"They're sort of like the canary in the mine," said Dr. Tina Tan, an infectious disease specialist at Chicago's Lurie Children's Hospital.
State regulations in Illinois and elsewhere generally require vaccinations for older children in day care centres, but measles shots are not recommended for children under age 1. And like most states, Illinois does not require vaccinations for day care centre staffers.
"Unfortunately, there is no requirement. But this is on our radar," said Melaney Arnold, spokeswoman for the Illinois Department of Public Health.
The cases are among more than 100 nationwide this year, most of them linked with a Disneyland outbreak. Ten other young children at the suburban centre were exposed and are being monitored for symptoms.
This year's cases also include an infant at a Santa Monica, California, day care centre that closed temporarily this week. Fourteen infants from that centre have been quarantined at home for three weeks.
Dr. Julie Morita, acting commissioner of the Chicago Department of Public Health, said this year's outbreaks highlight the major reasons for immunizations against a rare disease. The shots are not just for self-protection. They also provide what experts call "herd immunity" — protection for those too young or too sick to be vaccinated, including infants in day care.
"We have always felt like this was a vulnerable population ... in a potentially risky setting because there are a lot of kids who are together," Morita said.
Measles can cause a cough, runny nose and rash. Infants are vulnerable to rare but dangerous complications that include pneumonia, deafness, permanent brain damage and death.
Dr. Saad Omer, an Emory University vaccine specialist, offers this advice to parents with children in day care: "Unless there is an ongoing outbreak in the specific day care, I don't see any reason for keeping your child at home. But make sure when they become eligible for vaccines, they get the vaccine on time, on schedule."
Illinois authorities were seeking the source of the day care outbreak but said there was no evidence it's linked with the Disneyland cases. Possible sources include unvaccinated older children or adults who recently travelled overseas.
Most U.S. measles cases in recent years stem from contact with someone who has been abroad, since the disease is still common in many countries. The highly contagious virus was declared eliminated in the United States in 2000, but U.S. cases have been reported every year since then, including more than 600 last year.
The government recommends the first dose of measles vaccine for children aged 12 months to 15 months, with a second dose before the start of kindergarten. The shots are not advised for younger children, mainly because the vaccine is less effective for them. But it can offer partial protection, and the outbreak has led some experts including Tan to suggest that concerned parents discuss possible infant immunization with their pediatricians.
Most parents choose to vaccinate their older children, but health authorities and ethicists are concerned about clusters of families who reject vaccines for personal or religious beliefs, and the risk they pose to those who cannot be vaccinated.
"There is clearly a very weak link in the system or else we wouldn't be having these outbreaks," said Lawrence Gostin, an ethicist and public law specialist at Georgetown University.
Public health authorities "absolutely should be looking more closely" at how to protect unvaccinated children in day care, through vaccine requirements for staff or fewer exemptions for children of parents who oppose vaccines, Gostin said.
"I'm fine with the idea of individual freedom and parental rights so long as they don't put the community at risk," Gostin said.
KinderCare Learning Centers, which runs the Palatine, Illinois, centre where the five infected infants were enrolled, announced this week that it will start requiring measles vaccinations for staff members at its 1,500 locations nationwide who work with children under age 15 months.
"We are being vigilant about enforcing our policy of excluding children from care when they are sick," KinderCare spokeswoman Colleen Moran said Friday. "We are also working with families and staff members in our centres to double-check and update their immunization records."
Parents like Andrew Hires are watching the outbreaks closely. He lives near the Santa Monica day care centre with one infected infant and 14 others at risk. His unvaccinated 10-month-old daughter does not attend day care, but Hires said he's worried she might come in contact with an exposed child.
Hires, a neuroscientist at the University of Southern California, said his vaccinated 2-year-old daughter attends a preschool where most children are up to date on their shots, but some local preschools have low immunization rates.
"I don't live in fear of it, but I am keeping a very close eye on where outbreaks are occurring, and where they might be predicted to occur in the future," he said.
Associated Press video journalist Alex Sanz in Atlanta contributed to this report.
CDC and measles: http://www.cdc.gov .