Any ambiguity in texts used in the communication about vaccines can not only interfere with comprehension, but also generate safety and liability issues. Within a survey on the quality of written protocols for at-risk interventional procedures and sanitary crises, we analyzed documents relating to vaccination, and among them, the "package-leaflet" of an anti-H1N1 influenza vaccine, widely disseminated to the public in 2009-2010. Among the most common mistakes, we observed that 1) language was not always adjusted to the non-specialist's level of knowledge; 2) chronology, logic, consistency, and homogeneity were often missing; 3) crucial pieces of information were disseminated all over the text, 4) use of the passive voice did not distinguish between instructions and information; 5) use of synonyms could be misleading and impair translation. We propose the use of "Controlled language" (CL) to improve the situation. By constraining lexicon, grammar and syntax, CL is a way to write documents that are clear, accurate and devoid of ambiguity. However, the set of rules necessary to write in CL is difficult to memorize. We thus developed authoring software (Rédacticiel Prolipsia) to make the creation of a CL by linguists and its use by health professionals easy and adapted to any domain. It may considerably improve the writing of vaccine package inserts/leaflets. It could be used to write information documents about vaccines and their safety, and operating procedures for professionals to prepare, store, and administer vaccines, decide upon proper indication of vaccines, and follow patients after vaccine injection.