Levels and trends of immunization coverage are used to monitor the performance of immunization services at local, national and international levels; to guide eradication, elimination and control strategies for vaccine preventable diseases [8–10]; to identify areas of immunization systems that may require additional resources and focused attention [1, 11]; and to inform decisions as to whether new vaccines should be introduced into national and local immunization systems . Moreover, measles immunization coverage is one of the indicators for tracking progress towards achievement of the Millennium Development Goals .
Although coverage targets should be kept in perspective and not distract from technically appropriate and sustainable immunization programme activities , high levels of immunization coverage are important to prevent and control vaccine-preventable diseases. Our review of national routine immunization coverage estimates demonstrates that substantial progress has been made: more than three-quarters of countries, accounting for more than two-thirds of surviving infants globally in 2010, achieved at least 90% coverage during 2010 or were on track to achieve ≥ 90% coverage for DTP3 and MCV by 2015, and 118 countries maintained DTP3 coverage ≥ 90% for four of the first five years of GIVS. The data suggest that by 2015, 45 countries will not have reached the 90% target for DTP3 (46 countries for measles) if the trend identified for 2005-2010 is correct and maintained. These 45 countries account for an estimated two-thirds of surviving infants unvaccinated with DTP3, most are developing or least developed countries and roughly half are located in Africa. Thus, developing and least developed countries continue to struggle to attain and maintain high coverage levels. Moreover, we identified 28 of 68 priority countries with either insufficient or no progress towards reaching the GIVS goal of 90% coverage for DTP3 and for MCV coverage as of 2010; 22 of the 28 priority countries were located in Africa.
The role of GIVS to address the challenges of vaccination programmes has been described [1, 2, 15]. GIVS provides countries with a mechanism to identify critical areas and resource needs. While many of the activities supported by GIVS began prior to its development, there is some evidence that these strategies (e.g., RED , child health days , among others) are resulting in improved immunization system outcomes. Ultimately, GIVS will be measured by how well countries, particularly the developing and least developed countries, are able to reduce vaccine preventable deaths, the total costs of which have been estimated at US$76 (range: US$23-110) billion including US$49 billion for maintenance of current systems and $27 billion for scaling-up in order to attain the GIVS goal of reducing mortality due to vaccine preventable diseases by two-thirds by 2015 .
The review was based on the 2010 revision of the WHO and UNICEF estimates of national immunization coverage (completed in July 2011). The limitations of these data have been described . Perhaps most importantly, the quality of the estimates is determined by the quality and availability of empirical data. Vaccination coverage is comparatively easy to measure and two methods - administrative reports and surveys - have been developed, each of which, when properly designed and implemented, provides accurate and reliable direct measures of coverage levels. Implemented jointly (using each measure for the same population), they provide a validation of coverage levels. However, both methods are subject to biases. In some instances, these may be identified and corrected. In no instance are the WHO and UNICEF estimates based on complete, consistent, multiple measures for an entire country and vaccine time series. In some instances the WHO and UNICEF estimates are based on complete administrative data validated by periodic or occasional consistent survey findings. In others, data are available from a single source - usually administrative data - and appear internally consistent over time and across vaccines, while in some countries, administrative data and survey results are inconsistent and in others the administrative time series is incomplete, internally inconsistent or both. The method also does not attempt to triangulate coverage levels with disease incidence data. For example, some countries may have high MCV coverage level estimates but suffer at the same time severe measles outbreaks.
The WHO and UNICEF estimates are limited by the absence of any articulation of uncertainty; as presented, they appear equally precise and certain. The uncertainty in the estimates is rooted in the accuracy and precision of the empirical data (described above) and in the choice and application of the heuristics. Because the estimates are not based on a probability sample and multiple measures are not considered as random variants of a single population measure, limiting the uncertainty to the amount of variation in the empirical data is a challenge.
It is also important to emphasize that the WHO and UNICEF estimates reflect (to every extent possible) coverage levels attained through routine immunization system. Some countries may implement immunization programme activities (e.g., child health days  or campaigns) outside of the routine immunization system that target children missed by routine immunization systems. As stated above, such doses are not included in the estimated coverage levels and actual doses administered would therefore be higher than estimated by the coverage estimates.