As Washington state combats a whooping-cough epidemic, a rise in cases among teens suggests the vaccine may be wearing off sooner than expected, officials from the Centers for Disease Control and Prevention say.

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A rise in whooping-cough cases among teens suggests the pertussis vaccine may be wearing off sooner than expected, officials from the Centers for Disease Control and Prevention said Thursday.

After Washington state declared an epidemic in April, the CDC dispatched a team to research the spike in cases.

The investigation revealed an increasing number of cases among adolescents ages 13-14 in Washington and nationwide, leading researchers to believe that pertussis vaccines can start to wear off by age 10.

In the U.S., the CDC found 10-year-olds contract pertussis at a rate of 25 cases per 100,000, higher than any other group from ages 1-19.

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At ages 11 and 12, children can get the adult version of the vaccine. Still, by ages 13-14, officials saw the rate go up again.

“Vaccines have done a good job in protecting against pertussis, but our vaccines are not perfect,” said Dr. Anne Schuchat, director of the National Center for Immunization and Respiratory Diseases during a Thursday teleconference with state Health Secretary Mary Selecky. “They don’t last as long as we would like them to.”

Schuchat said the vaccine still offers effective protection for several years.

The state Department of Health has recorded 3,000 cases of pertussis this year — one-sixth of the 18,000 cases around the country. Last year, Washington state saw 965 cases. In 2010, the state recorded 608 cases.

Nine people have died from the illness around the country this year, though none in the state.

Pertussis, also known as whooping cough because of the sound of the coughing, is a contagious respiratory illness caused by a bacterial infection. It can lead to difficulty breathing, vomiting and disturbed sleep. Pertussis can be fatal for infants because their bodies cannot support the strong, consistent coughing.

Infants and children under age 7 are immunized against pertussis with the DTaP vaccine. Children who miss a shot of the DTaP five-dose series receive the booster shot called Tdap, as do teens and adults. Infants and young children often get pertussis from adults who have not been vaccinated.

The DTaP vaccine is relatively new — replacing the DTwP vaccine in 1997 — so the wave of teens experiencing an uptick in cases used DTaP.

The switch to DTaP came after DTwP caused common side effects like fever and swelling at the injection point. Some children experienced chronic neurological issues, but studies failed to prove the vaccine was at fault, Schuchat said.

In Washington, more than 80 percent of children by age 3 are up to date on their DTaP shots.

Also a newer vaccine, Tdap, was introduced in the past several years to better protect teens and adults from pertussis. Statewide, 70 percent of teens ages 13-17 had their Tdap shots.

Investigators plan to continue looking at the protection time of both vaccines.

Since vaccines do eventually wear off, Selecky said, adults should get vaccinated with Tdap. Only 8 percent of adults nationwide have been vaccinated.

“With this type of outbreak, you can’t pinpoint the cause, but one cause is that the vaccines wear off, and people are not protected from disease,” said Michele Roberts, spokeswoman for the state immunization office. “There are still a lot of adults who don’t have it.”

To combat the epidemic, the Department of Health has spent more than $1 million in state and federal funds to make 41,400 free and low-cost vaccines available to lower-income adults.

In June, the state spent $727,000 in federal funds for 27,400 doses of Tdap for underinsured and uninsured adults — 26,400 for counties, 1,000 for tribal health clinics. This month, the state spent more than $371,000 in state and federal funds to make 14,000 more doses available to counties on an as-needed basis.

“The current price (of vaccine) is $60, and for some people that’s not doable with the economy right now,” said Marci Reynolds, clinical-care coordinator for QFC.

QFC and several area pharmacy chains have partnered with counties including King and Snohomish to offer the free-to-low-cost vaccine with a $10 service fee. Most insurers cover the cost of the vaccine. But with the state-supplied vaccine, the price drops to a $10 to $15 service fee if the underinsured or uninsured customer can pay. Otherwise, the fee is waived.

As pertussis cases rose this year, immunization rates in adults rose as well. The state recorded administration of 111,382 doses of Tdap to adults 19 and older between March and June, more than twice the 51,456 doses recorded in the same period in 2011.

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