State law requires children to get two dozen vaccines against a dozen different diseases from birth to age 5, though parents may object for medical or religious reasons or choose to delay some of the shots. The number who have refused any vaccines for their children has doubled in recent years, though the figure is still under 1 percent, said Dr. Paul A. Offit, chief of infectious diseases at Children's Hospital of Philadelphia and an expert on vaccines.
"We ask a lot and the public generally has responded well," he said.
But there remains a vocal minority who say vaccines are unsafe for children. Some say the large number of shots given to children in a short period of time causes autism - a belief promoted on national television by actresses Jenny McCarthy and Holly Robinson Peete, who blame vaccinations for their sons' autism.
The belief has been rejected by mainstream science, yet it has been widespread enough to make many parents think twice about giving their children so many shots at once. Dr. Robert Sears, son of parenting guru Dr. William Sears, advocates spreading them out.
Meanwhile, there is a generation of parents who didn't see firsthand the devastating impact of the many vaccine-preventable diseases. That allows fear of the vaccine to displace fear of infections, doctors said. Whooping cough is highly contagious and deadly. Prior to the vaccine, pertussis killed about 8,000 people in the United States every year. Now the number is down to about 10.
The vaccine "has kept young children out of the hospital and out of the morgue," said Offit, the author of Autism's False Prophets: Bad Science, Risky Medicine and the Search for a Cure.
Still, pertussis has not been eradicated; it is constantly circulating. Immunity typically wanes in adulthood, so pertussis is more common than most of the other vaccine-preventable diseases. Often, adults pass the disease to their children, who in recent outbreaks have borne the brunt of serious infection, Offit said, just as they did in the pre-vaccination era.
In recent years, health officials have recommended adolescents and adults get a booster shot that includes protection from pertussis.
When there is a decrease in immunizations, Offit said, pertussis is the first disease to come back, making it sort of a proverbial canary in the coal mine.
