to quarantine themselves at home.

On Wednesday, the CDC said the number of cases had surpassed the previous high of 667, set in 2014. This year's outbreak is the largest since the disease was declared eliminated in the United States in 2000. In 1994, there were 963 cases.

Elimination in 2000 meant measles virus was no longer circulating in the United States as it presumably had since European settlers first brought it to this hemisphere in the 15th or 16th century. Each year after 2000, a few cases arrived from overseas, either in immigrants or in returning tourists, but each outbreak was snuffed out.

More than 94 per cent of American parents vaccinate their children against measles and other diseases, Dr Robert Redfield, director of the CDC, said on Monday.

His agency "is working to reach the small percentage of vaccine-hesitant individuals," he said. "Vaccines are safe and do not cause autism."

About 100,000 children in this country under age 2 have not been vaccinated, he said, meaning they are vulnerable in this outbreak.

Some infants are not immunised because their parents avoid vaccination. Others cannot be protected either because they are allergic to components of the vaccine or are, for example, taking cancer or organ-transplant medications that suppress their immune systems.

"We must join together as a nation to once again eliminate measles," Dr Redfield said.

This year's widespread outbreak was sparked by people infected with measles who have come into this country since last year, the CDC said. The measles strains detected were most frequently from Ukraine, Israel and the Philippines.

Communities have begun to take extraordinary measures to slow the infection rate and crack down on resistance to immunization.

New York City Mayor Bill de Blasio declared a state of emergency and threatened residents of four Brooklyn ZIP codes with US$1,000 fines if they refused to vaccinate. City officials closed a yeshiva preschool for violating vaccination orders.

Rockland County, New York, the center of another outbreak, initially barred unvaccinated children from all indoor public places, including schools, malls, supermarkets, restaurants and houses of worship.

After a court blocked that order, the county instead barred from public spaces anyone who had measles symptoms or who had recently been exposed to the disease, threatening them with fines of up to US$2,000 a day.

There have been no confirmed measles deaths in this country, but officials have said it is just a matter of time.

Even with modern medical care, the disease normally kills about 1 out of every 1,000 victims, according to the CDC.

Pneumonia and encephalitis - swelling of the brain - are the most common severe complications and epidemics among malnourished children who cannot get modern hospital care have mortality rates of 10 per cent or more, according to the World Health Organisation.

Measles is among the most contagious of diseases. Virus-laced droplets can hover in still indoor air for up to two hours after someone infected has coughed or sneezed. Up to 90 per cent of people who are exposed will catch the virus if they are not immunised.

The vaccine is considered very safe and two doses are about 97 per cent effective at conferring immunity. The vaccine is normally given at ages 1 and 5, but during outbreaks pediatricians may give it to healthy children as young as 6 months old.

Around the world, measles cases fell 80 per cent between 2000 and 2016, with deaths dropping to 90,000 a year from 550,000. But two years ago, cases began rebounding, driven by a combination of poverty, warfare, tight vaccine supplies and, in some countries, hesitation about vaccination.

Earlier this month, the WHO said there were three times as many measles cases around the world this year as there were in the first three months of 2018.

Outbreaks of tens of thousands of cases have occurred recently in poor or war-torn countries like Madagascar, Ukraine and Yemen. But case numbers are also climbing in wealthy countries with modern health care systems, like Israel, Britain, France and Italy. Deaths from measles have occurred in those countries.