PT-IgG levels according to time since symptom onset in children aged 0–2 years with pertussis, by vaccination status: a cross-sectional study

Background Both pertussis vaccination and natural infection induce PT-IgG production. However, few studies have described PT-IgG levels according to time since symptom onset after natural infection in children with different vaccination doses. Methods We retrospectively analyzed clinical data from 586 children aged 0–24 months hospitalized at the Children’s Hospital Affiliated to Chongqing Medical University between April 1, 2018, and July 31, 2019. Patients were categorized into vaccinated and unvaccinated groups based on immunization records, with further stratification by vaccination doses (0, 1, 2, 3, or 4 doses). PT-IgG levels were compared between vaccinated and unvaccinated children within each week after onset. Results Among 586 patients (46.25% female), vaccination rates increased with age, exceeding 90% for ≥ 1 dose in children aged 12–24 months. In 3–6-month-olds, median PT-IgG levels were significantly higher in vaccinated than in unvaccinated children at weeks 2–4, with the most pronounced difference at week 4. At week 5 the difference was not significant, and at week 6 median levels were higher in unvaccinated children, although this difference did not remain significant after correction for multiple testing. For 6–12-month-olds, in vaccinated children, the highest median level was observed at week 4. Unvaccinated patients (0 doses) showed low median antibody levels during weeks 1–4, with the highest median level observed at week 6. Among children with 1 dose, the highest median level was observed at week 4, with lower median levels at weeks 5–6; median levels among 2-dose recipients were similar across weeks 2–5; median levels among 3-dose recipients were highest at week 5 and lower at week 6. In the dose-group analysis, all vaccinated dose groups (1–3 doses) differed significantly from unvaccinated children, before correction for multiple testing, only at weeks 2 and 3; no differences remained at weeks 1, 5, or 6 after correction for multiple testing. In a multivariable analysis adjusted for age, sex, and sampling week, children with 1, 2, or 3 doses had higher PT-IgG levels than unvaccinated children (adjusted geometric mean ratios 4.79, 10.54, and 7.36; all P < 0.001). Conclusion Vaccination status was associated with differences in PT-IgG levels during the first 6 weeks after infection, with higher median levels in vaccinated children during weeks 2–4. As children were sampled cross-sectionally without pre-infection baseline measurements, these group-level differences may partly reflect pre-existing vaccine-derived antibody and do not establish within-individual antibody kinetics or a faster response to infection. Clinical trial number Not applicable.