Timing of vaccine-associated anaphylaxis and implications for post-vaccination observation period: A systematic review and Canadian surveillance data analysis

Background The post-vaccination observation period ensures the rapid identification and clinical management of immediate adverse events following immunization, including anaphylaxis. Recommended observation periods vary internationally. In Canada, a 15 min observation period is standard. This systematic review assesses the empirical basis for these recommendations and the potential impact of varying observation period durations. Objective To describe time to onset of anaphylaxis following immunization and provide an empirical basis to evaluate variable post-vaccination observation period durations. Methods This study combined mixed methods to capture the available evidence base. A systematic review following PRISMA guidelines searched Embase and MEDLINE from inception to March 6, 2023, for studies reporting time to onset of anaphylaxis within 30 min of vaccination. Anaphylaxis reports submitted to the Canadian Adverse Events Following Immunization Surveillance System (CAEFISS) were also analyzed. Results Of 44 studies reporting 1365 cases of anaphylaxis, 1229 (90%) had recorded onset time, of which 802 (65%) occurred within 30 min. Among cases with sufficient temporal detail ( n = 305), 236 (77%) had onset within 15 min; of 113 cases with 5-min interval data, 61% had onset within 5 min. Findings were broadly consistent across COVID-19 and non-COVID-19 vaccines, age groups, and levels of diagnostic certainty. Among 2193 CAEFISS reports, the median onset time was 12 min; 78% ( n = 1702) occurred within 30 min, 80% ( n = 1363) of which occurred within 15 min, and 46% ( n = 628) of those within 5 min. Results were similar across age groups and vaccine types. Conclusions Vaccine-associated anaphylaxis most commonly presents within 15 min of vaccination. A routine 15-min observation period would be expected to capture most (77–80%) events occurring within 30 min.