The New York Times reports on public health concerns about the growing number of parents who refuse vaccinations for their children.

Children who are not vaccinated are unnecessarily susceptible to serious illnesses, they say, but also present a danger to children who have had their shots — the measles vaccine, for instance, is only 95 percent effective — and to those children too young to receive certain vaccines.

Measles, almost wholly eradicated in the United States through vaccines, can cause pneumonia and brain swelling, which in rare cases can lead to death. The measles outbreak here alarmed public health officials, sickened babies and sent one child to the hospital.

Every state allows medical exemptions, and most permit exemptions based on religious practices. But an increasing number of the vaccine skeptics belong to a different group — those who object to the inoculations because of their personal beliefs, often related to an unproven notion that vaccines are linked to autism and other disorders.

Twenty states, including California, Ohio and Texas, allow some kind of personal exemption, according to a tally by the Johns Hopkins University. . . .

In 1991, less than 1 percent of children in the states with personal-belief exemptions went without vaccines based on the exemption; by 2004, the most recent year for which data are available, the percentage had increased to 2.54 percent, said Saad B. Omer, an assistant scientist at the Johns Hopkins Bloomberg School of Public Health.

While nationwide over 90 percent of children old enough to receive vaccines get them, the number of exemptions worries many health officials and experts. They say that vaccines have saved countless lives, and that personal-belief exemptions are potentially dangerous and bad public policy because they are not based on sound science.

"If you have clusters of exemptions, you increase the risk of exposing everyone in the community," said Dr. Omer, who has extensively studied disease outbreaks and vaccines.

It is the absence, or close to it, of some illnesses in the United States that keep some parents from opting for the shots. Worldwide, 242,000 children a year die from measles, but it used to be near one million. The deaths have dropped because of vaccination, a 68 percent decrease from 2000 to 2006.

"The very success of immunizations has turned out to be an Achilles' heel," said Dr. Mark Sawyer, a pediatrician and infectious disease specialist at Rady Children's Hospital in San Diego. "Most of these parents have never seen measles, and don't realize it could be a bad disease so they turn their concerns to unfounded risks. They do not perceive risk of the disease but perceive risk of the vaccine."