A child visits a pediatrician, who fails to diagnose that the young patient has pertussis, also known as whooping cough. The child infects the doctor, who then passes the respiratory tract infection on to others.

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That's what happened in September in the Allentown-Lehigh area of the state. As a result, 20 cases of pertussis were confirmed and the Pennsylvania Department of Health issued an advisory to doctors, alerting them to be on the lookout for the disease.

That's more easily said than done. Whooping cough is hard to diagnose, and most adults aren't aware that they're even susceptible -- immunity from the childhood vaccine wanes after 10 years. Primary care providers don't tend to look for it, and even if it's suspected, it's hard to test for.

And while pertussis is usually more of a nuisance in adults and adolescents than anything else -- manifesting itself as a cough that hangs on and on -- they can pass the bug on to unvaccinated infants, who can die from the disease.

That danger may become less of a threat in the near future. Drug maker Aventis Pasteur has developed a vaccine for adults and adolescents to address the waning immunity problem. It expects to submit it to the Food and Drug Administration for approval "very very soon," said Dr. Michael Decker, the company's vice president for scientific and medical affairs.

Once that vaccine is approved, a study that began this past spring at Children's Hospital in Pittsburgh and other sites in the United States and Canada should help provide some guidance on how to use it.

Led by Dr. David Greenberg, director of the Center for Vaccine Research at Children's, the study will determine how infants six months and younger who have been diagnosed with pertussis were infected. Those children are still getting their first course of the DTaP (diphtheria, tetanus and acellular pertussis) vaccine and are still susceptible to infection.

One example of a use for the new vaccine, Greenberg said, might be an ob-gyn advising a woman due to give birth within a couple months to have family members vaccinated to reduce any chance of transmitting pertussis to the newborn.

"The ultimate goal is to reduce pertussis in infants because they are the ones who suffer the most from it," he said.

The Children's study may be responsible for a slight uptick this year in reported pertussis cases in Allegheny County, an increase that's mirrored at the state and national levels.

In the county, 34 cases through Nov. 4 have been reported, up from 26 in all of 2002, said Health Department spokesman Guillermo Cole. Twenty of this year's cases were in children 5 or younger. However, those figures are down from the 50 or so reported each year from 1998 through 2001.

In Pennsylvania, there have been 120 cases with no deaths in 2003, compared with 117 cases with one death in 2002, according to the state Health Department. Nationally, the Centers for Disease Control and Prevention reported 9,771 in 2002, compared with 7,580 in 2001. Twenty-two U.S. children died of the disease in 2002.

The increase in cases may be part of a natural upswing in the disease that occurs every three or four years, although public health officials don't know why.

"Maybe it has to do with the building up of a number of susceptibles ... but I don't think anyone knows for sure," said Dr. Margaret Cortese of the epidemiology and surveillance division of the National Immunization Program, part of the CDC. Numbers for 2003 aren't available, she said, but this year there have been somewhat fewer cases reported than at the same time last year.

In fact, said Decker, cases have been on the rise for the past 25 years and the CDC reports that since 1990, there has been a disproportionate increase of the disease in adults and adolescents.

Although the new vaccine would go a long way toward providing better protection for infants, Greenberg suggested that the simple fact of approval won't mean people will automatically start getting it. Most adults, for example, couldn't tell you when they last had a tetanus booster, which is recommended every 10 years.

How would it be recommended for use in the United States?

"At a minimum, it is expected that doctors could use the new vaccine whenever they otherwise would have used the old vaccine, Decker said. Doctors also might be actively encouraged to use the new vaccine in adolescents, as well as in any persons having continued, close contact with young infants, such as parents, grandparents, siblings and day care workers

"In doing this, you are putting a cocoon of protection around the baby so the bug can't get at the baby,"

Getting adults and adolescents on a course of vaccination would be a last step in defeating pertussis permanently, said Decker, who has studied the disease for more than 20 years.

"Before the vaccine, adults didn't get pertussis because every kid got it and adults were constantly having enough exposure to maintain their immunity.''

The vaccine has saved a lot of babies, but it's left older persons susceptible because they're no longer being constantly exposed.

In the meantime, the best protection for young children, Cortese said, is for parents to strictly follow the vaccine schedule at two, four and six months of age.

"If parents can get their children immunized right on time, there's a good chance they would only get a mild case."