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Laboratory tests confirmed two cases, with results for three others still pending, officials said. However, the remaining three have been diagnosed with measles based on clinical and epidemiological criteria.
Health officials said that five children under the age of 1 were infected after attending the KinderCare Learning Center, in Palatine, Illinois, a suburb of Chicago.
Health officials warned at a news conference that the current outbreak could grow.
"We’re talking about a highly contagious disease, one that is particularity threatening for those people who have not been vaccinated," said Dr. Terry Mason of the Cook County Department of Public Health. "There’s likely to be more cases. We shouldn't be surprised by that."
Infants younger than 12 months old are particularly susceptible to the measles virus because infants are not traditionally given the first dose of the vaccine until they are a year old. The cause of the outbreak has not been determined, according to health officials.
A measles outbreak that started in mid-December in the Disneyland theme park in California has already infected more than 100 people in 14 states. Health officials said they were trying to determine if this new measles cluster was related to that outbreak.
Although an adult in the same county was diagnosed with measles earlier this month, health officials said that the cases were likely not linked.
In addition to the children who were diagnosed, health officials are monitoring another seven to 10 children who were also exposed to the virus at the day care center and were too young to have received their first measles vaccine.
Prior to this outbreak, Dr. Rachel Rubin of the Cook County Health Department said there had been only 10 measles cases in the last five years.
"These were sporadic cases," she said at a press conference. "We haven’t seen anything like this in many years."
A spokeswoman for KinderCare said it was working closely with the state and county health departments to respond to the outbreak and was informed of the first two infections Thursday.
"We wish both children a speedy recovery and are hopeful they will be able to rejoin us in the center soon," read part of a statement from KinderCare. "We are following Public Health officials’ guidance and excluding unvaccinated children and staff who may have been exposed to the virus from our center."
Any staff or students at the facility who have not received the measles vaccine have been asked to stay home for 21 days until Feb. 24.
Measles is an extremely contagious virus that infects 90 percent of unprotected people who are exposed. Symptoms of measles include fever, runny nose, conjunctivitis and a red rash.