Five new cases of pertussis have been diagnosed on Vashon so far this month, bringing the total number of confirmed cases on the Island to 15 since Dec. 1.

Public health officials first noticed a county-wide increase in the bacterial illness in December, according to James Apa, the communications manager for Public Health-Seattle & King County. Since Dec. 1, 101 cases have been reported in the county; this number compares with 98 cases in all of 2011.

While pertussis is on the rise throughout the area, Vashon has been harder hit than the rest of the county, according to Dr. Tao Kwan-Gett, a medical epidemiologist with the public health department. Vashon has less than one percent of the population, he noted, and has accounted for 16 percent of the 79 cases reported so far this year.

Whooping cough, as the bacterial illness is also known, ebbs and flows as part of its natural cycle, Kwan-Gett said, but he cautioned that communities need to be aware of factors that affect that cycle, including how many people are immunized against the disease. Historically, Vashon has had high numbers of people who are not immunized. Of Vashon’s 15 cases, he noted, only one person was up to date on the pertussis vaccine.

Neither the disease itself nor the vaccine is a magic bullet, he said, and neither provides lifelong immunity, but public health officials believe the the vaccine is vitally important.

“Lower vaccination rates make it easier for pertussis to spread in the population,” he said.

The spread of pertussis is worrisome because it can be deadly for infants, and public health officials are taking this outbreak seriously.

“Because pertussis can be so dangerous for babies, we always get very concerned when there is an increase,” he said.

There is no set pattern to how pertussis outbreaks unfold, he noted, and it is not possible to accurately predict what will occur from here.

“We take things week by week and month by month,” he said.

Officials at the public health department work directly with individuals and communities when pertussis is reported, he said. They call the families or individuals to try to determine when they might have been infected, who might be at risk of infection and who would benefit from preventative treatment, including other family members, friends, classmates and health care providers. They also look for links between the newly reported cases and previous cases, and if people do not have access to health care, public health officials arrange for appropriate treatment.

To stop the spread of the illness, Kwan-Gett encourages people to be immunized, get diagnosed and treated if they suspect they might have the illness and to stay home if they are sick. He noted that patients who seek care might have to insist on a culture for whooping cough because pertussis frequently resembles many other common respiratory infections, and health care providers must have a “high degree of suspicion” that someone has the disease in order to test for it. With antibiotics, people are no longer contagious after five days, Kwan-Gett noted; without antibiotics people can be contagious up to 21 days after their symptoms appear.

Also important to understand, he said, is that the reported numbers only tell part of the story, and it is likely pertussis is more prevalent than the numbers suggest.

“The number of reported cases might seem small,” he said in a recent email, “but it’s important to remember that the reported cases are just the tip of the iceberg. Many people with pertussis never get diagnosed or reported.”

At the Vashon Island School District, nurse Sarah Day said there have been confirmed cases at Chautauqua and Vashon High School but none so far at McMurray. She, too, encourages people to be responsible if they are ill and to remember that there are many vulnerable people in the community, including infants, pregnant women and people receiving chemotherapy.

“They’re our neighbors and friends,” she said. “We all shop at the same grocery stores.”