Based on the patterns of other recent whooping cough outbreaks, the disease could continue to infect more people through much of the rest of this year.
"It's continuing to grow and grow and grow," said Dr. Yuan-Po Tu, who closely monitors infectious diseases at The Everett Clinic.
"We have not seen the peak of this."
There were 225 cases reported in the county last year. So far this year, 105 cases have been confirmed. One adult and one infant have been hospitalized.
"This clearly is the biggest infectious disease epidemic since H1N1," Tu said, referring to the swine flu epidemic of 2009-10.
No one really knows why Snohomish County is being hit so hard.
"We have the disease circulating in our state at all times," said Michele Roberts, spokeswoman for the state Department of Health's immunization program. "Not every county gets it at the same time."
About 92 percent of the county's kindergarten students are vaccinated against the disease, she said. The 88.4 percent of sixth graders immunized in the 2010-11 school year is higher than the state average of 83.8 percent.
The disease continues to spread among teens and adults, who either never got the shot or got it long enough ago that its protection has waned.
"People may have been vaccinated, but the vaccine won't protect forever," she said.
In kids, it wears off in about five years.
That's why health officials are urging adults to get vaccinated. But nationally, only 10 percent of adults are immunized, said Dr. Gary Goldbaum, health officer for the Snohomish Health District.
Even older adults, such as grandparents, who may have been infected with the disease in the past need to get the shot, he said.
"That doesn't confer immunity," Goldbaum said. "I'm still asking every adult to be vaccinated."
The shot is recommended for pregnant women after reaching their 20th week of pregnancy, parents of infants who aren't old enough to be vaccinated, child care workers and grandparents.
Adults who have close contact with infants are urged to get the shot because babies can't be immunized until they're 2 months old.
To help reach all of these groups, the Snohomish Health District will provide 320 doses of the vaccine for free on Feb. 25 at the Group Health's Everett Medical Center. The vaccines were paid for through a $10,000 grant from the Group Health Foundation.
This is the second time this month the local public health agency has provided the shot for free to low-income and uninsured adults who may not be able to pay for the vaccine.
More than 300 people were vaccinated at two free-shot clinics Feb. 4.
Since the beginning of the year, one or more cases have been reported in schools, child care centers or other community centers in Marysville, Bothell, Everett, Granite Falls, Lake Stevens, Lynnwood, Stanwood, Monroe, Arlington, Snohomish, and Mukilteo.
The central, north and eastern portions of the county have recorded the most cases, Goldbaum said, while relatively few cases have been reported from south Snohomish County.
The disease can cause a number of problems in infants younger than 2 months who haven't had their first dose of vaccine, including pneumonia, seizures and troubles with breathing.
Last year, eight Snohomish County infants were hospitalized with whooping cough. A 27-day-old Lake Stevens infant died from the disease in August.
Children's symptoms often include the whooping sound that earned the disease its name. Adults, however, typically have coldlike symptoms that seem to linger with either no fever or a low-grade fever.
Since adult symptoms often mimic those of a cold, there may be far more people who are ill with the disease but never go to the doctor and so are never tested. That means the number of cases may be greater than the number reported to public health agencies.
"We know the reported cases are just the tip of the iceberg," Roberts said.
The largest recent outbreak of whooping cough occurred in 2010 in California, with 9,143 cases reported.
Last year, 938 cases were reported in Washington, the highest number in six years.
Free immunizations to protect adults from whooping cough will be offered from 9 a.m. to 1 p.m. on Saturday at the Group Health Everett Medical Center, 2930 Maple St., in Everett. Walk-ins are welcome, but appointments are recommended by calling 425-339-8694 from 9 a.m. to 1 p.m. Feb. 21-24. The shots are being offered to low-income adults without health insurance, pregnant women in their third trimester, and adolescents and adults who have close contact with infants, young children, or pregnant women in Snohomish County.
Sharon Salyer 425-339-3486; email@example.com.
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