Five-year-old twins Solania and Enzo are getting their fifth dose of the DTAP vaccine, which protects against diphtheria, tetanus and pertussis, also known as whooping cough.

"I think it's important to protect the kids from all the illnesses out there," mom Teresa Barrera said.

A new study in the New England Journal of Medicine raises concerns about that final dose of whooping cough vaccine and just how long it protects children.

Kaiser Permanente researchers found the shot's effectiveness fades about 40 percent in the five years after the vaccination.

"Every year it gets a little less effective, a little less effective," said Dr. Amy Porter, a pediatrician at Kaiser Permanente Los Angeles Medical Center.

This is a record year for the whooping cough. Doctors are reporting at least 26,000 cases so far in the United States. In North Carolina, there were 354 reported cases by mid-August.

Infants under 6 months old are most susceptible because they haven't been fully vaccinated.

In the study, 80 percent of kids who had whooping cough caught it from an older relative.

"As time goes on and the vaccines get less and less effective. It's time for those booster shots," Porter said. "Experts recommend older people get a booster if they didn't have one in their teens."

Barrera feels good knowing her family is protected.

"I think it's important. I think it's better some protection than no protection at all," she said.

Her twins will need another whooping cough booster in about 6 years.