Universal administration of the same group of vaccines to the entire population, in the absence of contraindications, is based on a number of fairly approximate assumptions: every vaccinated individual would respond immunologically identically (antibody titer and cellular response) and would experience minimal side effects. Each individual is exposed to a similar risk from the infectious agent he or she is vaccinated against, so the titre and number of doses administered to achieve a protective response could be identical. This "simple" paradigm underpins the Expanded Program on Immunization (EPI), adopted by most nations. Nevertheless, in the twentieth century, it helped to build widespread immunization coverage, leading to the control - and even eradication, in the case of smallpox - of several infectious endemics, including in disadvantaged areas of the planet where it is the basic operational model. It has worked well overall, except for BCG's failure to eliminate pulmonary tuberculosis, particularly in underprivileged regions with a high prevalence of HIV infection. Will it remain the model for vaccination in the 21st century? Will it undergo modifications, shifts towards "personalization", and to what extent? The strength of the universal vaccination paradigm lies in the fact that it achieves exhaustive coverage of the population, reducing the number of individual cases and inducing "herd immunity" with reduced carriage, thus blocking the circulation of the pathogen and making it possible to envisage the elimination of the disease, or even the eradication of the etiological agent. There are, however, conditions for success: side effects must be kept to a minimum to guarantee acceptability ("zero risk" vaccine), and the number of doses must not require numerous adjustments according to the medical and vaccination history of the individuals vaccinated. This paradigm also reveals a number of weaknesses that need to be addressed:
- an individual reality: variety of individual risk in the face of the infectious agent, variety of quality of individual immune response to the vaccine, variety of individual risk of secondary reaction to the vaccine.
- a medical societal reality: the various components of society require a certain degree of personalization of vaccines, particularly for newborns, women of childbearing age, immunocompromised patients and the elderly.
- a socio-anthropological reality: society is evolving, with a tendency for the individual to take precedence over the collective, a loss of generational memory of the seriousness of major endemics, a questioning of the universal vaccination obligation, a fear of side effects, and the primacy of the precautionary principle. The persistence of poverty (Third World, Fourth World in industrialized countries, migration) creates large pockets of non-vaccination. The recent measles epidemic in France should be analyzed in this light.
- a microbiological reality: the emergence of serotypic variants that escape current vaccines, as observed with pneumococcus and influenza viruses.
