2012 is shaping up to be the worst year for whooping cough in more than five decades, and a new study from California may help explain why.

When researchers studied a 2010 outbreak in that state, they found that protection among children vaccinated against whooping cough (also known as pertussis) waned dramatically during the five years after their last immunization.

Vaccines Safe, but Protection Doesn’t Last

In the U.S., most children receive five doses of the combined diphtheria, tetanus, and pertussis (DTaP) vaccine, beginning at around 2 months of age and ending between the ages of 4 and 6 years.

Researchers have long suspected that a newer version of the whooping cough vaccine, introduced in 1997, does not protect as long as the older version. And this, they think, has contributed to the increase in cases across the nation.

The new analysis of the California outbreak appears to confirm this.

“The current available vaccines are the safest that we have ever had, and they do protect against pertussis,” says researcher Nicola P. Klein, MD, PhD, of the Kaiser Permanente Vaccine Study Center. “They just don’t last as long as we would like.”

26,000 Whooping Cough Cases Reported

As of last week, slightly more than 26,000 cases of whooping cough had been reported to the CDC since the first of the year, says CDC medical epidemiologist Thomas A. Clark, MD.

That’s twice as many cases as were reported by this time last year.

“We are seeing 1,000 new cases a week right now, so they are still coming at a pretty brisk clip,” Clark says.

Other than infants, the rise has been greatest among children between the ages of 7 and 10, followed by young teens.

The spike in these age groups first alerted investigators to the possibility that the newer vaccine may not be as long-lasting as the older one, which was replaced for safety reasons.

To test this theory, the Kaiser Permanente investigators focused their research on children who had been vaccinated with the newer combined vaccine.

Protection Wanes Each Year

They concluded that the odds of developing whooping cough rose by about 40% per year among fully vaccinated children during the five years after the last DTaP dose was given.

In addition to waning protection, Clark says better reporting of cases may be contributing to the whooping cough epidemic, especially in states with recent outbreaks that were widely reported, such as Wisconsin, Washington, Montana, and Minnesota.

These four states have the highest number of whooping cough cases, compared to the national average.

A booster shot of pertussis, tetanus, and diphtheria vaccine, known as Tdap, is recommended for pre-teens and unvaccinated adults. Clark says a second booster shot may be needed.

The CDC recommends vaccinating pregnant women after their third trimester, if possible, to protect their infants.

Most whooping cough deaths happen in young babies who have not been vaccinated.

Klein says new whooping cough vaccines are needed that are as safe as the current ones, but longer lasting. The researchers question whether giving an earlier booster shot around age 8 should be considered, too.