The booster shot that is supposed to protect adolescents from whooping cough isn’t working as well as public health officials would hope, according to a study published Friday in the journal Pediatrics.

The study concludes that for every year that passes after a child receives the tetanus, diphtheria and pertussis vaccine, known as Tdap, their protection against the respiratory disease wanes by about 35 percent.

While some other vaccines are known to wear off over time, that’s unusually fast. And it’s distressing because whooping cough, or pertussis, spreads easily and can be deadly in infants, researchers said.

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Outbreaks Lead to Study

After a 2010 outbreak in California that and resulted in the deaths of 10 infants, the state mandated the Tdap booster for all kids entering 7th grade.

Despite that effort, another outbreak erupted in 2014, affecting even more people and causing another three infant deaths.

Researchers with Kaiser Permanente in Northern California looked back through medical records to see if and when vaccinated kids came down with whooping cough during the two outbreaks.

The researchers had previously used the same data to analyze the effectiveness of the initial round of childhood shots against whooping cough, known as the DTaP series. They concluded that after the last DTaP shot is administered — usually around age 5 — immunity wears off at a rate of 42 percent a year.

Their new study is a follow-up that reports on the effectiveness of the booster shot, which is given to kids at age 11 or 12.

“We’re all trying to figure out what’s special about pertussis and how we can induce more durable immunity,” Dr. Kathryn Edwards, a pediatrician and researcher who was not involved in the study, told Healthline.

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Whooping Cough on the Rise

Whooping cough is caused by a bacteria and spread by coughing or sneezing. Patients are often seized by fits of coughing, which can make a characteristic “whooping” sound.

In the United States, the prevalence of the disease dropped after a vaccine became available in the 1940s. But recently, it has been on the rise again, with reported cases increasing sixfold between 2000 and 2012, .

That uptick coincides with a change in the vaccine’s recipe, which was introduced in the late 1990s. The original vaccine, known as DTP, worked well but caused side effects such as rash and fever. So manufacturers designed a new vaccine, DTaP, which has fewer side effects.

However, as the first cohort of children vaccinated solely with DTaP grows up, doctors are finding that the new shot doesn’t offer the same long-term protection as the old shot. In 2006, the Tdap shot was offered as a means of boosting the waning immunity experienced by kids vaccinated with DTaP.

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Why Does Vaccine Lose Strength?

It’s possible that the whooping cough bacteria may be evolving, rendering the current vaccine obsolete. That’s one explanation offered for a recent outbreak among pre-schoolers in Florida.

But Dr. Nicola Klein, FAAP, a Kaiser Permanente research scientist who was lead author on the Kaiserstudy, doesn’t think that explains the California outbreaks.

“I think the primary driver of these epidemics has been the waning of DTaP overall,” she told Healthline.

And because the Tdap booster wears off at a similar rate, it has been only moderately helpful.

That’s not to say that the current vaccine is not effective at all.

“It does work. It just doesn’t work for very long,” Klein said.

Even in , the year with the highest number of cases since 1955, the prevalence of whooping cough was less than a quarter of what it had been during outbreaks before the vaccination era.

Still, the short-lived performance of the revised vaccine indicates that the new recipe lost a vital ingredient.

The current vaccine contains just a handful of proteins extracted from the bacteria. In contrast, the old vaccine contained the entire organism and all its thousands of proteins.

Perhaps one or more of the proteins found in the intact bacteria is the missing ingredient that triggered a longer-lasting immune response.

Until scientists find that ingredient, Klein suggests we use the vaccine we have more strategically. Instead of giving the booster routinely, she says, it might be better to wait until the beginning of an outbreak. That will afford more protection to kids when they really need it.

One strategy that does seem to be working, Klein said, is giving Tdap to pregnant women late in their pregnancies. That gives newborn babies some protection against the disease during that fragile time before they are old enough to be vaccinated themselves.

Edwards, who advises the American Academy of Pediatrics on infectious diseases, agrees that reworking the vaccine and its delivery is necessary to protect communities from whooping cough. But it won’t be an easy puzzle to solve.

“I wish we had more of the answers,” she said, “but I’ve been working on pertussis for 30 years and I still have more questions than answers.”