To assist health authorities in developing recommendations regarding new vaccines against chickenpox and pertussis, the French Institute for Public Health Surveillance assessed, under various scenarios, the impact of these vaccinations on adolescents and young adults. Two cohorts of individuals aged 11 and 18 were hypothetically followed until age 80, and the number of health events prevented by vaccination was calculated. The three strategies studied were vaccinating all subjects, those with no known history of chickenpox, and those with no known history and who were seronegative for chickenpox. The latter strategy proved to be the most cost-effective. However, it would prevent, at best, between one-third and two-thirds of expected events each year—that is, fewer than 7 deaths and 385 hospitalizations—provided that an initial catch-up vaccination campaign for the entire adult population is undertaken. Data from the Renacoq surveillance network were used to estimate the number of pertussis cases occurring each year among children under 5 months of age that could be prevented by vaccinating their contacts. The two strategies studied were vaccination of all 25-year-olds with the combined diphtheria-tetanus-polio-pertussis vaccine and vaccination of the immediate family during pregnancy. With 100% coverage, vaccinating 25-year-olds would prevent, on average, 76 to 148 hospitalizations and 1.3 to 2.6 deaths each year. Family vaccination would prevent twice as many. The higher expected vaccination coverage for the family vaccination strategy would make it even more advantageous compared to the 25-year-old booster strategy (a 24% to 47% reduction in deaths with 80% coverage for family vaccination versus a 6% to 12% reduction with 40% coverage for vaccination at age 25). (R.A.)