• All studies like lung function tests and chest x-rays (save those described below) come back normal.
• Afflicts ALL ages
In fact, most teens and adults lack the classic symptoms for whooping cough and their only symptom may be a bad lingering cough. In fact, some estimate that 10% to 30% of prolonged coughs in adults are caused by pertussis.
Even if a patient has been vaccinated for whooping cough, you can STILL get it mainly because it is not 100% effective. Furthermore, vaccine protection fades with time. Given incomplete ineffectiveness in combination with more and more parents electing to not vaccinate their kids, it is no surprise there are more cases being diagnosed every year. It's just that physicians may not recognize the cough for what it is.
Here's what the CDC has to say about whooping cough vaccines:
Vaccine protection for pertussis, tetanus, and diphtheria can fade with time. Before 2005, the only booster vaccine available contained tetanus and diphtheria (called Td), and was recommended for adolescents and adults every 10 years. Today there are boosters for adolescents and adults that contain tetanus, diphtheria, and pertussis (called Tdap). Pre-teens going to the doctor for their regular check-up at age 11 or 12 years should get a dose of Tdap. Adults who didn’t get Tdap as a pre-teen or teen should get one dose of Tdap instead of the Td booster. Most pregnant women who were not previously vaccinated with Tdap should get one dose of Tdap postpartum before leaving the hospital or birthing center. Getting vaccinated with Tdap is especially important for families with and caregivers of new infants.
The easiest thing for adults to do is to get Tdap instead of their next regular tetanus booster—that Td shot that they were supposed to get every 10 years. The dose of Tdap can be given earlier than the 10-year mark, so it’s a good idea for adults to talk to a health care provider about what’s best for their specific situation. [link]
Does this coughing adolescent or adult patient have pertussis? JAMA. 2010 Aug 25;304(8):890-6. doi: 10.1001/jama.2010.1181.
No big whoop: Adult pertussis may not produce the whooping cough. Harvard Health Blog 9/13/10