More people had measles infections in the first seven months of this year than during any comparable period since 1996, and public health officials blamed growing numbers of parents who refuse to vaccinate their children.
Many of these parents say they believe vaccines cause autism, even though multiple studies have found no reputable evidence to support such a claim. In Britain, Switzerland, Israel and Italy, measles outbreaks have soared, sickening thousands and causing at least two deaths.
From January through July, 131 measles cases were reported to the Centers for Disease Control and Prevention from 15 states and the District of Columbia. Fifteen people, including four infants, were hospitalized. There were no deaths. Nearly all the cases resulted when people traveling abroad or visiting from a foreign country spread the illness to others. In Illinois, 30 people were sickened in one outbreak.
Most of those who were sickened were unvaccinated or had an unknown vaccination status. Sixteen were younger than a year old, too young to have been vaccinated. But two-thirds of the rest — or 63 people — were unvaccinated because of their or their parents' philosophical or religious beliefs.
Public health advocates have become alarmed in recent years over a growing number of people who contend that vaccines cause illnesses, particularly autism. The number of parents who claim a philosophical exemption to mandatory vaccine laws has grown.
Nonetheless, vaccination rates have remained relatively high in the United States. In 2006, 95 percent of school-age children received at least one shot of the combined measles, mumps and rubella vaccine, according to the C.D.C. But such surveys are often years behind vaccination trends, and government officials say the growing number of measles outbreaks suggests that overall vaccination rates may be on the decline.
Because it is virulently contagious, measles is often the first vaccine-preventable disease to reappear when vaccination rates decline. In the decade before the measles vaccination program began, each year nearly 4 million people in the United States were infected, 48,000 were hospitalized, 1,000 were chronically disabled and nearly 500 died.
Autism and antivaccines advocates are unapologetic about the return of measles.
"Most parents I know will take measles over autism," said J. B. Handley, co-founder of Generation Rescue, a parent-led organization that contends that autism is a treatable condition caused by vaccines.
It is an attitude that pediatricians say they are increasingly having to confront.
"All pediatricians are spending more time speaking with parents about the rationale for vaccines," said Dr. Andrew D. Racine, director of the division of general pediatrics at the Children's Hospital at Montefiore in the Bronx.
Responding to parents' concerns, manufacturers in 2001 almost entirely removed a preservative containing mercury from all routinely administered childhood vaccines. The incidence of autism has shown no drop.