Authorities urge anyone over 6 months old to get the vaccine if they haven't already, especially the elderly and those with chronic health conditions.
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The flu season typically reaches its peak in February.
"It's not too late to get a flu vaccine, even if you get it today, the chances that you are going to protect yourself and your family are great," said Dr. Gil Chavez with the California Department of Public Health. "If you get the vaccine, particularly the fourth shot, you should be protected against all the strains circulating."
Health officials said people who get the shot and still get the flu usually experience less severe symptoms.
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