Introduction
In France, the vaccination schedule (1) for children includes the following:
• BCG: from the first month. At this age, immunisation is reserved for children living in environments at risk. It becomes mandatory when children enter a community (nurseries, schools, and when cared for by a nurse).
• Diphtheria, tetanus, pertussis, poliomyelitis, invasive infection of Haemophilus influenzae b (combined vaccine): starting at 2 months of age, 3 injections at one month interval, and a first booster dose at 16-18 months. A second booster is planned at 6 years of age for diphtheria, tetanus, and poliomyelitis.
• Hepatitis B: from 2 months of age, 2 injections at a month interval, then a third injection between 5 and 12 months after the second injection.
• Measles, mumps and rubella: from 12 months of age, a 1st dose, and a second dose between 3 and 6 years of age. Measles immunisation can be administered from the age of 9 months for children living in community followed by a revaccination with the triple vaccine 6 months later.
• Immunisation against diphtheria, tetanus and poliomyelitis (DT polio) and BCG are mandatory. All the others are subject to recommendations.

The results of an immunisation programme are assessed not only through the surveillance of the incidence of target diseases, the population's serological status, the follow-up of potential adverse effects of vaccines, but also by the regular evaluation of vaccine coverage rates in the population, allowing to monitor progress of the objectives set. Moreover, for diseases in the phase of elimination, this measure should allow to identify rapidly any decrease of vaccine coverage that could lead to a disease re-emergence.
The evaluation of vaccine coverage is different depending on age and antigens as shown by the work coordinated by the Institut de veille sanitaire in 2000 assessing the methods used (2). The results of the main measures taken in children up to 6 years of age are described in this article.

Vaccine coverage assessment
The assessment of vaccine coverage is carried out from health certificates of the 24th month (CS24), filled by each doctor during the examination that is mandatory before the 24th month of age (3). Since 1985, the antigens concerned are: DT, pertussis, polio (3 and 4 doses for these 4 antigens), BCG, measles and rubella, and since the update of health certificates to the new immunisation schedule in 1996, mumps, hepatitis B, Haemophilus influenzae type b infections.
Data is analysed at the local authority level through the services of maternal and infant protection (PMI), then at the national level by the Directorate of Research, Studies, Evaluation and Statistics (DRESS) of the Ministry of health (4).
Another calculation is carried out by the PMI services in children aged between 3 and 4 from the data collected on the children's health records during the mandatory health assessment for nursery schools. This assessment concerns BCG, measles, and mumps and rubella (3).
Until 1999, a biennial survey was carried out in 6-year-old children attending their last year in nursery school by doctors and nurses from the National Education Ministry. The size of the sample was calculated to allow for representative results at national, regional and local levels. The analysis concerned antigens for measles, mumps, rubella, and the last two surveys included antigens for BCG too (5,6). Since 2000, a triennial cycle of surveys has been set up in schools for three generations of children (6, 10, and 15 years). These surveys deal with all the antigens that the child should have received from the immunisation schedule. The first survey on six-year-old children is still ongoing.
Population surveys were carried out on samples by research companies: one of them was repeated every six months until the end of 1998 to evaluate hepatitis B vaccine coverage (7). The sample was calculated to be representative by age groups, one year old in children (5-10 years in adults), for 20 000 families. This method was also used in 1998 to estimate MMR vaccine coverage in children aged 0 to 15 years together with the number of doses received (8).

Results
The results are presented here antigen by antigen. They come from DRESS data for 2000, unless otherwise specified. Table 1 describes the trends of results for the coverage of children aged less than 24 months for all antigens from 1992 to 2000. Between 363 000 and 444 000 certificates for vaccination coverage, depending on the year, were analysed over this period, representing from 51% to 60% of all expected certificates for the following cohorts of children aged two (57% in 2000).

• BCG
BCG vaccination is administered to 83% of children aged 24 months, and 93% of children aged four in nursery schools. The survey carried out in 1997 in schools shows that 95% of children were vaccinated before they were six (5).
• DT Polio and pertussis
DT Polio vaccination: 98% of children received three doses before two years of age, and 88% received three doses and a booster. Pertussis coverage was 97% for 3 doses and 87% for 3 doses and one booster. Figure 1 shows the evolution of vaccine coverage in 24 months old children for DT Polio vaccination from 1985 to 2000 with a high coverage level for both 3 and 4 doses. Figure 2 shows the coverage distribution (4 doses) by department with a few areas where coverage is under 85%, in particular in the south of the country.

• Measles-Mumps-Rubella
Vaccine coverage for measles, mumps and rubella (1st dose) was 84% at 24 months of age and 91% at 4 years in 2000. The last survey carried out in school children aged 6 showed a 90% coverage in 1999 (for the same generation of children aged 4 years, it was estimated to be at around 88% in 1997 by the PMI services) (3,6).
Figure 3 shows the evolution of vaccine coverage in 24 month old children for measles and rubella from 1985 à 2000, with a rapid increase of coverage until 1994 which now stabilises. Figure 4 shows differences in MMR coverage by department in 2000 with rates of coverage higher in the northern half of France.

