Vaccination coverage refers to the proportion of people vaccinated in a population at a given time.
It is the ratio of the number of people who have been properly vaccinated—that is, who have received the required number of doses by a given age—to the total number of people who should have been vaccinated in that same population. For a vaccine requiring multiple doses, we refer to “1-dose” coverage, “2-dose,” “3-dose,” or “booster.” Thus, vaccination coverage for a single dose of the measles-mumps-rubella (MMR-1) vaccine at 24 months in France corresponds to the proportion of children in France aged 24 months who have received at least one dose of the MMR vaccine.

Why measure vaccination coverage?

Vaccination coverage is one of the indicators used to monitor and evaluate, along with incidence and mortality data and seroepidemiological data, the impact of a vaccination program.
After a recommendation is introduced into the vaccination schedule (or an existing recommendation is modified), vaccination coverage data make it possible to determine whether these new recommendations have been effectively implemented in practice, that is, whether they have been put into practice among the population groups for which they are intended. These data are therefore useful for determining whether a vaccination program is being properly implemented. They are essential because maintaining high vaccination coverage is a key element in controlling infectious diseases, enabling the protection of a population against a given disease. An example is polio, for which no indigenous cases have been reported in France since 1989, and for which vaccination coverage levels are very high.

Individual Coverage and Population Coverage

At the individual level, a person who is adequately covered by a vaccine (i.e., fully vaccinated) is one who has received the recommended number of doses for their age by a given age. In a population, vaccination coverage refers to the proportion of that population that has been vaccinated. For an infectious disease transmitted strictly from person to person to be controlled or even eliminated through vaccination, it is necessary to achieve a certain level of vaccination coverage, which depends primarily on the disease’s transmissibility. For example, the elimination of measles requires a vaccination coverage rate of 95% among young children. In France, this level has never been reached since the vaccine was added to the routine immunization schedule, which explains the epidemic that caused thousands of cases between 2008 and 2011 and the new epidemic that began in late 2017.

Reported Coverage and Confirmed Coverage

To estimate vaccination coverage, a person’s vaccination status (vaccinated or unvaccinated) is most often determined by information provided by the person themselves, or by their parents for children. This information is provided either orally (e.g., via telephone survey) or in writing (e.g., a questionnaire mailed and completed at home by the person themselves). These are self-reported data, collected in numerous surveys, particularly in large population surveys conducted in France. Their reliability depends on the individual’s ability to recall their vaccination status and to answer truthfully. For this reason, we prefer to use confirmed data, that is, information recorded in a document (vaccination record, health record, etc.). This is the method used in France to estimate vaccination coverage among infants and older children. Confirmed data are more reliable than self-reported data but are not always available or accessible.

In which population groups should vaccination coverage be measured?

Vaccination coverage is rarely measured across the entire population, but rather in specific groups for which vaccination is recommended. Thus, vaccination coverage can be measured:

• in an age group (influenza among people aged 65 and older);

• in an occupational group (hepatitis B among healthcare workers);

• in a social category (BCG in children at high risk of tuberculosis);

• in a geographic region (yellow fever in French Guiana, the only region in France where this vaccination is mandatory).

What are the main sources of vaccination coverage data?

Vaccination coverage can be estimated using various data sources.

• Administrative data reflect the number of vaccinated individuals relative to the population that should have received the vaccination. In France, this method is used annually to estimate influenza vaccination coverage by comparing the number of vouchers for free vaccination coverage used to the number of vouchers sent to the target population for this vaccination by the various health insurance plans.

• In survey studies, often conducted to address a specific need when no other method can provide the required information, data are collected from a sample of the population. The results can then be extrapolated to the entire population when the sample has been selected randomly.

• In some countries—but not in France, where this tool does not exist—vaccination coverage can be estimated using computerized registries that systematically or comprehensively collect vaccination data provided by vaccination staff.

• A fourth data source consists of vaccine sales and reimbursements. Vaccine sales to pharmacies serve as a trend indicator because the lack of a denominator prevents the estimation of vaccination coverage from this source. Conversely, the analysis of vaccine reimbursements in the Health Insurance databases allows for a direct estimation of vaccination coverage. Since 2013, Santé publique France has had access to the nearly exhaustive database of healthcare reimbursements by the National Health Insurance system, the Inter-Regime Consumption Datamart (DCIR), as part of the National Health Data System (SNDS), and routinely uses this data source to monitor vaccination coverage.

What are the main systems used in France to collect data?

In France, vaccination coverage is estimated using various methods that vary by age.

• For infants, the child health certificates issued at 9 and especially 24 months of age by the primary care physician are the main sources of vaccination coverage data. This is a routine system through which vaccination data is reported from the departments and then aggregated and analyzed at the national level.

• For children aged 2 to 15, vaccination coverage is estimated through a cycle of school-based surveys, conducted alternately among children in the final year of preschool (age 6), the second year of middle school (age 11), and the third year of high school (age 15).

• For adults, there is no routine data collection system. Vaccination coverage data come from Santé publique France’s Health Barometers and, in the past, also came from two major population surveys conducted by the Institute for Research and Documentation in Health Economics (IRDES) and the National Institute of Statistics and Economic Studies (INSEE) in close collaboration, for the vaccination component, with Santé publique France. In the future, the assessment of vaccination coverage in this population group will rely on new tools that are currently being evaluated.

Finally, for all age groups, and outside of sample surveys, vaccination coverage is estimated using vaccine reimbursement data, made available to Santé publique France through the health insurance databases. This latter system is particularly useful for quickly detecting changes in vaccination coverage and, in children, for supplementing the data available from health certificates and school-based surveys.