Breaking news from the 51st ICAAC meeting in Chicago: Physicians from the Kaiser Permanente Medical Center in San Rafael, California say they have found an unexpected amount of pertussis — whooping cough — in pre-teen children who had received their mandatory school-entry vaccination booster at age 5 and should have been protected from the disease until about the time of their final childhood booster dose at age 12.

The physicians said case records suggest the vaccine may have protected children for less time than it was expected to. They speculate that the cases they recorded — along with additional cases that are likely to have occurred outside Kaiser’s  patient population — helped amplify California’s 9,000-person pertussis epidemic in 2010.

“The vast majority of these were fully-vaccinated children,” Dr. David Witt, the medical center’s chief of infectious diseases, said in a press briefing during the meeting. “That was the surprise.”

Here are details of their report: The researchers reviewed the records of 171 patients who were initially treated for a prolonged coughing illness and then were as diagnosed as having pertussis by PCR. Of the 171, 132 were between 8 and 14 years old. The incidence of pertussis in that age group (see their chart to the right) was “almost 20-fold that of the preschool age group” and “the rate plummeted in 13-year-olds,” Witt said.

The schedule of pertussis vaccination and boosters for US children goes like this: 2 months, 4 months, 6 months; between 15 and 18 months; at 5 years, and again at 12 years. (A booster is recommended for adults, especially ones in professions that bring them in close contact with children, but is not legally required.) Witt and his fellow researchers, who include his son and research assistant Max Witt and Kaiser senior pediatrician Dr. Paul Katz, noted that the spike in cases fell in the longest gap between boosters, and began at almost exactly 3 years after the age-5 dose.

“We found very low rate of clinical pertussis up to age 8, and at that point it skyrocketed, with the  attack rate peaking in the 10-12-year-olds,” Dr. Witt said.

In their ICAAC paper, the researchers concluded: “Acellular pertussis boosters are effective but less durable than previously thought. Vaccine guidelines and pertussis control measures need to be reconsidered.”

The greatest concern, Witt said, is that the children who were presumed to be protected, but apparently were not, could have served as a transmitter of infection to others in their families and neighborhoods who could not be vaccinated and thus were vulnerable to whooping cough infection. Grandparents with waning immune systems and infants with undeveloped ones would be particularly at risk, he said; children younger than 2 months cannot be vaccinated because their immune systems are immature.

Eleven infants died of whooping cough in California’s epidemic, which peaked in 2010 but has not yet ended. “Anything else that killed 11 infants would be a national crisis,” he said.

The further problem, Witt said, is that the lapse of immunity in the vaccinated children punched one more hole in the general “herd immunity” of the local population, which was already undermined by families who refused to vaccinate their children as well as by families who did not strongly oppose vaccines but who vaccinated their children late.

“In California we have ‘personal belief exemptions’ [from mandatory vaccination] of 3 percent to 10 percent per county,” he said. “If you have 10 percent unvaccinated, and the vaccine efficacy is 85 percent, then you’re talking about only 75 percent” of the population at best who are protected, he said.

The Kaiser team has warned the California health department of their findings, and the Centers for Disease Control and Prevention has been alerted, Witt said. When asked if vaccine schedules need to be reworked to protect children in the 8-12-year-old group, for instance by adding or moving up a booster, he replied that it would be important first to find whether the same results have been observed elsewhere. In the meantime, he added, health officials might consider administering boosters to pre-teens during outbreaks, in order to repair any gaps in immunity such as Kaiser observed.

Coincidentally, thanks to the timing of the ICAAC meeting, the Kaiser group made their presentation just as children in California are returning to school — and in many cases, being sent home again. As Nancy Shute reported today on NPR, this fall marks the first time that California and other states have required the age-12 pertussis booster for school attendance, in large part because of the epidemic last year.

Here’s some video of Witt discussing the Kaiser research during ICAAC:

Cite: Witt, MA, Katz PH, Witt DJ. Marked Acellular Pertussis Vaccine Failure in 8-14 Year-olds in a North American Outbreak. 51st ICAAC Chicago, Sept. 20, 2011.

Update: Mike Stobbe of the AP covers the study here.

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