Five infants have died of whooping cough this year in what may be the state's worst epidemic in 50 years, California health officials have revealed.

As of June 15, the state of California recorded 910 cases of pertussis, or whooping cough, four times the number from the same period last year when 219 cases were reported, said Dr. Mark Horton, director of the California Department of Public Health. Horton further said that in addition, 600 possible cases of pertussis are being investigated by local health departments.

Horton said in a statement, "Whooping cough is now an epidemic in California. Children should be vaccinated against the disease and parents, family members and caregivers of infants need a booster shot."

The pertussis vaccination is begun when infants are 2 months old, but infants are not adequately protected until all three shots are completed when the infant is 6 months old.

It was also reported that Pertussis is highly contagious and non-immunized or incompletely immunized young infants are particularly vulnerable. Since infants cannot be completely vaccinated the only way to protect them from this potentially fatal disease is to vaccinate family members, healthcare workers and anyone who has contact with the infant.

According to Horton, the series of pertussis shots most children receive wears off by the end of middle school and neither vaccination nor illness from pertussis provides lifetime immunity, so people in California are being asked to get a pertussis booster. (With inputs from Agencies)