Editor’s note: This story was corrected and differs slightly from an earlier online version and the printed version.

Six students on Vashon have confirmed cases of whooping cough, prompting King County health officials to issue a letter urging parents and teachers to be on alert for the highly contagious disease.

The public health agency, in a letter to all parents and staff at Vashon’s three public schools, says adults should pay attention to cold symptoms among children, noting they could be the first signs of pertussis, or whooping cough.

Those adults who have contact with babies or young children — considered the most vulnerable to severe infection from the disease — should take extra precautions to ensure they don’t get the illness, the letter adds. Babies, according to the letter, signed by Eileen Benoliel, a public health nurse in the Communicable Disease Epidemiology & Immunization Program at Public Health – Seattle & King County, can die from pertussis.

The letter was issued late Friday after officials learned there are four confirmed cases of pertussis at the Vashon School District and two confirmed cases of the disease in school-age children who attend a private school on Vashon. School officials believe there’s a linkage to those cases — or “evidence of contagion,” as Sarah Day, Vashon’s school nurse, put it — which led to the county’s letter.

Vashon School Superintendent Michael Soltman said the letter from the county underscores the seriousness of the situation on the Island, where the number of children who have not been vaccinated against whooping cough is considered high by many health care professionals.

According to recently tabulated numbers, 33 percent of the students in the Vashon school district have not been fully vaccinated against whooping cough. What’s more, according to health officials, the immunization is not highly effective or long-lasting. Health officials believe the pertussis vaccine is only 59 to 89 percent effective in preventing the disease.

“I think we have a collective responsibility to be immunized,” Soltman said. “We need to be thoughtful about not putting other people at risk.”

If the Island were to experience an outbreak of pertussis, he added, the county could order all children who have not been vaccinated to remain at home or, in a worse-case scenario, close the schools altogether.

“I’m concerned about (pertussis) being a risk to people’s health and also extremely disruptive to the schools,” he said.

For most people, whooping cough begins like a cold — with a runny nose or scratchy throat — and a dry cough that often lasts one to two weeks. But for babies too young to be immunized or small children who have yet to receive the vaccine, the cough can prove fatal.

According to the state Department of Health, there were 728 cases of pertussis in Washington from Jan. 1 to Dec. 17 last year. This compares with 529 cases in the same time period in 2010. Eighty-six infants under 1 had the illness and 29 were hospitalized, including 23 who were under 3 months. Two infants died. In King County, there were 98 confirmed cases last year, according to James Apa, the communications manager for county health department.

By law, students are required to either be immunized to attend school or have a waiver on file signed by a health care professional that verifies the child’s parent or guardian was informed about the benefits and risks of immunization.

Vashon School District currently has 75 students who don’t have either a record of their immunizations or a signed waiver on file, Soltman said. Next week, he said, the district will issue letters to the parents or guardians of those students, telling them that they have 30 days to submit documentation of immunization or a waiver to the district. Those students whose families don’t comply, he said, won’t be allowed to attend public school.

“In light of the concerns from the county health department,” Soltman said, “we need to get Vashon’s schools in compliance with the state’s immunization laws.”