July 21, 2010 -- Six infants have died in California in what looks like the state's worst whooping cough epidemic in 50 years.

To date, the CDC says South Carolina is the only other state where whooping cough cases have exceeded the "epidemic threshold" -- a statistical measure that means there are significantly more cases than usual for the time of year.

After declaring an official epidemic of pertussis, the medical term for whooping cough, California health officials announced a broadened vaccination campaign for teens and adults of all ages. Anyone who comes into contact with babies is particularly urged to get the vaccine -- even pregnant women and the elderly.

"Teens and adults should be vaccinated, especially anyone who is going to have contact with infants who are too young for vaccinations," CDC epidemiologist Stacey Martin, MSc, tells WebMD. "Those California deaths were all in infants less than 3 months old. They don't have the benefit of vaccination yet, so we have to vaccinate around them."

Infants get three doses of the vaccine but are not fully protected until after they are 6 months old.

Neither the pertussis vaccine nor natural infection gives a person lifelong immunity to whooping cough. Outbreaks tend to occur in five-year cycles, suggesting that immunity wanes within that time.

Pertussis is one of the diseases covered by the three-way DTaP (diphtheria/tetanus/acellular pertussis) vaccine for children under age 7 years and by the three-way booster Tdap vaccine for older children, teens, and adults. There is no standalone pertussis vaccine.

Although a person needs a tetanus vaccination only once every 10 years, it's not a problem to get the Tdap vaccine at shorter intervals. Adults who get the tetanus and Tdap shots within two years may have more redness and soreness at the place the needle went in, but no significant safety issues.