The U.S. faces the biggest measles outbreak in seven years, the federal Centers for Disease Control and Prevention said on Thursday.

According to CDC, there were 64 cases from January through April 25, more than in all of 2006 and the highest number during that four-month period since 2001 (116 cases). Moreover, four outbreaks are ongoing in Arizona, New York, Michigan and Wisconsin. Fourteen people have been hospitalized as a result of the outbreak. However, no death has been reported so far.

“I am concerned. This is different from what we have been seeing in the last few years,” said Anne Schuchat, director of the CDC’s National Center for Immunization and Respiratory Diseases, according to the Washington Post.

Measles can be a severe illness that causes symptoms like rash, high fever, cough, runny nose and red, watery eyes. In rare cases, patients face more serious complications, including pneumonia, encephalitis and even seizure and death.

The federal officials have long been stressing the importance of immunizing children against measles, mumps or rubella, but many parents refuse to vaccinate their children for religious reasons or because they think the shots may cause autism or other health problems.

These findings add to others revealed by another federal report released at the beginning of this week, according to which, one in four children does not comply with official vaccination recommendations because of missed doses of vaccines or vaccine lapses.

“We are concerned…about the population of people who are choosing not to be vaccinated, and whether we may be on the verge of facing larger-scale outbreak in the United States,” said Jane Seward of the CDC’s division of viral diseases, according to Reuters.

Sixty-three of the measles cases occurred in people who were unvaccinated or whose vaccination status was unknown. Unvaccinated persons are at risk of acquiring measles themselves and also transmitting the disease to others, including children who are too young to be vaccinated.

The outbreaks seem to have been triggered by 10 cases imported into the U.S. from other countries, particularly Switzerland and Israel, which are fighting very large outbreaks, Schuchat said. Forty-four other cases have been directly linked to the imported cases. The ages of those infected ranged between 5 months and 71 years.

“We do expect many more cases this year than in 2001, based on what’s going on today. We have multiple near-simultaneous outbreaks with different importation sources. And we have gone on the third, fourth or possible fifth generation of transmission in some of these circumstances. So I think that there is reason for concern that we haven’t seen the end of this,” Schuchat warned.

Before the measles vaccination program, about 3–4 million persons in the U.S. were infected each year, of whom 400–500 died, 48,000 were hospitalized, and another 1,000 developed a chronic disability from measles encephalitis.