Risk of Immune Thrombocytopenic Purpura Recurrence Following Second Measles-Containing Vaccine Dose

Although the cause of immune thrombocytopenic purpura (ITP) is often unknown, viral infection has been associated with ITP,1 especially in young children. Several studies have also reported an elevated risk of ITP after the first dose of a measles-containing vaccine (MCV).2–6 Whether ITP recurs after the second MCV dose among children who developed ITP after the first MCV dose has not been examined in a population-based study.We conducted a cohort study from 2000 through 2022 at 8 sites within the Vaccine Safety Datalink (VSD),7 a collaboration between the US Centers for Disease Control and Prevention and integrated health care systems across the United States. We included children aged younger than 7 years who were members in their respective health plans 6 months before their first MCV through 42 days after the second MCV dose. We excluded children if they had another diagnosis leading to thrombocytopenia in the prior 6 months (Table 1). Potential ITP cases were identified using International Classification of Diseases (ICD) 9th and 10th Revision codes 287.31 or D69.3 and platelet counts <100 000/μL. All potential ITP cases were confirmed by medical record review.We calculated the incidence of confirmed ITP that occurred 1 to 42 days after the first MCV dose. We also calculated and compared the proportion of ITP 1 to 42 days following the second dose among children who were diagnosed and not diagnosed with ITP after the first dose.The study included 960 000 children who received 2 doses of MCV. We electronically identified 60 children with probable ITP 1 to 42 days after the first dose. Upon medical record review, we excluded 8 children (3 did not have ITP, 1 ITP occurred before vaccination, and 4 had missing medical records) resulting in 52 (86.7%) confirmed cases. The incidence of medical record-confirmed MCV-associated ITP was 5.4 per 100 000 children (95% CI, 3.94–6.89) after the first dose.Of the 52 confirmed MCV-associated ITP cases, 30 occurred in male children. All but one received the first dose before age 2 years, and most (n = 42) received measles, mumps, rubella, and varicella vaccines (MMR+V) separately (Table 2).After the second dose, none of the 52 children who previously had ITP developed ITP during the 42 days postvaccination (95% CI: 0%–6.8%). Among those who did not have ITP after the first dose (n = 959 940), 26 probable ITP cases were identified in the 42 days following the second dose. Of these, 10 were confirmed, corresponding to an incidence of 1 per 100 000 children (95% CI: 0.56–1.91 per 100 000).In this study, which followed nearly one million children after 2 doses of a MCV, we found that none of the children who had ITP after the first dose developed ITP after the second dose. ITP incidence after the second dose (1 per 100 000 children) was much lower than the incidence after the first dose (5.4 per 100 000 children).Although we found no recurrence of ITP after the second dose, the number of ITP cases after the first dose was too small to rule out a risk of recurrence of less than 7% (the upper bound of the 95% CI for the zero recurrence was 6.8%). Yang and colleagues reported that the risk of ITP recurrence independently of MCV was 6.7% in children aged under 18 years.8 Our results are similar to 2 small previous studies, one with 2 children and the other with 24 children who had ITP after the first MCV dose; none developed ITP 1 to 42 days after the second MCV dose.9,10Our study was strengthened by its large sample size, ability to follow children over time, access to vaccination data for both doses, and ability to confirm ITP by medical record review. The study was limited by the small number of ITP cases after the first dose which limited the ability to estimate potential recurrence incidence with precision.In this study, we did not find evidence of ITP reoccurring following the second dose among children who had ITP after their first MCV dose. The study may provide some reassurance to pediatricians and parents who are hesitant to revaccinate children who had ITP after the first MCV dose.