The 502 cases tallied so far this year are more than double the number of people sickened in the county during all of last year -- 224.
Nine people have been hospitalized, including six infants.
The number of new cases has declined some over the summer, with kids out of school. Yet if the current rates of infection continue, the case count could hit 700 by year's end, said Dr. Gary Goldbaum, health officer for the Snohomish Health District.
"The number of cases is sufficient to sustain a pretty high level of (disease) spread in the community," he said.
Statewide, nearly 4,000 cases have been reported.
Public health officials will be watching closely to see whether the number of infections increases or declines now that school is back in session, he said.
"We know at some point epidemics like this burn themselves out," Goldbaum said.
In the meantime, he urged all people, particularly adults, to get immunized against the disease, also known as pertussis. The shot provides protection against whooping cough, tetanus and diphtheria.
The shot is included in the list of immunization required for children to attend public schools in Washington.
Many adults, however, have not been immunized. The symptoms of the disease in adults, such as a runny nose, often mimic a cold, so it's easy for them to unintentionally spread the disease.
This can be particularly dangerous for infants. Whooping cough can cause a number of problems, including pneumonia, seizures and trouble breathing. Infants can't get their first whooping cough shots until they're about 2 months old.
A Lake Stevens infant, Kaliah Jeffery, died from whooping cough last year when she was just 27 days old.
Five immunizations are recommended for children by the time they are age 7.
In July, federal health officials said they found that 13- and 14-year-olds are getting the disease at higher-than-expected rates, but were unsure why the vaccine's effectiveness seemed to be waning.
In Washington, nearly a quarter of all cases are among 10- to 13-year-olds, according to the state Department of Health.
In south Snohomish County, the nonprofit Verdant Health Commission is trying to rally attention to the issue of getting adults immunized.
"The biggest issue is lack of awareness," said George Kosovich, Verdant's director of programs. Only an estimated 10 percent to 20 percent of adults have been vaccinated, he said.
Three meetings have been scheduled, starting Sept. 19, to get the word out to nearly 300 people representing area nonprofits, churches, employers and schools.
They will be provided with information on how to set up vaccination clinics.
The hope is that employers will use organizations such as Visiting Nurse Services of Seattle to provide immunizations to employees, similar to workplace flu shot campaigns often scheduled in the fall, he said. If the company has health insurance for employees, the plan usually covers the cost of the shot, Kosovich said.
About a dozen area medical clinics and other health care organizations have volunteered to speak to community groups about the whooping cough epidemic.
"The primary push of what we're doing is educating the community that people really do need to get vaccinated," Kosovich said.
Sharon Salyer 425-339-3486; firstname.lastname@example.org.
Read The Herald's story about Chelsey Charles, of Clearview, whose infant died last year from whooping cough: http://tinyurl.com/HeraldCase.
The Public Broadcasting Service interview with Charles: http://tinyurl.com/PBSOneStory
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