Persistent cough: pertussis rises in young adults and infants.

Nearly a century after a pertussis vaccine became available, the disease appears to be rebounding in adolescents and adults, a variety of studies shows. This trend could explain the increase in mortality among infants, who are infected through contact with family members and other caretakers.

Rarely fatal after the age of 2 years, pertussis can cause vomiting and "a cough that most adolescents or adults say they will never forget," says Carl-Heinz Wirsing von Konig of the hospital Klinikum Krefeld in Krefeld, Germany. The coughing spasms can cause urinary incontinence or break ribs, and the sound and the duration of the illness account for its nicknames, whooping cough and hundred-days cough.

As part of a childhood-vaccination program, begun in the 1960s, that also prevents tetanus and diphtheria, pertussis immunization has cut the disease's incidence among children. But immunity wanes within several years, and booster shots have not been routine. In the late 1970s and early 1980s, pertussis cases in many industrialized nations began rising among people whose immunity had waned. A rise has been apparent, too, among infants under 2 months old, who are too young to receive the vaccine.

What's more, in recent years, U.S. infant mortality from pertussis has approximately doubled, to about 20 cases per year. That's probably because parents and people who have contact with newborns are accidentally spreading the bacteria.

To understand changing infection patterns, Italian researchers analyzed several years' worth of data on European pertussis eases in different age groups.

Alberto Tozzi of Bambino Gesu Hospital in Rome and his colleagues found that adults and adolescents over 14 years old were more than twice as likely to get infected in 2002 as people of the same age had been in 1998. The researchers also found that infants are at far higher risk of infection than are people of any other age.

The researchers reported their findings on Oct. 30 at the annual Interscience Conference on Antimicrobial Agents and Chemotherapy in Washington, D.C. There, other scientists discussed what to do about the resurgence of pertussis.

To combat the problem, vaccination practices need to change, says pediatrician Scott Halperin of Dalhousie University in Halifax, Nova Scotia. He recommends booster shots for all adolescents, especially because new pertussis vaccines are less likely than older ones to produce side effects such as fever and swelling. Canada, Australia, and many European countries recently instituted booster shot programs for students.

Researchers should also study whether vaccinating newborns or pregnant women could prevent infections in infancy, Halperin says. A potentially safer alternative for protecting infants, he says, is to revaccinate family members and other people likely to have contact with the babies.

Some researchers have been concerned that changes in the genetics of Bordetella pertussis, the most common pertussis-causing microbe, could underlie the disease's resurgence. Those bacteria have evolved since the 1960s, but they don't appear to have become more dangerous or more resistant to vaccines, says Nicole Guiso of the Institut Pasteur in Paris. Instead, she says, recent mutations in the bacteria may reflect their ongoing adaptation to living exclusively in people, which they appear to have done for fewer than 500 years.