Whooping cough vaccine fades in pre-teens: study

During a whooping cough outbreak in California in 2010, immunized children between eight and 12 years old were more likely to catch the bacterial disease than kids of other ages, suggesting that the childhood vaccine wears off as kids get older, according to new research.

"We have a real belief that the durability (of the vaccine) is not what was imagined," said Dr. David Witt, an infectious disease specialist at Kaiser Permanente Medical Center in San Rafael, California, and senior author of the study.

Whooping cough, or pertussis, is caused by Bordetella pertussis bacteria. The infection produces an intense cough that lasts weeks and can lead to pneumonia, an inability to breathe or death - although most cases don't reach such extremes.

According to the Centers for Disease Control and Prevention (CDC), about nine out of every 100,000 Americans get pertussis each year, and while that number is considerably smaller than before the vaccine was introduced, it has been rising for the past two decades.

The pertussis vaccine, a five-shot series referred to as DTaP, is recommended for children at ages two-, four-, six- and 18-months, and at four to six years old.

The CDC recommends that at age 11 or 12 kids get the booster shot called Tdap.

In early 2010, a spike in cases appeared at Kaiser Permanente in San Rafael, and it was soon determined to be an outbreak of whooping cough - the largest seen in California in more than 50 years.

Witt had expected to see the illnesses center around unvaccinated kids, knowing they are more vulnerable to the disease.

"We started dissecting the data. What was very surprising was the majority of cases were in fully vaccinated children. That's what started catching our attention," said Witt.

To figure out just how well the vaccine was working, Witt and his colleagues collected information on every patient who had tested positive for pertussis between March and October, 2010.

Of the 132 patients under age 18, 81 percent were up to date on recommended whooping cough shots and eight percent had never been vaccinated. The other 11 percent had received at least one shot, but not the complete series.

The rate of cases for each age, two through 18 years old, peaked among kids in their pre-teens.

Among fully immunized kids, there were about 36 cases for every 10,000 children two to seven years old, compared to 245 out of every 10,000 kids aged eight to 12.

"The longer you went from your last vaccine, the greater your risk of disease," Witt told Reuters Health.

At age 13, the number of cases dropped, presumably because that's the age when children are eligible for their booster shot.

Comparing the kids who got pertussis to the more than 22,000 kids in the medical center's database who didn't, Witt's group wrote in the journal Clinical Infectious Diseases that the vaccine is effective about half of the time for all kids, and just 24 percent of the time in the eight to 12 year old age group.