Pertussis--whooping cough--was once primarily an illness of early childhood--half of the children who had it were infected before beginning school. But introduction of  vaccines for infants in the 1940s and the 1990s markedly reduced the number of early-childhood cases.

That was significant progress from the time in the 19th century when whooping cough was one of the leading causes of infant death, but public health officials are now worried about something else that seems to be happening to pertussis, says an article in the New England Journal of Medicine. 

Specifically, in the year 2006, almost a dozen outbreaks of whooping cough were reported in United States. And the age at which individuals contract whooping cough is advancing, with many new cases now reported in adolescents and young adults.

Taken together, those factors are sparking a new emphasis on re-immunization after infancy. For the year 2007, recommendations from the Advisory Committee on Immunization Practices call for a booster shot at age 5 or 6 to children who received the infant schedule of three vaccinations, and a combined tetanus/diphtheria/pertussis vaccine at ages 11 to 12 for children who had the infant immunizations but did not get the booster shot before entering school. The committee also recommends that adolescents 13 to 18 years old who were immunized as infants but missed the 11-12-years booster dose should receive a single dose of Tdap.

To understand why all of this is happening, says Dr. Scott Halperin, we need to understand that after vaccination, immunity to pertussis is of limited duration. For the  vaccine introduced in the 1940s, immunity is known to begin to diminish after three to five years, with no demonstrable protection by 10 to 12 years. How long the improved vaccine introduced in the 1990s will be effective is not yet established, but studies show that immunity to that vaccine also appears to decline after four to five years.

And that’s not all. Recent epidemiologic studies show that the immunity people were believed to have after coming down with whooping cough in childhood is not lifelong and may not be much more reliable than the immunity achieved by vaccination.

Adding to the pertussis puzzle, Dr. Halperin points out, is the fact that a laboratory test long considered the gold standard for identifying pertussis involves a lengthy process of culturing the causative agent, the bacterium Bordetalla pertussis. This can take time, and the results may not be available soon enough to allow hospitals, communities, or schools to know that they have a whooping cough epidemic and to take preventive steps. And because the culture test has relatively low sensitivity to the infection in older children, adolescents, and adults, it may miss cases.

On the other hand, questions are arising as to whether newer, more sensitive tests such as serologic analysis are producing "false positives" that may actually be leading to overestimates of pertussis incidence.  One of the largest 2006 outbreaks, for example, happened at Dartmouth-Hitchcock Medical Center in Lebanon, New Hampshire, where more than 4,500 hospital employees were given doses of acellular vaccine in response to laboratory-reported cases of pertussis. When samples were submitted to the more lengthy culture analysis, however, not a single case of whooping cough was found.

That leaves public health and other medical offices in a quandary—whether to go with the quicker, more sensitive molecular tests or rely on the long-standing culture option. The former may give incorrect impressions that a petussis epidemic is under way, but the older test can cause delays in corrective or preventive action.

But we shouldn’t dismiss the return of pertussis as just the effect of increased laboratory testing, Dr. Halperin cautions. "The 'marching cohort' of infected preadolescents and adolescents indicates that the shift in epidemiology is not just a consequence of changing patterns of laboratory testing."

While those questions are being sorted out, Dr. Halperin suggests, there is one thing we do know. "In view of the clear burden of pertussis among older age groups, more attention should be paid to following the recommendations of the Advisory Committee on Immunization Practices that all adolescents and adults be given a dose of an acellular pertussis vaccine combined with diphtheria and tetanus toxoid (Tdap)."

"Given the demonstrated safety and efficacy of such a vaccine in a clinical trial involving adolescents and adults and early evidence of the effectiveness of adolescent-vaccination programs, broader use of the vaccine may prevent pertussis disease and circumvent the difficulties of establishing its diagnosis."

The article, "The Control of Pertussis—2007 and Beyond," was published in the January 11, 2007, issue of the New England Journal of Medicine.

The 2007 Recommended Immunization Schedules for 0-6 and 7-18 Years are available online at http://www.cispimmunize.org or http://www.cdc.gov/mmwr/PDF/wk/mm5551.pdf

See also: "FDA Approves Pertussis Booster for Adolescents,"  http://www.healthinschools.org/2005/may5_alert.asp