What's Behind Washington's Whooping Cough Spike?

Washington state’s whooping cough outbreak continues to grow. So far this year, there have been 397 confirmed cases, compared with 85 last year.

Whooping cough is cyclical; it peaks every 3 to 4 years.

But there’s more to it than the natural cycle. To better understand that, researchers went back to 2012, when Washington state was in the middle of an epidemic. At the time there were nearly 5,000 reported cases; babies and young teens were the most affected.

Health experts were puzzled. Teens were getting infected even though they’d been vaccinated. So they looked at their immunization records. They wanted to understand how much protection the vaccine provided over time.

Chas DeBolt is senior epidemiologist with the state health department. She, along with researchers from the Centers for Disease Control and Prevention conducted the study.

They found that the benefit of this newer vaccine, in use since 1997, faded over time.

“Within the first 12 months it was 73 percent effective,” DeBolt said. “But then as you go out over time, since getting vaccinated, that level of effectiveness drops down.”

The older version of the vaccine didn’t wane as quickly, but it was replaced because it caused adverse reactions in kids, like high fevers.

DeBolt said the current vaccine may not be as robust over time, but it still offers some protection. “There is some memory there, so the body knows what it’s dealing with,” she said. “And they don’t get as sick if they’ve had their vaccine series.”

And the CDC isn’t ready to change vaccine recommendations yet. Dr. Manisha Patel, of the division of bacterial diseases, said more research is needed before that happens -- like how the vaccine affects transmission of the bug.

“We know that it’s spread through respiratory droplets, so how does the vaccine change that, or does it at all?" Patel said. "There’s a number of questions that, about the bacteria, about immunology, about changes in the epidemiology that we are continually re-evaluating.”

What hasn’t changed is that babies are most vulnerable to whooping cough, which can be fatal. Health officials urge pregnant women to get vaccinated during their last trimester so they can pass on the protection to their babies.

The study is published in this month's journal Pediatrics.