Background: Vaccine effectiveness estimates of pH 1N 1 vaccine were often based on surveillance data using assumptions of equal exposure between vaccinated and unvaccinated groups.Moreover, due to viral mutation, these estimates are only valid < 1 year after vaccination.On March 10, 2011, a primary school reported an influenza liked illness (ILI) outbreak, which was confirmed as due to pH 1N1.We investigated this outbreak to evaluate pH1N1 vaccine effectiveness 12 months after vaccination.Methods: We defined a suspected case as onset of ILI from February 21 to April 4, 2011, in the students and teachers of this school.A confirmed case was a suspected case with pH1N1 virus identified by real-time RT-PCR.We searched for cases by interviewing the students and teachers by telephone.We conducted a retrospective cohort study in the classrooms that had cases.We divided students and teachers who were at school from February 21,2011 (i.e., beginning of semester) to April 6, 2011 (i.e., end of the outbreak) into vaccinated and unvaccinated groups, determined by their vaccination records.We excluded persons who had a history of pH1N1 infection before this outbreak and those with unknown vaccination history.Results: We identified 61 case-persons (12 confirmed).The overall attack rate of the school was 15% (61/403).Case-patients were in 5 out of 8 classrooms in this school.Among 115 persons vaccinated 12 months earlier in the 5 classrooms, 14% (16/115) develop a case compared to 31% (34) among 111 persons who were unvaccinated (RR=0.45, 95% CI=0.27-0.77).The pH1N1 vaccine effectiveness in this outbreak was estimated to be 55% (95% CI: 23%-73%) after 12 months.Conclusions: The pH1N1 vaccine maintained a moderately protective effect 12 months after vaccination.