BackgroundMaintaining and assuring excellent immunisation safety is essential. Suspected adverse events are passively reported to the UK Medicines and Healthcare Products Regulatory Agency (MHRA). We developed a supplementary system to proactively identify recent potential adverse events resulting in admission to hospital and applied this system to rotavirus vaccine, offered to all infants across Scotland from July, 2013, at age 2 and 3 month visits. MethodsAll general and acute inpatient and day cases in hospitals across Scotland were included. Records for predefined potential adverse events were extracted with International Classification of Diseases 10 codes. For rotavirus vaccine, inclusion criteria were age less than 2 years, with codes of intussusception, Kawasaki disease, or anaphylaxis; and haematochezia, an intussusception marker. Primary outcome was the number of infants per month meeting inclusion criteria. Historical data were extracted, enabling presentation with Statistical Process Control methodology, with lower and upper control limits. New data points exceeding upper control limits were signals that needed further investigation. These included extracting all associated diagnostic codes, age breakdown to assess plausibility in relation to vaccination age and hospital locations, and, after formal approval, linking admissions to hospital to vaccination and laboratory reports. FindingsEvery month in Scotland over 9000 rotavirus vaccine doses are administered. Admissions to hospital for intussusception, Kawasaki disease, and anaphylaxis did not exceed upper control limits after vaccination. Admissions for haematochezia exceeded the upper control limit in August, 2013, 1 month after vaccine introduction. Further investigations showed that 45 infants had been admitted with the condition since vaccine implementation, 23 had been vaccinated, but only two had been admitted within 7 days of vaccination, the specific risk period. Neither of these two infants had received the vaccine outside the recommended age period, and both had other compatible reasons for haematochezia. InterpretationWe have developed a timely active system for monitoring adverse events potentially associated with vaccination in Scotland. The system is limited to specified events, but it supplements MHRA reporting to provide added assurance of robust monitoring. Signals of interest can be rapidly further investigated, including near-time data linkage to the national childhood immunisation records, leading to formal epidemiological studies, if required. FundingNHS National Services Scotland.