Adverse events following immunization (AEFI) with the novel influenza a (H1N1) 2009 vaccine: findings from the national registry of all vaccine recipients and AEFI and the passive surveillance system in South Korea.

This study characterized the adverse events following immunization (AEFI) with the novel influenza A (H1N1) 2009 vaccine in Korea. Data on immunization and AEFI were collected between October 27, 2009 and March 15, 2010 through the national immunization registry and passive surveillance systems. The frequency of AEFI and serious adverse events (SAEs) were calculated according to age, sex, priority group, and vaccine type. In 13,758,527 vaccine recipients aged 6 months or older, 2,530 AEFI were reported (18.4 per 100,000 immunizations). The AEFI reporting rate was highest among people aged 10-19 years (29.6 per 100,000 immunizations) and was higher in female recipients than in male recipients (20.0 versus 16.7 per 100,000 immunizations). Most AEFIs were nonspecific systematic reactions that occurred within 24 h (77.4%) after vaccine administration. A total of 178 vaccine-related SAEs were identified, and vaccine-related mortalities were not reported. This study showed that the AEFI reporting rate after influenza A (H1N1) 2009 vaccinations was relatively high, especially in the younger population. Mild systemic reactions accounted for the majority of reported AEFI, and fatal SAEs were rare. This study also implied that passive surveillance might be an efficient safety monitoring system that can detect relatively rare AEFI.