An analysis of Washington state’s 2012 pertussis epidemic, the worst since 1942, found that the vaccine to prevent the disease waned sharply and quickly in teens who were fully inoculated.

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The last time Washington state had an outbreak of whooping cough, it was 2012 and nearly 5,000 people — mostly babies and young teens — got sick.

Now, a new analysis of that epidemic finds the vaccine used to prevent pertussis waned quickly and sharply in adolescents, likely contributing to a surge of cases among those who already had their shots.

Effectiveness of the Tdap vaccine — tetanus, diphtheria and acellular pertussis — was only about 64 percent overall, and it dropped to about 34 percent within two to four years after it was given, according to a study led by Dr. Anna Acosta, an epidemiologist with the Centers for Disease Control and Prevention (CDC).

Who needs a vaccine?

CDC recommendations call for children to get five doses of the childhood DtaP vaccine — Diphtheria, Tetanus, acellular Pertussis — at 2 months, 4 months, 6 months, 15-18 months and 4 to 6 years. Kids ages 11 to 18 should receive a single dose of Tdap vaccine — Tetanus, diphtheria, acellular pertussis. One dose of Tdap is also recommended for adults aged 19 and older who didn’t get Tdap as adolescents. Tdap also should be given to kids aged 7 to 10 who are not fully protected against the disease, the CDC says. Pregnant women should get a pertussis vaccination in the third trimester of every pregnancy.

Centers for Disease Control and Prevention

That helps explain why even kids who received all the CDC-recommended doses by age 11 were part of a spike in cases during the epidemic, the worst in Washington since 1942.

“Basically, there was this huge uptick,” said Acosta, who is with the CDC’s National Center for Immunization and Respiratory Diseases. “We saw a rise in cases in 13- and 14-year-olds. This outbreak really let us take a look at why that might be happening.”

The study, published Monday in the journal Pediatrics, comes just as Washington state is seeing another uptick in pertussis. As of April 25, nearly 400 cases had been reported statewide, compared with nearly 90 at the same time last year, health officials reported.

Despite the waning protection, Acosta and other health officials said pertussis immunization remains important. It reduces the severity of illness when people do get sick and, when given to pregnant women, can induce immunity in newborns who are most likely to develop serious complications.

“We immunize against pertussis to protect babies,” said Dr. Ed Marcuse, a professor emeritus of pediatrics and a vaccine expert at the University of Washington. “Babies under three months of age die from this disease.”

The new study is one of the largest case-controlled studies to date to examine the effectiveness of the Tdap vaccine, Acosta said. It required 60 researchers working frantically for four weeks during the epidemic to track down records for all pertussis cases reported from Jan. 1, 2012, to June 30, 2012, in the seven Washington state counties with the most cases.

“It was a huge collaborative effort,” Acosta said. “We were very fortunate. The Washington state Department of Health is a great partner.”

Researchers found that in 450 adolescents who got pertussis during the outbreak, compared with 1,246 kids who didn’t get sick, the overall vaccine effectiveness was about 64 percent. It was 73 percent effective in the first year — but declined to 34 percent effective in years two through four.

The study confirms what others suggested, that a switch from whole-cell pertussis vaccine to acellular types in 1997 took a toll on the vaccine efficacy. The change was made because there was an “unacceptably high” level of reactions to the whole-cell shots, including febrile seizures, Marcuse said.

“We do have a safer vaccine, and it’s a vaccine that works, but it doesn’t work for very long,” he said.

Vaccine critics like Michael Belkin of Bainbridge Island question the wisdom of giving people shots with limited efficacy.

“The push for more and more doses of the failed pertussis vaccine brings to mind the quote attributed to Albert Einstein: The definition of insanity is “doing the same thing over and over again and expecting different results.”

But Marcuse emphasized that some coverage is far better than none.

“While we’re seeing more cases now than when we had whole cell vaccine, we’re seeing far fewer cases than when we didn’t have a vaccine,” he said.

Until a better vaccine is developed, families should follow the CDC guidelines, with an emphasis on pertussis vaccination in the third trimester of pregnancy every time a woman is pregnant, Marcuse added.

Only about 14 percent of pregnant women get the pertussis vaccine, according to a 2014 study in Michigan, a CDC report found. Washington rates are likely similar, Marcuse said.

Pertussis is a highly contagious disease that causes severe coughing that can make it difficult to breathe. It’s often characterized by a distinctive “whooping” sound made by children trying to get air, leading to the nickname “whooping cough.”

Health providers don’t expect this year’s outbreak to rival the last epidemic, said Chas DeBolt, senior epidemiologist for vaccine preventable diseases with the department of health. But they’re gearing up just the same.

“We certainly are watching it very carefully,” she said. “We’re trying to be prepared in case of repeat of 2012.