The 24-month health certificate is the primary tool for estimating vaccination coverage among young children. Until 2004, county councils reported aggregated data; since 2004, they have reported individual data. In 2004, both data collection methods were used. The objectives of this study were to estimate vaccination coverage at 24 months from 2004 to 2007 and to assess the impact of the change in the data reporting system on these estimates. In 2004, 88 departments submitted their aggregated data. Depending on the year, 64 to 74 departments submitted their individual data, representing between 26% and 36% of births. At the national level, the estimates obtained in 2004 using both data collection methods are generally consistent. Coverage rates for diphtheria, tetanus, polio, pertussis, and Haemophilus influenzae type b (Hib) remained stable between 2004 and 2007 (approximately 90% for 3 doses plus a booster). Coverage rates for measles, mumps, and rubella (MMR) and hepatitis B were increasing. They stood at 90% for MMR coverage and 42% for hepatitis B coverage in 2007. BCG coverage decreased to 78.2% in the same year. The same trends are observed at the departmental level. Our analysis allowed us to update vaccination coverage data for children at 24 months of age. Individual data offer the advantage of enabling more in-depth analyses, particularly of vaccination coverage according to sociodemographic characteristics. However, improvements could be made, particularly in terms of data participation and the time required to make data available. (R.A.)