Children Hospitalized for Pertussis Despite Vaccination

Children hospitalized for Bordetella pertussis include those who have been vaccinated against the infection, in a retrospective cohort studyconducted in Poland, which employs both whole-cell (wP) and acellular pertussis (aP) vaccines.¹

Dominika Lachowicz, PhD, University Center for Laboratory Medicine, Medical University of Warsaw, Warsaw, Poland, and colleagues found that cases of hospitalization for pertussis within 5 years of wP vaccination suggest the need for "population-based and molecular studies to evaluate potential factors such as waning immunity or antigenic changes in circulating B pertussis strains."

Lachowicz and colleagues identified 269 children hospitalized with laboratory-confirmed diagnosis of B pertussis infection between January and December 2024.The highest proportion of cases were infants under 1 year of age (32.9%); with 17% in those aged 1-5 years, 19% in 6-11 years, and 10% in 12-15 years.There were 2 cases (2.6%) in adolescents over 15 years of age.

Unlike other EU countries and the US, Poland has not transitioned to aP.The aP vaccines are available through the private market, however, and while the investigators identified which vaccine was used in each case, the study was not powered to distinguish between them to infer duration or degree of protection. They did note that there was no apparent difference in clinical symptoms or severity in children receiving wP or aP vaccines.

The interval between disease onset and the last vaccine dose across the cohort ranged from 1 month to 12 years, with a trend toward increasing clinical severity corresponding to longer periods from the last booster. There were 12 cases of severe symptoms, with pertussis severity score (PSS) >5, all occurring in children younger than 12 years of age: 4 patients in each age group of <12 months, 1-5 years, and 6-11 years.

"Severe disease was observed among both vaccinated and unvaccinated children," the investigators reported.

They also observed that children receiving the wP vaccine more than 5 years before hospitalization exhibited "more heterogeneous clinical presentations" compared to those with more recent vaccination. These symptoms included dyspnea and requirement for intravenous fluids and/or oxygen therapy.

Lachowicz and colleagues suggest that the manifestation of pertussis infection within 5 years of vaccination with wP reflects the possibility of circulating B pertussis strains that differ from the vaccine targets. They cite recent studies of strains with mutations in genes encoding pertactin Prn and PT promoter ptxP.

"Pertactin-deficient strains, which may evade immune response induced by aP vaccines, are now widespread in many countries," they warn.

In addition to vaccination status, the investigators considered the relation of PCR (real-time polymerase chain reaction) and serology testing with clinical symptoms. In 50 PCR-confirmed pertussis cases, the majority (74%) of patients had cough duration of ≤3 weeks, and 14% were confirmed between weeks 4 and 6.

The investigators point out that current WHO guidance indicates that PCR sensitivity is highest during the first 2 to 3 weeks of symptoms onset, and that it recommends PCR-testing in young children.

In cases where serology results were equivocal in their cohort, Lachowicz and colleagues found that PCR allowed confirmation in 4 out of 5 cases, "supporting the complementary use of both methods."

Reference

1. Lachowicz D, Jastrezebska B, Bukowska E, et al. Diagnostic performance and clinical outcomes of pertussis in hospitalized children in relation to available vaccination status: A retrospective cohort study. Infect Dis Ther 2026, published online April 3. https://doi.org/10.1007/s40121-026-01349-5.