Pertussis in New South Wales surged in 2024 and moved into older school-age children
New South Wales faced a strikingly larger pertussis epidemic in 2024 than it did in 2015, and the age profile changed in a way that matters for public health planning and school-based transmission control. Notifications more than doubled, but the more consequential shift was demographic: the burden moved away from younger children and concentrated in older school-age groups, especially those 10 to 14 years old.
The pattern fits a familiar biologic problem. Protection against pertussis does not last indefinitely, whether it comes from vaccination or from prior infection. When circulation of a highly contagious respiratory pathogen drops for several years, the immune system loses the periodic reminders that normally help sustain population-level protection. In a setting where school-aged children sit several years beyond their last scheduled vaccine dose, the gap becomes wide enough for transmission to accelerate.
A larger outbreak with a narrower age band
New South Wales recorded 25,695 confirmed pertussis cases in 2024, compared with 12,007 in 2015. Across 2015–2024, a total of 68,842 confirmed cases were reported in the state. The 2024 notification rate reached 314.7 per 100,000 population, almost twice the 157.7 per 100,000 seen in 2015.
The epidemic curve was not simply larger; it was more concentrated. In 2015, the largest share of cases fell in children 5 to 9 years old, who accounted for 25.2% of notifications, followed by those 10 to 14 years old at 22.2%. In 2024, the center of gravity shifted upward. Children 10 to 14 years old made up 38.0% of notifications, while the 5 to 9 year group accounted for 23.2%.
One sentence captures the clinical consequence: pertussis in 2024 looked less like an infant-centered problem and more like a school-age outbreak.
The proportion of cases in children younger than 5 years fell from 15.1% in 2015 to 8.3% in 2024. Among infants younger than 6 months, the percentage declined from 1.4% to 0.9%. Those smaller shares do not suggest that infants were immune to risk; rather, they indicate that the epicenter of transmission had shifted upward, with older children likely serving as the main amplifying reservoir.
Waning immunity and disrupted boosting offer a plausible explanation
Vaccination schedule timing helps explain why 10- to 14-year-olds emerged as the dominant group. In New South Wales, pertussis vaccination is funded at 6 weeks, 4 months, 6 months, 18 months, and 4 years of age, with a booster at 12 to 13 years through the school vaccination program. By 2024, many children in the 8- to 12-year range were several years removed from their last routine dose, yet still below or approaching the adolescent booster window.
The notification rate in 10- to 14-year-olds rose from 594.2 per 100,000 population in 2015 to 1,904.4 per 100,000 in 2024. That increase produced an incidence rate ratio of 3.21, with a 95% confidence interval of 3.07 to 3.35. Rates did not rise to the same extent in the older age groups, especially those older than 25 years.
The biology is straightforward even if the public health consequences are messy. When Bordetella pertussis circulation nearly disappears for several years, natural immune boosting also disappears. Vaccine-derived protection continues to wane, but the recurring environmental exposure that can temporarily reinforce immunity is gone. School-age children then move through classrooms with increasingly fragile protection, and transmission can spread quickly once the bacterium re-enters circulation.
The age curve suggests school transmission did the heavy lifting
The median age at notification remained 11 years in both epidemics, but the dispersion changed. The interquartile range narrowed from 7 to 33 years in 2015 to 9 to 17 years in 2024, showing that cases were packed more tightly into a younger, school-age band during the later epidemic.
Age-specific rates climbed year by year among school-aged children in 2024, particularly in those 8 to 12 years old. That gradient is epidemiologically important because it tracks the period after early childhood vaccination but before full adolescent boosting. In practical terms, it identifies a vulnerable corridor in which a large number of children can still be susceptible enough to sustain spread.
The male-to-female notification ratio also shifted, from 0.88 in 2015 to 1.08 in 2024. The article does not attribute a mechanism to that change, and it should not be overinterpreted. The more robust signal lies in the age distribution and the large increase in notifications among children approaching adolescence.
Pandemic-era transmission suppression likely widened the immunity gap
New South Wales reported historically low pertussis activity from 2020 to 2023, with fewer than 600 notifications annually. COVID-19 mitigation measures likely reduced transmission of many respiratory pathogens during that period, and pertussis appears to have been no exception. The result was not merely fewer diagnoses; it was fewer opportunities for immune reinforcement in the community.
The 2024 resurgence therefore likely reflects two forces acting together: reduced circulation during the pandemic years and waning immunity in cohorts that had not been naturally re-exposed. That combination would be expected to affect school-age children first, because they sit at the intersection of diminishing vaccine protection and dense social contact. The state’s adolescent booster program likely helped blunt spread in older teens, which may explain the relative reduction in rates from age 13 onward.
International experience points in the same direction. Several countries reported increased pertussis incidence among older children after COVID-19 restrictions eased, suggesting that this was not an isolated regional phenomenon but part of a broader post-pandemic rebound in respiratory pathogen transmission.
Why the 2024 epidemic matters for prevention strategy
This outbreak pushes pertussis control back toward a basic but stubborn question: how much protection can routine childhood vaccination sustain once natural circulation has been interrupted for several years? The answer appears to be less than many systems assumed. Children 8 to 12 years old emerged as the group where waning immunity became most visible, and they are old enough to amplify classroom spread but young enough to be missed by infant-focused prevention messaging.
For clinicians and public health teams, the message is not abstract. A pertussis surge centered in older children can still spill into households, reach infants, and intensify risk for the youngest patients, who remain the most vulnerable to severe disease and death. The 2024 NSW epidemic showed that once the age structure shifts upward, surveillance and prevention need to follow the transmission pathway rather than the traditional image of whooping cough as an infant disease.
The analysis is limited to notification data, and increased laboratory confirmation during the pandemic recovery period could have influenced the total number of reported cases. Even so, the age shift is hard to dismiss. It points to a real change in susceptibility patterns and leaves one concrete question for future seasons: how durable will pertussis control be in cohorts that passed through the pandemic years with little or no natural immune boosting?