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"I am concerned that people do not have accurate information," said Dr. Jonathan Fielding, L.A. County Health Dept.
Fielding says that part of the problem is measles vaccinations have been so effective. In recent times many parents have not even seen measles and do not understand how serious it is.
"One in every 30 gets pneumonia from measles. About one in 1,000 gets encephalitis, which is the swelling of the brain. You can even get one death per thousand. This is not a cold," said Fielding.
The measles vaccine is given to children at 1-years-old. They get their booster shot between the age of four and six.
A growing number of parents have opted out of in recent years believing vaccines can cause serious illness and disorder, including autism.
"The link between getting vaccinations and autism for any child, that is otherwise normal, is just not there," said Fielding.
The 12 sick children in San Diego had not been vaccinated because their parents opted out.
"A disease an infection that I haven't seen and isn't very likely to get does my child really need the vaccine? Well I think what is going on in San Diego County really answers that question," said Dr. Bretten Pickering, Pediatrician.
"I am afraid that if people start thinking wrongly about the benefits then the risk we will have more cases. That is really the concern that is being voiced," said Fielding.