Comparative safety assessment of measles-containing vaccination schedules in Jiangsu Province, China: A population-based pharmacovigilance study (2014-2024)

China's immunization policy was updated in 2020, replacing the sequence of MR (at 8-12 months) and MMR (at 18-24 months) with a comprehensive two-dose MMR schedule.This study aims to evaluate the safety of this policy shift by comparing the incidence of Adverse Events Following Immunization (AEFI) before and after the adjustment.Methods: A retrospective analysis was conducted using AEFI surveillance data from Jiangsu Province. The study compared two cohorts: the "pre-adjustment" group (MR at 8 months, MMR at 18 months) and the "post-adjustment" group (MMR at both 8 and 18 months). Incidence rates of non-serious reactions (fever, fatigue, local redness, and swelling) and serious reactions (allergic rash, febrile seizures, etc.) were analyzed using Chi-square tests and rate ratios (RR). Results: The transition to a two-dose MMR schedule was associated with an increase in reported mild AEFIs. Specifically, at the 8-month dose, the incidence of fever, fatigue, local redness, and swelling was significantly higher in the MMR8 group compared to the MR8 group (P < .001). The incidence of allergic rash also showed a 31% increase in the MMR8 group (45.39 per 100,000) compared to the MR8 group (34.71 per 100,000), though it remained below global safety thresholds. Crucially, there were no statistically significant differences in the incidence of serious AEFIs, such as thrombocytopenic purpura (P = .670) or febrile seizures (P = .915), between the two strategies. Furthermore, serious reactions as a whole category showed no statistically significant difference between MR8 and MMR8 (P = .710). The implementation of a two-dose MMR immunization schedule in Jiangsu Province is safe. While the shift was associated with an increase in mild, self-limiting reactions, it did not lead to an increase in serious adverse events. These findings support the continued use of the two-dose MMR strategy to provide broader protection against measles, mumps, and rubella without compromising vaccine safety.