The article describes the characteristics and classification of adverse events after immunization (AEFI) according to the latest WHO recommendations. The registration systems for AEFI in the USA and in Russia are described. Particular attention is paid to the interpretation of the convulsive syndrome that developed after vaccination. Four clinical cases of the development of convulsive syndrome in children hospitalized at the Pediatric Research and Clinical Center for Infectious Diseases of Russia (St. Petersburg) in the postvaccination period are presented. The criteria for differential diagnosis of the described diseases are indicated. Only a search for the etiology of the disease allows us to assess the relationship with vaccination, timely conduct adequate therapy and generate objective information on the safety of vaccines. There is a need to register all episodes of seizures after vaccination and introduce new methods for registering PPI, as well as creating a system of statistical accounting of background health conditions (convulsions, allergies) of the population of the Russian Federation of various age groups.