The increase in illness among young teens is different than what's been seen in previous waves of the disease, said Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases.
The trend was reported by the federal Centers for Disease Control and Prevention as part of a larger study on the spread of the disease, both in Washington and nationally.
Schuchat was joined by Mary Selecky, Washington state's secretary of health in a news conference on Thursday, calling attention to a disease that has continued to spread at rates not seen in decades.
In Snohomish County, health officials declared the disease, also known as pertussis, an epidemic in January. During the first six months of this year, 405 cases have been reported, compared to 225 during 2011.
In Washington, the rates are three times greater than in 2011, with more than 3,000 people sickened this year.
Nationally, 18,000 cases have been reported. "We may be on track for record high pertussis rates this year," Schuchat said.
Federal health officials came to Washington in the spring as part of its examination of why another wave of whooping cough was spreading across the nation.
Scientists are trying to determine why more young teens are getting whooping cough. It may be caused by a change in the vaccine used for young children, Schuchat said.
The switch to the acellular type of the vaccine occurred in 1997, following concern about possible side effects caused by the previous version of shot, Schuchat said.
These included short-term effects, such as fever and swelling in the area where the shot was given. There was also public concern over whether it caused rare but more serious side effects, she said, such as chronic neurologic problems.
The change in the vaccine, while causing fewer reactions, might have done something to affect how long the vaccines are effective, she said. The current group of 13- and 14-year olds is the first to have all their immunizations with the newer vaccine.
Higher rates of whooping cough also were found among 10 year olds, Schuchat said.
The increase in cases may be caused by a waning in immunity provided by the vaccine, she said.
Initially, it provides about 95 percent protection in young children. But that declines to about 70 percent five years after the initial five-shot childhood series is completed at age 6.
Unvaccinated children are at eight times higher risk of getting pertussis compared to fully vaccinated children, she said.
Vaccinated children who still get pertussis usually have milder symptoms, a shorter illness and a reduced risk of hospitalization, Schuchat said.
In 2006, physicians began recommending that youngsters get a booster shot by their 12th birthday. That shot is now a requirement for the state's public school students.
Federal health officials want to take a more in-depth look at how well that booster shot is working to better determine just how long it remains fully effective and whether adults will need more booster shots, Schuchat said.
Health officials continue to urge pregnant women to get immunized, since infants aren't vaccinated until they're about 2 months old and children aren't fully protected until they have completed the five-shot series.
Nationally, nine babies have died from whooping cough this year.
More than half of babies younger than a year old who get the disease are hospitalized. In Snohomish County, six infants have been hospitalized this year.
Dr. Gary Goldbaum, health officer for the Snohomish Health District, said that the number of local whooping cough cases declined some in June, but not as dramatically as last year.
It's unclear whether the spread of the disease has begun to slow, since the number of cases has jumped up and down each month, ranging from 75 in January to 55 in June.
"It will take a few more months for us to believe we're headed in the right direction," Goldbaum said.
Sharon Salyer: 425-339-3486 or email@example.com
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