• Haemophilus influenzae b
Since the introduction of Hib immunisation in 1992, vaccine coverage has increased quite rapidly; it was 86% in 2000.
• Hepatitis B
Coverage in children under 2 years of age was 26%. At the end of 1998, the latest estimation of vaccine coverage for hepatitis B in the population showed that 35% of children ages from 1 to 6 were vaccinated (7).
• Comparison with the European Union countries and the United States
Table 2 compares data available on vaccine coverage in children under 2 years of age (3 doses for DTPC, Hib and HBV, 1 dose for measles), without any details of the calculation method used. The mandatory or recommended nature of vaccination is given (9,10). The comparison of this data reveals that France has a high level of vaccine coverage, similar to that of Nordic countries for diphtheria, pertussis, tetanus and poliomyelitis. However, coverage levels are not as good for vaccination against Haemophilus influenzae b infections, and in particular measles and hepatitis B.

Discussion
The use of CS24 forms to evaluate vaccine coverage raises some concern about factors that may affect the results, causing a lack of completeness: the quality of information given in forms, their transmission by the doctor who administers vaccines to the health department, their analyses by each department, and their dispatching at national level. For each year, only 50 to 60% of expected certificates could be analysed at the national level. The results of local cluster sample surveys (WHO method) carried out at home in several districts (11) and the cohesion with survey analyses carried out at 4 and 6 years enable validation of the data analysed routinely on the CS24 (3,5,6).

Vaccine coverage for 2 year old children is high, in particular for DT, poliomyelitis. Except for a case of diphtheria which occurred in 2002 one month after a young woman arrived from China, no case of diphtheria or autochthonous poliomyelitis has been reported since 1989 (probably linked with a good group immunity). However, persisting cases of tetanus in older population1 suggest a bad practice of boosters in adults. We observe a persisting circulation of pertussis bacteria with the description of cases in older children and adults. This is not explained by the lack of coverage, which is high, but by a deficient duration of protection given by the vaccine. This caused a change in the immunisation schedule with the introduction of a booster dose at 11-13 years in 1998. For BCG, the coverage rates reflect the vaccination policy, with a catch-up dose occurring when children enter school.
Satisfying coverage rates have been reached for vaccination against Hib infections, associated with a sharp decrease of meningitis incidence in infants under one year of age2.

Vaccine coverage for measles, mumps and rubella is insufficient, remaining around 83% for several years. The situation is even more worrying in southern districts with coverage under 80% and even under 70%. The continual lack of coverage allows for viruses to circulate resulting not only in insufficient control of the diseases in children but also in a shift of cases from childhood to adolescence and adult age and an increased risk of complications 3.
In 1994, an immunisation programme for adolescents and infants against hepatitis B was set up. The coverage rates of the 1998 survey show that the mass immunisation policy led between 1994 and 1998 for children entering secondary school had reached and even exceeded its objectives (7). However, the reporting of demyelinising pathologies in persons vaccinated against hepatitis B raised serious concern. Although no study has ever revealed any causal association between vaccination and the occurrence of a first demyelinising stroke, and despite the fact that infants immunisation was not questioned, vaccine coverage of 24 months old children does not exceed 26%.

The analysis of the vaccine coverage data in France shows higher numbers in the north of the country compared to the south. A study published in 1999 showed that doctors from the south of France were less sensitive to vaccination practices and that anti-vaccine leagues are more present (12). The results are in favour of a higher coverage for mandatory vaccines (diphtheria, tetanus, polio, BCG) than for recommended vaccines (MMR, hepatitis B), with 83% to 98% for mandatory vaccines versus 26% to 84% for vaccines recommended before the age of 2. Pertussis and Hib vaccines, although recommended, have a similar level of coverage as vaccines for diphtheria, tetanus and poliomyelitis as those 5 vaccines are combined in a unique vaccine preparation.
However, the conclusions of such a comparison as regards the influence of mandatory immunisation on vaccine coverage have to be considered cautiously. Indeed, diphtheria, tetanus and polio immunisation was introduced in the 1940s to 1960s, when these diseases represented very dangerous infections and were perceived as such by medical staffs and families. Immunisation against measles, mumps and hepatitis B, introduced more recently in the schedule as a recommendation, has not reached comparable levels of coverage in infants. These diseases don't seem to be considered as representing a large-scale threat. However the launch of a hexavalent vaccine could have a positive effect on Hepatitis b vaccine coverage.

International comparisons must be interpreted with care considering that socio-cultural factors and the organisation of health systems (in particular the proportion of private and public sectors) influence considerably the vaccine coverage. The analysis of MMR vaccine coverage shows that northern European countries where no mandatory immunisation policy was ever implemented and where vaccines are administered through public health facilities, have on average better performances than southern European countries where most of mandatory immunisation was set up and where the proportion of the private sector is higher. Some countries without any legal obligations have implemented incentive programmes like in England, active follow-up of children from birth in the Netherlands, or the obligation to be immunised before school entry like in the USA.

Conclusion
Adapting the immunisation schedule according to the latest immunological and epidemiological findings, and the appearance of new vaccines for children reveal the need to reconsider the measures of vaccine coverage and the 'key' ages to that end. In France, the implementation of surveys in schools for children aged 6, 10, and 15, will enable to complete the available data and to evaluate the administration of the second MMR dose as well as the boosters for other vaccines.
Finally, regarding MMR and hepatitis B immunisation, it is necessary to increase vaccine coverage in infants and to ensure an efficient catch up immunisation in children and in young girls for rubella in the near future.