Washington declared a statewide pertussis epidemic in April 2012. That is where this post started, and the declaration is history now. The reason to keep writing about whooping cough is that the thing which caused it has not gone away, and the pattern repeats every few years somewhere.

Pertussis is one of the few vaccine-preventable diseases that still circulates freely in a highly vaccinated country. Understanding why explains most of what follows.

Immunity wears off, and that is the whole story

Protection from the acellular pertussis vaccine fades over several years. So does protection from having had the disease. Neither one is for life.

What that produces is a population of adolescents and adults whose immunity has quietly lapsed. They catch pertussis, they do not get the dramatic illness a baby gets, and most of them are never diagnosed. They have a cough that will not quit for a month and they carry on going to work.

Then one of them holds a newborn.

That is it. That is the mechanism.

That chain is the reason infant pertussis happens, and it is why the vaccine strategy is aimed as much at the adults around a baby as at the baby.

What it actually looks like, by age

The illness runs in three phases. It opens looking like an ordinary cold, runny nose and a mild cough, for one to two weeks. Then the paroxysmal phase, where the cough comes in violent fits that stack on top of each other without a breath in between. Then a long convalescent tail.

The traditional name for it is the hundred-day cough. That is not poetic license.

In infants under six months, the classic picture often does not appear at all. The whoop requires enough air movement to produce it, and small babies may not manage that. What you see instead can be apnea, pauses in breathing, or colour change, or simply a baby who has stopped feeding well. Pertussis in that age group is the version that puts children in hospital, and it is the version that kills them.

In older children the whoop is more typical. Often vomiting straight after a fit.

In adolescents and adults it is usually just a cough. Prolonged, worse at night, no whoop, no fever to speak of. If someone has been coughing for three weeks with paroxysms and post-tussive vomiting and no other explanation, pertussis belongs on the list whether or not they had the vaccine.

Testing has a window

PCR from a nasopharyngeal swab is the practical test, and it works best within the first three to four weeks of the cough, because after that the organism load in the nasopharynx falls away and the test begins returning negatives in people who genuinely have the disease and are still coughing hard enough to crack a rib.

This matters because the people most likely to be tested, adults with a lingering cough, are usually the people who present late. A negative PCR at week six does not clear the diagnosis.

The uncomfortable truth about treatment

Azithromycin, or another macrolide, is the treatment. What it does depends entirely on when you start it. Timing is everything here.

Given early, in the catarrhal phase, it can shorten the illness. Given once the coughing fits are established, it does very little for the patient in front of you. The cough is being driven by damage already done to the airway lining, and an antibiotic cannot undo that.

So the honest framing is that we mostly treat pertussis to stop the person giving it to someone else. That is not a reason to skip it. Interrupting transmission in a household with an infant in it is worth a great deal. But I would rather tell someone that plainly than let them believe the antibiotic is going to fix their cough this week, because it will not, and they will lose faith in the next thing I tell them.

Close contacts get prophylaxis on the same logic, and the threshold drops sharply when there is a baby, a pregnant woman in her third trimester, or anyone immunocompromised in the house. Practising by video, the household question is one I have to ask rather than observe, and it changes what I do more than almost anything else in the history.

The vaccine schedule, as it stands now

These are the current recommendations from the AAFP and AAP schedule (1). They have shifted since this post was first written, and the catch-up rules in particular are more specific than they used to be.

Children get DTaP as a five-dose series: three doses at two, four and six months, then boosters at fifteen to eighteen months and again at four to six years.

Adolescents get a single Tdap booster at eleven to twelve years.

Anyone aged seven to eighteen who is not fully vaccinated with DTaP gets one dose of Tdap as part of catch-up. There is a wrinkle worth knowing: a child who receives that dose between seven and nine still needs the routine adolescent booster at eleven to twelve, while a child who receives it at ten does not.

And the one that protects newborns: Tdap in every pregnancy, preferably between twenty-seven and thirty-six weeks. Timing inside that window is not arbitrary. It balances how long the mother takes to mount an antibody response against how efficiently those antibodies cross the placenta, and that balance is still being refined in the literature (2).

A newborn cannot be vaccinated. Maternal antibody is the only protection a baby has for the first two months of life, and it is the single highest-value thing on this page.

For patients

A cough that has run past three weeks in fits, especially with vomiting afterward, is worth getting checked, and worth mentioning pertussis by name.

If you are pregnant, get Tdap in the third trimester of each pregnancy, including pregnancies close together. If you are going to be around a newborn and cannot remember your last tetanus-containing shot, that is a reason to ask.

If someone in your household is diagnosed, expect to be offered antibiotics even if you feel fine.

For colleagues

Test within the window or accept that a negative PCR does not settle it.

Say out loud that macrolides are mostly about transmission once the paroxysmal phase is underway. Patients handle that honestly framed, and it protects your credibility when the cough is still there in a month.

Ask about the household before you decide how aggressive to be with prophylaxis. The answer changes the calculation entirely.

The Bottom Line

Whooping cough persists in a vaccinated population because immunity fades, in adolescents and adults who then look like they have an ordinary lingering cough. They are the reservoir, and infants are who pays. Test early, because PCR has a window. Treat knowing the antibiotic is mostly stopping spread rather than fixing the cough. And of everything here, the intervention that saves the most infant lives is a Tdap shot given to a pregnant woman between twenty-seven and thirty-six weeks.

Related Reading

Is Pertussis Contagious? Symptoms and Treatment Explained Flu Prevention: Vaccines, Symptoms, and Treatment Options Why Won’t Antibiotics Cure a Viral Infection? Doctor Explains Cold vs Flu vs COVID vs RSV: How to Tell Them Apart

Scott Rennie, D.O.
Board Certified in Obesity Medicine and Family Medicine

This blog is for educational purposes only and does not constitute individual medical advice. Always consult your own physician before making changes to your health, medications, or treatment plan.

Sources

1. American Academy of Family Physicians. Childhood and adolescent immunization schedules. https://www.aafp.org/clinical-insights/immunizations-and-vaccines/immunizations-schedules-resources/childhood-vaccine-schedules

2. De Weerdt L, et al. Timing of maternal Tdap vaccination: A dynamic balance between antibody induction and placental transfer efficiency. Vaccine. 2026. PMID 42472495. https://pubmed.ncbi.nlm.nih.gov/42472495/