(USA TODAY) -- About one-quarter of U.S. toddlers haven't received all of their recommended vaccines, "and they're like dry wood ready to catch fire" if serious diseases break out, a federal researcher said Monday.

Immunization rates aren't increasing and seem to be slipping in some regions, says Philip Smith, an epidemiologist at the National Immunization Program of the Centers for Disease Control and Prevention.

About 1.4 million children 19 months to 35 months old have been given at least one dose of a vaccine but not all the shots advised, and 18,000 haven't received any vaccines, a CDC survey shows. Smith presented the vaccine data at the American Academy of Pediatrics meeting in New Orleans.

Most kids, 98%, have been fully vaccinated by the time they start school, when parents must show proof of immunization. But preschool years "are a window of vulnerability that gives us a lot of concern," says Duke University pediatrician Samuel Katz of the National Network for Immunization Information.

Undervaccinated children tend to come from very different kinds of families from those who are given no shots, Smith says. Those who don't get all of their shots are disproportionately poor and minority children. They're often uninsured or not fully covered for childhood immunizations, he says.

The unvaccinated tend to have white, well-educated and affluent parents who doubt vaccine safety. "These are intelligent, yuppie parents who chase the Internet and read all the misinformation that's there," Katz says.

Toddlers not receiving any vaccines tend to cluster in certain regions: Southern California; Cook County, Ill.; Westchester County, N.Y.; Dallas; Salt Lake City; and King County, Wash.

Recent outbreaks of pertussis (whooping cough), a potentially fatal disease, in wealthy Westchester County and Seattle show the danger of holes in the vaccine "safety net," Smith says. "The unvaccinated get the disease, and those not fully vaccinated can catch it from them."

Costs can be a great concern for the working poor, Smith adds. For example, the four doses required of a vaccine to prevent pneumonia cost $360, he says.

More than 50% of 2-year-olds qualify for free vaccines under a federal program. But states vary widely in efforts to inform and find eligible families, Katz says. Most New England states and North Carolina provide free vaccines to all children, and they have the highest vaccination rates, he says.

Parents' safety concerns should have been eased by recent federal reports that found no proof of a link between vaccines and autism, diabetes or attention deficit hyperactivity disorder, Katz says.

But vaccine studies have been poorly designed and done by scientists with financial conflicts of interest, such as ties to drugmakers, says Barbara Loe Fisher of the National Vaccine Information Center, a group opposing mandatory vaccination. "Even the federal reports couldn't totally rule out an autism problem . . . We need basic science to identify the genetic and other high-risk factors that make vaccines dangerous for some children, but there is no will to do this kind of research," Fisher says.

Copyright 2003 USA TODAY, a division of Gannett Co. Inc.