(KCEN) -- Whooping cough cases reached peak levels over the past couple years.
Now experts are learning what's behind the spike.
In a new study, researchers studied cases of pertussis in fifteen 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 the vaccine loses effectiveness over time.
Experts say children who are fully vaccinated are protected from the virus more than ninety percent of the time, however that number drops to seventy percent after five years.
Experts say a booster shot is recommended around age eleven.
Pregnant women and anyone caring for infants should also get a booster.