East Lyme - What seemed like an ordinary cold, with coughing, sneezing and a runny nose, turned out to be a potentially dangerous and highly contagious illness for four students in town schools this month who were diagnosed with pertussis, commonly called whooping cough.

"It's not something to take lightly," said Dr. Joseph Gadbaw, chief of infectious diseases and chairman of the Department of Medicine at Lawrence + Memorial Hospital in New London. "It's usually not a big problem for adults, but it can be for pediatric-aged patients, especially babies."

The illness had all but disappeared in this country for decades after a vaccine was developed in the 1940s, but it has resurged in the last five years, with large outbreaks nationally in 2010, 2011 and 2012, according to the Centers for Disease Control and Prevention. Nationally, cases peaked at a 58-year high in 2012, when there were 50,000 cases, including 20 deaths. In Connecticut that year, there were 156 cases.

"The vaccine seems to be wearing off after three to five years," said Amy Pisani of Waterford, executive director of a national nonprofit vaccination advocacy group, Every Child By Two, that's based in Mystic. Less potent vaccine, along with parents not having their children vaccinated or failing to get needed booster shots, she said, is largely responsible for the resurgence of the illness.

"It really is a deadly disease for infants," she said. "It's a horrible, horrible death. The outbreaks need to be taken seriously." Babies and children with the illness can develop pneumonia, encephalitis and pulmonary complications.

In adults, the illness can last three to four weeks, but usually isn't serious, Gadbaw said. The real danger, he emphasized, is adults passing it to children, especially infants.

Because the infection can spread rapidly, Pisani and other public health advocates are urging all pregnant women to be vaccinated in the third trimester, which will protect newborns until they are old enough to be vaccinated.

In addition, grandparents, siblings and others who have contact with infants should make sure they're up to date with booster shots, which are recommended every 10 years. Her group currently has a campaign underway to increase pertussis vaccination rates among pregnant women, only 30 percent of whom are currently getting the shot.

"This is our number one priority right now," Pisani said. "We're trying to get the word out to OB-GYNs and pregnant moms."

Last week, East Lyme schools Superintendent James Lombardo sent a letter to parents of district students, informing them that three high school students and a fourth at Flanders Elementary School had contracted the disease. Lombardo said town schools are "aggressive" about making sure its students are up-to-date on vaccinations against pertussis and other contagious diseases, but used the letter as a chance to remind parents to make sure their children have all the required shots and boosters.

State law requires elementary school-aged children to have four doses of the whooping cough vaccine, and a booster for students entering seventh grade.

"We sent the letters out the same day we got the diagnosis," he said.

While he didn't have information about the status of the four students' illnesses, the school's medical advisor said that once the children begin taking antibiotics, they are no longer contagious and can return to school, according to Lombardo. Typically, the illness starts with upper respiratory symptoms, but after two weeks, the cough worsens. Children whose lungs are still growing often develop the "whooping" sound as they try to catch their breath.

Lombardo said the four students were all up-to-date with immunization shots against pertussis, which had been administered in the DTaP vaccine that protects against diphtheria, tetanus and pertussis.

"It's unusual but not improbable," to get the disease even if vaccinated, Lombardo said.

Pisani said the four cases in East Lyme illustrate the concern that the vaccine's protection is wearing off after a few years, meaning booster shots are more important than ever.

Also, she added, "vaccines are not 100 percent effective, so a child who got vaccinated who is surrounded by the disease lacks what we call herd immunity and remains vulnerable," she said.

Gadbaw at L+M said that in addition to getting the vaccine and boosters, anyone who's had close contact with people sickened by whooping cough should consider asking their doctors to prescribe antibiotics as a preventive measure. That's especially true for anyone around infants, he added.

"We try to put our arms around anyone who's been exposed," he said.

The state Department of Public Health also is urging pregnant women to get the vaccine, spokesman William Gerrish said. While clusters of whooping cough cases are "not uncommon" in the state, Connecticut is on track to have its lowest number of cases in a decade. So far in 2013, just 39 cases have been reported, he said.

j.benson@theday.com

Pertussis cases

U.S. in 2012: 48,277

Connecticut in 2012: 156

Connecticut, January to October 2013: 39