Health officials have confirmed another case of whooping cough among Montgomery County students, 16 of whom have become sick since an outbreak in the first week of school.

The newest case was confirmed Wednesday at Gaithersburg High School, the eighth county public school in less than three weeks with a whooping cough diagnosis. A letter was sent to parents notifying them of the highly contagious illness, officials said.

Early symptoms of whooping cough, or pertussis, often include runny noses and coughs, and typically surface seven to 10 days after exposure, but they might not show up for as long as three weeks. Coughing can grow more severe over time and include a whoop sound.

The disease is most dangerous to infants and those with compromised immune systems. Most young children get a series of immunizations that include a pertussis vaccination, and a booster is recommended at age 11 or 12.

All 16 of the students who have fallen ill in Montgomery were vaccinated, according to health officials. Experts say the protective effects of immunizations wane over time.

The whooping cough outbreak in Montgomery has come as boosters are being required for the first time in Maryland for all seventh-graders.

Montgomery health officials said that by noon Thursday, 1,491 seventh-graders in public schools had not shown documentation of a booster, about 13.4 percent of the more than 11,000 enrolled. Immunization efforts are ongoing.

After Friday, students who have not had a booster could be excluded from school, officials said. “We want to continue working with families to make sure they get the immunizations that are required by the state,” said Montgomery schools spokesman Dana Tofig.

Unlike most of the other recent Montgomery cases, health officials said the Gaithersburg student had no link to a summer session at Capital Camps, near Waynesboro, Pa., about 60 miles north of Rockville.

Most of the students recently diagnosed in the county had spent time at the camp, which is popular in the Washington area. All of those who have fallen ill are ages 9 to 18 and include students at public and private schools. Amid the outbreak, the only other case unrelated to the camp was at Sherwood High School, health officials said.

In addition to Sherwood and Gaithersburg, the public schools with identified whooping cough cases in recent weeks are Walter Johnson and Thomas S. Wootton high schools; Robert Frost, Julius West and Cabin John middle schools; and Cold Spring Elementary School.

Frances Frost, president of the Montgomery County Council of PTAs, said because many parents are not very familiar with whooping cough, the distribution of information is key.

“I think it’s important just keeping parents informed, knowing what the symptoms are, knowing what the concerns are and what you should do about it,” she said.

With the new Gaithersburg case, Montgomery has had a total of 36 this year. The county had 40 cases all of last year. In 2012 — when numbers of whooping cough cases peaked nationally — Montgomery had 78 cases. The county saw 18 cases of pertussis in 2011 and nine cases in 2010.