Recurrence risk of a hypotonic hyporesponsive episode in two Australian specialist immunisation clinics

Background: A hypotonic hyporesponsive episode (HHE) is a well-described adverse event following immunisation (AEFI) in young children. There is limited data regarding recurrence post re-vaccination. Method: A retrospective analysis of HHEs reported to two tertiary paediatric hospitals in Australia: The Royal Children's Hospital, Melbourne [2006–11] and the Children's Hospital Westmead, Sydney [1997–2014]. HHE definition level of confidence was allocated according to Brighton Collaboration (BC) criteria and defined immediate if within 30 min post vaccination. The Australian Immunisation Register (AIR) was utilised to document current immunisation status. Results: 235 HHE cases (135 Melbourne, 100 Sydney) were identified: 47% were female and 67% (157/235) occurred following the routine dose one vaccines at 6–8 weeks of age. Median time following immunisation was 120 min (range 1 min to 14 days) An immediate HHE occurred in 43% (102/235) and by BC criteria, 74% (173/235) were level 1 (definite). Subsequent vaccines were administered under supervision in hospital in 37% overall (86/235); 43% (58/135) in Melbourne and 28% (28/100) in Sydney. HHE recurrence rate was 3% (7/235) [95% confidence interval 1–6%]. AIR records were available in 94% (221/235). At a median age of 3.1 years, 84% (186/221) were up-to-date with recommended vaccines. Conclusion: This study highlights the importance of specialist immunization clinics in supporting the National Immunisation Program, through follow-up and management of serious adverse events following immunization.