Mass vaccination was a key component of the public health response to the 2009 influenza A(H1N1) pandemic, complementing other measures such as symptomatic treatment of cases, isolation of patients, the use of face masks by those who were sick, hand hygiene, and other preventive measures, as well as the administration of antiviral treatments when necessary. The decision to implement mass vaccination was based on both epidemiological and health-economic arguments (disruption of work, absenteeism). According to the World Health Organization, a significant proportion of deaths attributed to the 2009 H1N1 virus occurred among young adults with no risk factors. Vaccination was therefore a preventive measure aimed at conferring immunity to a population against a new virus.