The number of people sickened by the highly contagious, potentially deadly disease increased by 71 during the third week of April, with 22 states reporting cases.

Health officials said they expect 2019′s case counts to jump in the coming weeks because of increased disease spread as a result of Easter and Passover gatherings. Officials in New York City, location of the largest outbreak in the country, are especially worried. At least 303 cases have been reported this year, virtually all in Brooklyn. New York City Health Commissioner Oxiris Barbot said she is bracing for an increase.

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“Because of measles’ long incubation period, we know this outbreak will get worse before it gets better," she said last week.

The Centers for Disease Control and Prevention’s figures, updated Monday, report cases as of April 18, before the start of the holidays.

The states that have reported cases to the CDC are Arizona, California, Colorado, Connecticut, Florida, Georgia, Illinois, Indiana, Iowa, Kentucky, Maryland, Massachusetts, Michigan, Missouri, Nevada, New Hampshire, New Jersey, New York, Oregon, Tennessee, Texas and Washington.

The outbreaks are linked to travelers who brought measles back from countries such as Israel, Ukraine and the Philippines, where large measles outbreaks are occurring.

In 2014, the United States had a record 667 cases, including one large outbreak primarily among unvaccinated Amish communities in Ohio that accounted for more than half of the cases that year.

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This year as in the past, the majority of people who have fallen ill were unvaccinated, officials said. In some communities, anti-vaccine activists have spread false claims about the measles, mumps and rubella vaccine, fueling refusal or hesitancy among parents about immunizing their children. When many people in a community have not been vaccinated, the disease can spread quickly. It can cause serious complications among all age groups, especially young children, adults with weakened immune systems and the very elderly.

The CDC defines an outbreak as three or more cases. In addition to New York City, there are outbreaks in California; Rockland County, N.Y.; New Jersey; and Michigan, where almost all 43 cases are linked to one man who traveled to the Detroit area from Brooklyn, unaware that he had measles.

California has 23 cases, including four in San Mateo County, which include an adult who visited Google headquarters in Mountain View, and 13 cases in Butte County in northern California, the region devastated by the most destructive wildfire in history.

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In Washington state, 74 people contracted the infection, including 63 who were unvaccinated. Health officials are expected to declare that outbreak over if no more cases are reported by Wednesday. That’s two incubation periods (42 days) without new cases.

Meanwhile, the rise in measles cases prompted the Food and Drug Administration on Monday to stress the importance of the measles, mumps and rubella vaccine, saying large, well-designed studies confirmed its safety and effectiveness long ago and demonstrated it is not associated with the development of autism, false information that anti-vaccine groups have claimed for two decades.

“We cannot state strongly enough – the overwhelming scientific evidence shows that vaccines are among the most effective and safest interventions to both prevent individual illness and protect public health,” Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, said in a statement.

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The FDA approved the vaccine nearly 50 years ago. Two doses, beginning at 12 months of age, are 97 percent effective against measles, 88 percent effective against mumps and 97 percent effective against rubella, sometimes called German measles, Marks noted. Potential side effects such as rash and fever are generally mild and short-lived.

"It’s an urgent public health priority to monitor these diseases and raise awareness of the importance of timely immunizations, especially as outbreaks are taking hold among unvaccinated populations in this country,” Marks said.

Laurie McGinley contributed to this report.