A federal government program launched 20 years ago to increase vaccinations for low-income children in the United States will prevent more than 700,000 deaths, but measles remains a stubborn adversary, with more than 129 cases so far this year, a federal agency said on Thursday.

Most of the U.S. measles cases are linked to unvaccinated travelers from abroad, the Centers for Disease Control and Prevention (CDC) said.

The first four months of this year have brought more measles cases than any similar period since 1996, in part because of serious outbreaks in countries such as the Philippines, the agency said. There have been no deaths from the disease reported in the United States this year, the CDC said.

The importation of measles from overseas makes vaccination even more important for children in the United States, the CDC said.

"Borders can't stop diseases anymore, but vaccinations can," CDC Director Tom Frieden told reporters.

A national measles outbreak in the late 1980s that involved 50,000 cases and more than 100 deaths prompted the CDC to launch the Vaccines for Children program, which provides free vaccinations to children whose parents and care givers are unable to afford them.

The vaccinations are for a variety of diseases, including measles, mumps, and rubella.

The program will prevent more than 21 million hospitalizations and 732,000 deaths among children born in the last 20 years based on estimates of how many illnesses there would had been without the extra immunizations, the CDC said.

The Vaccines for Children program will be expanded under the Affordable Care Act, the CDC said.

The 20th anniversary of the program has, however, been marked by 13 measles outbreaks in the United States so far this year, the largest in California and New York City.

Most U.S. measles patients either had not been vaccinated or were not sure if they had been, the CDC said. Those who had not been vaccinated included patients who opted out of vaccinations because of personal beliefs.

"Because measles can be spread so easily, unvaccinated people become very vulnerable once a disease is introduced," Anne Schuchat, U.S. assistant surgeon at the National Center for Immunization and Respiratory Diseases, told reporters.

Measles from abroad can also infect children who are too young to be vaccinated, she added.

The CDC recommends that starting at age 12 months, infants receive two doses of MMR (measles, mumps, and rubella) vaccine.

Infants aged 6 through 11 months old should receive one dose of MMR vaccine before international travel.