Introduction - During an outbreak in 2008, four healthcare professionals and trainees at the Reims University Hospital were infected with the measles virus; three of them were infected by hospitalized patients. Method - An initial immunization survey was conducted among contacts of the index cases and was gradually expanded to include other healthcare workers. Individuals with no clearly identified clinical history, who were unvaccinated or had received only one dose during childhood, were invited to undergo serological testing. Results - Of the 1,606 records investigated, 1,489 were documented, reporting 364 clinical histories and 681 vaccinations. Of these, 344 involved two-dose regimens, and 337 involved single vaccinations, 272 of which were administered in childhood. Among the 830 serological tests performed, 35 were negative (Ab titer <100 mIU/mL). Discussion - Conclusion - The viral susceptibility rate was 2.3% in the documented population. Among those who were seronegative, 40% had received a single vaccine dose during childhood. It is therefore necessary to check serology results in such contexts, such as in the absence of any medical history, in order to offer vaccination in the event of a negative result. Systematically documenting this information upon starting a new position or internship ensures its preventive effectiveness. (R.A.)