Nov 28, 2012 12:24 PM by NBC News
Whooping cough cases reached peak levels over the past couple years, and experts at the Centers for Disease Control and Prevention are now learning what's behind the spike.
In a new study, researchers at the CDC studied cases of pertussis in 15 counties in California in 2010.
They found children with whooping cough were less likely to have received all five doses of the vaccine.
They also found children who were vaccinated were coming down with the illness, as it appears the vaccine loses effectiveness over time.
Experts say children who are fully vaccinated are protected from the virus more than 90% of the time; however, five years after their last dose, that number drops to 70%.
Experts say a booster shot is recommended around age eleven.
Pregnant women and anyone caring for infants should also get a booster.
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