Washington state leads the nation in the percentage of parents opting out of vaccinations for their kindergarten-age children. This meant 6.2 percent of these children were missing one or more immunizations in the 2009-10 school year, according to the federal Centers for Disease Control and Prevention.
The immunization most commonly skipped for the state's kindergartners is varicella, which protects against chickenpox.
Since 1997, there's been a steady, statewide decline in the number of school children from kindergarten through high school who are fully vaccinated.
"The world has changed," said Dr. Jack Stephens, a pediatrician at . "It used to be the unimmunized child was the child of an economically disadvantaged family with poor access to health care.
"Nowadays, it's usually well-educated parents of higher social status who do their own independent research and tell you what they're willing to do."
And the reasons for skipping or delaying vaccinations vary from family to family. For 32-year-old Maria Rippo of Bothell, the issue is concern about what vaccinations could do to her children. She has chosen to have her four children receive only one vaccine, which protects against diphtheria, tetanus and whooping cough.
A new state law that went into effect July 22 seeks to close a loophole that many parents used to avoid providing proof of vaccinations to schools. All they had to do was sign a piece of paper saying they had religious, medical or personal objections to vaccines.
Now, parents must meet with a medical provider, get a signed letter confirming that the consultation took place, and provide the note to child-care centers or schools. That could mean an extra cost to parents of about $90 to $100.
Questions over the safety of vaccines came into the mainstream in the late 1990s, triggered by international debate over a claimed association between the vaccine for measles, mumps and rubella and autism.
Actress became a fiery advocate for the cause, blaming her own son's autism on the vaccine. The movement quickly gained momentum from parents who wanted to protect their children from a severe developmental disorder for which there is still no known cause.
Last year, the British Medical Journal, which originally published a study on the alleged autism-vaccine link in 1998, , citing the falsification of data in the study and calling its conclusions "fraudulent."
Still, individuals and organizations concerned about vaccine safety point to the as proof that vaccines sometimes have unwanted side effects.
Data from the federal website show that just over $2 billion has been paid out since 1989 in legal cases claiming problems associated with vaccines.
Rippo noted one such recent settlement, which could reach $61 million. "Yes, it does happen, people's lives are ruined," she said.