Risk Interval of Immune Thrombocytopenia in Children After Influenza Infection

The temporal association between influenza and immune thrombocytopenia (ITP) is poorly characterized. This study examined the risk interval between influenza infection and newly diagnosed ITP in children.

We enrolled individuals aged 1-15 years newly diagnosed with ITP between 2005 and 2024, using the Japanese large-scale administrative JMDC Database. We conducted a nested case-control study and self-controlled case series to evaluate the risk of ITP within 28 days (categorized into 7-day intervals) of influenza diagnosis. We estimated the risk interval from influenza diagnosis to ITP onset using conditional logistic and Poisson regressions.

We identified 2255 individuals diagnosed with ITP, of whom 1324 met the eligibility criteria. In the nested case-control analysis, the adjusted odds ratio of preceding influenza diagnosis was the highest within 1-7 days before ITP onset (3.13; 95% confidence interval 1.38-7.12) compared with the matched controls. In the self-controlled case series, the incidence rate ratio for ITP was the highest within 1-7 days after influenza diagnosis compared to the baseline period (2.44; 95% confidence interval 1.16-5.14). In the sensitivity and post hoc analyses for different definitions of influenza diagnosis and ITP onset dates, ITP onset peaked within 1-14 days of influenza diagnosis.

The risk of ITP appeared to be highest during the first 1-2 weeks after influenza diagnosis, particularly around the first week.