For the first time, the 2009 Swiss immunization plan includes detailed recommendations for the catch-up of missing immunizations--an increasingly frequent and complex situation which requires following as clearly defined rules as possible. Whether the immunization history is unknown or incomplete, the numbers of missing vaccine doses and their administration schedules depend upon a large number of parameters that are briefly described, practical guidelines being summarized into four tables. Alternatively, health care professionals may contact InfoVac experts (www. infovac.ch) for the most complex cases, or rely on viavac (www.viavac.ch), a new medical software that instantly establishes ready-to-use individualized catch-up plans including the most appropriate vaccine formulations.