How Common Is Whooping Cough in Infants?

Whooping cough, caused by the bacterium Bordetella pertussis, strikes infants harder and more often than any other age group. A large U.S. study tracking nearly 1.2 million infants found an overall incidence of about 118 cases per 100,000 person-years, with two- and three-month-olds bearing the highest rates, roughly double the average for infants overall. Those numbers, though, almost certainly understate reality: multiple studies using capture-recapture methods estimate that a third or more of infant cases never make it into official surveillance counts. The gap between reported figures and true burden shapes everything from how parents perceive the risk to how public health agencies set policy.

How Rates Change Month by Month

Not all months of infancy carry equal risk. In a study of over a million U.S. infants born between 2005 and 2010, pertussis incidence peaked sharply at two and three months of age, reaching roughly 235 and 248 cases per 100,000 person-years, respectively. From the fourth month onward, rates fell steadily through the first birthday.¹ The pattern makes biological sense: newborns arrive with whatever antibodies they received across the placenta, but those maternal antibodies wane quickly. The first dose of pertussis vaccine typically happens at two months, and meaningful protection does not build until after two or three doses. That leaves a window of several weeks during which infants have little defense of their own.

Globally, infants under one year consistently show the highest pertussis incidence of any age group. A 2021 analysis of the Global Burden of Disease data confirmed that children under one had the highest incidence, and girls had slightly higher rates than boys across all pediatric age groups.² A separate estimate for 2014 put the worldwide toll at about 24.1 million pertussis cases and 160,700 deaths in children under five, with Africa contributing roughly a third of cases and well over half the deaths.³ In low-income regions, limited vaccine access and weaker surveillance combine to keep both incidence and mortality far above what wealthier countries experience.⁴

Official Numbers Miss a Lot of Cases

One reason parents sometimes underestimate the risk is that surveillance figures consistently undercount infant pertussis. A German capture-recapture study estimated that about 39% of pertussis hospitalizations in infants went unreported to the national notification system.⁵ In Ontario, Canada, a similar analysis found that only about 37% of infant pertussis cases that turned up in hospital or emergency-department records had been reported to public health authorities.⁶

Diagnosis itself is part of the problem. In very young infants, pertussis often looks nothing like the classic “whoop.” Babies may present with apnea, slowed heart rate, or poor feeding rather than the dramatic cough adults associate with the disease.⁷ A systematic review of South and Southeast Asian countries concluded that diagnosis is challenging precisely because the symptoms are so nonspecific, leading to substantial underrecognition of neonatal pertussis across the region.⁸

How Infants Get Infected

Pertussis is extremely contagious, with household transmission rates estimated at around 80%.⁷ For babies too young to go to daycare, the household is where most infections come from. A review pooling data across multiple studies found that about 39% of identified sources were mothers, roughly 16% were fathers, and around 5% were grandparents. Estimates for siblings ranged from 16 to 43%, depending on the study, while non-household contacts accounted for 4 to 22%.⁹ For roughly a third to half of infant cases, no source was ever identified, which points to silent infections in the community that nobody thinks to test for.

An Australian study highlighted an often-overlooked source: vaccinated older siblings. During the 2011–2012 epidemic peak, siblings accounted for over half of identified pertussis sources for infants six months and under, up from about 14% in the preceding period. Most of those sibling sources were fully vaccinated two- and three-year-olds whose immunity had begun to wane.¹⁰ The finding underscores that being “up to date” on shots does not make a child permanently incapable of carrying and passing on the bacteria.

Why Pertussis Hits Infants So Hard

In older children and adults, whooping cough is deeply unpleasant but rarely dangerous. In infants, particularly those under three months, it can be life-threatening. One reason is the size of their airways: even moderate swelling or mucus buildup can cause serious breathing problems. Another is that infants are prone to apnea during pertussis infection. Up to 67% of hospitalized infants with pertussis experience apnea, a rate far higher than in older patients.¹¹

The most dangerous complication involves a chain reaction that starts with the white blood cell count. Pertussis toxin triggers a surge in white blood cells, and in some cases the count climbs to extreme levels. When those cells pack into the small blood vessels of the lungs, they can cause pulmonary hypertension, where the pressure in the lung’s blood vessels rises dangerously.¹² In a study of 38 infants admitted to intensive care with severe pertussis, 13 died, and the two strongest predictors of death were extremely high white blood cell counts and pulmonary hypertension. Babies under six weeks accounted for over three-quarters of the deaths.¹³ In a separate review of 197 hospitalized children in China, every patient admitted to the pediatric intensive care unit was an infant who was either unvaccinated or incompletely vaccinated, and premature infants or those younger than three months were the most likely to need intensive care.¹⁴

When pulmonary hypertension becomes severe, some infants require extracorporeal membrane oxygenation (ECMO), essentially an external machine that takes over the work of the heart and lungs. One reported case involved a five-week-old who needed prolonged ECMO support alongside inhaled nitric oxide therapy to bring pulmonary pressures down. She survived after three months in the hospital.¹⁵ Cases like these are rare, but they illustrate how rapidly a seemingly common respiratory infection can become a medical crisis in young infants.

