[Impact of vaccinations on the epidemiology of infective diseases]

Immunizations may progressively modify the epidemiology and the clinical features of some infectious diseases, partially explained by a shift of ages towards adults. For measles, the coverage in France is low (80%) and mean age of measles cases has raised from 7 to 9 years (40% of cases are more than 10 years old). Measles may be more severe diseases in adolescents and adults, and for this reason a second dose of MMR vaccine has been recommended using a predictive mathematical model of measles epidemiology. Nevertheless, an important focus of measles remains in France (17,954 cases in 1999) and the risk of outbreaks in adolescents and adults is always present if immunization coverage for 24 months infants and catch-up are not improved quickly. Rubella has the same profile, with two outbreaks in young adults (1993 and 1997) followed by a peak of congenital rubella. Pertussis immunization has always been well applied in France. Resurgence started in the 90's, with pertussis in young adults (some being immunized in infancy) who transmitted pertussis to very young infants, not yet properly immunized. The situation requires a better immunity for old children and adults. For this reason, a booster is recommended in France at 11-13 years, and perhaps in adult later, these late boosters are practicable with the new acellular pertussis vaccines. Varicella vaccine is not in the french immunization schedule, for the same argument of risk of varicella in adults in cases of insufficiency of the coverage. Now USA recommend the vaccine in children, but a strong surveillance has been established. Those four examples observed in the french recent immunization policy emphasize the necessity for a stronger and new surveillance of immunizations and infectious preventable diseases.