colleagues (Dec 19, p 2115) 1 highlight the importance of developing locally relevant background rates of disease to aid assessment of pandemic H1N1 vaccine safety. Taiwan has the capacity to monitor H1N1 vaccine safety during mass immunis ation campaigns. Beginning on Nov 1, 2009, Taiwan began administering 15 million doses of H1N1 vaccine sequentially according to a priority list, with 12 target groups. National Health Insurance (NHI) in Taiwan enrols more than 99% of citizens. 2 Using the same International Classifi-cation of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) codes listed in Black and colleagues' study, we searched the NHI claims database from Jan 1, 2004, to July 31, 2008, to estimate background rates of the selected medical events that might be temporally associated with the vaccine (table). Assessment of Guillain-Barré syn drome after vaccination is a high priority during an H1N1 vaccine campaign. We predicted that about 1, 5, and 27 cases of co-incident Guillain-Barré syndrome, respectively, would occur within 1 day, 7 days, and 6 weeks of vaccination after 15 million doses. As of March 16, 2010, 5·66 million doses of H1N1 vaccine had been administered and there had been four confirmed reports of Guillain-Barré syndrome occurring within 6 weeks of vaccination. The government has expanded its vaccine-safety monitoring systems. In addition to spontan-eous reporting of adverse events, a population-based active surveil-lance system that links the national H1N1 vaccine registry to NHI's daily updated computerised database is being used to assess potential associations of H1N1 vaccine with selected adverse events in real time. 3 We believe that infor-mation collected from the Taiwan's Female Male