Maternal Vaccination and the Timing Problem

Because the highest-risk window falls before infants can build their own vaccine immunity, the main strategy for protecting newborns is vaccinating pregnant women. When a mother receives a Tdap booster during pregnancy, her body produces antibodies that cross the placenta and give the newborn a temporary shield. A U.S. case-control study estimated that third-trimester Tdap vaccination was about 78% effective at preventing pertussis in infants under two months and roughly 91% effective against cases severe enough to require hospitalization.¹⁶

Uptake of maternal Tdap, however, remains uneven. An Italian study examining a pertussis outbreak in newborns found that missed maternal vaccination was a major contributing factor. The strongest predictors of not being vaccinated during pregnancy were misinformation about pertussis vaccination and fear of side effects.¹⁷ Broader reviews point to low trust in health authorities, practical access barriers, and disrupted routine services during the COVID-19 pandemic as additional reasons pregnant women skip the shot.¹⁸

Another strategy, sometimes called cocooning, involves vaccinating the adults and older children who surround a newborn rather than the infant directly. The idea is to create a ring of immunity. In theory it makes sense, but estimates suggest cocooning alone prevents only about 20% of infant pertussis cases.¹⁹ That modest figure reflects the reality that a large share of infant infections come from contacts who were never identified as close household members or who had no symptoms at all. Cocooning remains worthwhile as a complement to maternal vaccination, but it is not a reliable substitute.

Why Whooping Cough Keeps Coming Back

Pertussis incidence in many countries moves in cycles, and several forces drive those waves. The most discussed is waning immunity. After children complete their primary vaccine series, protection fades over the following years. One modeling study estimated that pertussis incidence in vaccinated children increased by roughly 43% for every year since the last dose.²⁰ The result is a growing pool of older children, teenagers, and adults who can catch and spread the bacterium, often with symptoms mild enough that nobody suspects pertussis.

The shift from whole-cell to acellular pertussis vaccines, which happened in many countries during the 1990s and 2000s, also plays a role. Acellular vaccines cause fewer side effects, which is why they replaced whole-cell formulations. But evidence from animal models suggests acellular vaccines are less effective at preventing people from carrying the bacteria in their nose and throat, even when they prevent clinical disease.²¹ Research on infants has also shown immunological differences: children who received acellular vaccines in infancy produced weaker cellular immune responses compared with those primed with whole-cell vaccine.²² In one Australian epidemiological study, children who received three doses of acellular vaccine in infancy had higher pertussis rates than those who received three doses of whole-cell vaccine.²³

On top of waning immunity, the bacterium itself is evolving. Most circulating strains in the United States have independently acquired mutations that disable pertactin, one of the key proteins targeted by acellular vaccines.²⁴ A similar pattern emerged in Spain, where about 38% of isolates collected after the switch to acellular vaccine lacked pertactin.²⁵ Whether these pertactin-deficient strains actually cause more disease is still debated, but the selective pressure from vaccines is clearly reshaping the bacterial population.

The Post-COVID Surge

During the pandemic, pertussis essentially vanished. Social distancing, mask mandates, and school closures dramatically reduced transmission of all respiratory pathogens, whooping cough included.²⁶ But the quiet period set the stage for a rebound. With fewer people encountering the bacterium, the background rate of natural immune boosting dropped. At the same time, some routine vaccination appointments were missed or delayed.

The result has been a dramatic resurgence since 2023 and 2024. Many countries have reported outbreaks that are larger than anything seen in recent pre-pandemic years, particularly in adolescents whose vaccine-derived immunity waned during the years of low circulation.²⁷ A mathematical modeling study of the 2024 surge in England concluded that reduced social mixing during the pandemic, combined with the relatively short duration of acellular vaccine protection, was enough to explain the size of the outbreak. The model also found that infants of unvaccinated mothers were among the groups hit hardest.²⁸

Getting the Diagnosis Right

Testing matters for infants because early antibiotic treatment can reduce the severity of illness and, critically, the likelihood of spreading the infection. The standard diagnostic tool has long been bacterial culture, but culture is slow and misses a lot of cases, with sensitivity as low as about 15% in one head-to-head comparison.²⁹ PCR-based testing is much more sensitive. In a large evaluation, real-time PCR showed sensitivity and specificity above 92%.³⁰ Timing still matters, though: the longer between symptom onset and specimen collection, the lower the sensitivity of any test.³¹

For families with a confirmed case, post-exposure antibiotic prophylaxis with a macrolide antibiotic like azithromycin is recommended for household contacts, especially if there is another infant or a pregnant woman in the home. A cost-effectiveness analysis found azithromycin prophylaxis to be the most efficient option for preventing secondary household infections.³²

When Pertussis Arrives Alongside Other Viruses

Because pertussis season overlaps with common respiratory virus season, co-infections are surprisingly frequent in hospitalized infants. One study found that about 16% of infants hospitalized with RSV also tested positive for pertussis on PCR.³³ The clinical significance of the double hit is not entirely settled. One study reported that infants with both RSV and pertussis had higher rates of wheezing and readmission, along with longer hospital stays, compared with infants who had pertussis alone.³⁴ Another found that viral co-infections, including rhinovirus and coronavirus, were common but did not clearly worsen clinical outcomes.³⁵

The practical takeaway is that clinicians who test for common viruses and get a positive result should not necessarily stop looking. A baby diagnosed with RSV might also have pertussis, and the pertussis component is the one that benefits from antibiotic treatment. This is especially relevant during winter months, when both pathogens circulate heavily and coinfection is most likely to be missed.