Coverage Confusion! Trying to Make Sense of It
Vaccination coverage rates are the most commonly
used indicators of immunization program performance.
Have you ever encountered different vaccination coverage
estimates for the same country? In this issue of
Snap Shots, we explain where these different coverage
estimates come from and how they should and should
not be used. We also point you to other references
that we hope will help to clear up the coverage confusion.
DTP3 and Routine Immunization
Although coverage is calcuated for all vaccines,
DTP3 coverage by one year of age is now widely accepted
as the performance indicator for routine immunization.
This is because DTP vaccine is most often delivered
in routine immunization sessions versus campaigns.
Also, completion of the DTP series before the first
birthday has been a serious problem in some countries,
so adoption of the DTP3 indicator is helping to focus
global attention on this problem. We use DTP3 coverage
throughout this issue to describe coverage trends.
Regional Coverage Trends
After increasing rapidly in the 1980s, vaccination
coverage in many countries held steady or decreased
during the 1990s. Since 2000, coverage has slowly
begun to increase again. Globally, WHO/UNICEF estimate
that DTP3 coverage increased from 75% in 2000 to
78% in 2004. The Africa region, where coverage fell
sharply after 1990, experienced the most dramatic
gains (54% in 2000 to 66% in 2004). Coverage in the
Eastern Mediterranean region also improved (81% in
2000 to 86% in 2004), but in Southeast Asia coverage
remained low (69% in 2004). The fact that it has
not changed significantly since 1995 is also cause
Common Sources of Coverage Data
WHO/UNICEF's estimates of DTP3 coverage are used
in Figure 1 above. Other common sources of coverage
data are administrative reports and household surveys.
How do these three sources compare?
- Administrative reports:
National coverage rates are usually based
on administrative data collected during vaccination
sessions. Administrative data are used monthly,
quarterly and annually to estimate coverage,
calculate left-outs and drop-outs, and monitor
trends. Administrative data overestimate
coverage(1) when vaccinations
given to children after their first birthdays
are reported, census figures (coverage denominators)
are too low, or reports are falsified. On
the other hand, they underestimate coverage
when census figures are too high or reporting
- Household surveys:
Demographic and Health Surveys (DHS) and
Multiple Indicator Cluster Surveys (MICS) produce
national coverage estimates, but they are
conducted infrequently and do not, in most
cases, produce district coverage estimates.
The CORE Group's Knowledge, Practice and
Coverage (KPC) surveys produce reliable coverage
estimates and are often used with smaller
Coverage Cluster Surveys, which are appropriate
for use at both district and national level,
provide a wealth of data not only about immunization
coverage but also about service quality.
WHO/UNICEF Joint Reporting Form
- WHO/UNICEF estimates
of coverage: In April each year,
WHO and UNICEF collect coverage estimates
and other information from individual
countries on a Joint
Reporting Form, or JRF. The JRF contains
the official country estimates of coverage,
which are most often based on administrative
reports and census data. WHO and UNICEF
then adjust the official country rates
to reflect findings of population-based
surveys and known data-quality and/or
denominator issues.(2) The
WHO/UNICEF coverage estimates are now
used at global level to track country
performance and measure progress toward
global and regional immunization goals.
Where to Find Country Coverage Estimates
Both the official country estimates and the WHO/UNICEF
estimates of vaccination coverage can be accessed
Vaccine Preventable Diseases Monitoring System.(3) This
interactive web page contains country coverage estimates
since 1980. It also makes available many other items
of information from the annual JRF submissions that
may be useful in assessing country performance (see
box to the right).
Interpreting Coverage Estimates
When interpreting immunization coverage estimates,
keep the following points in mind...
- Coverage rates are averages. High coverage
rates almost always mask pockets of much
lower coverage. Coverage rates also tell
us little on their own about disparities
among socioeconomic groups.(4) To
truly understand a country's coverage situation,
one must have access to subnational coverage
data. To determine whether the poor and other
marginalized groups are being reached with
immunization services, whenever possible,
household surveys should be analyzed by wealth
- Like stocks and mutual funds, even very high
coverage this year does not guarantee the
same coverage next year. In one African country,
national DTP3 coverage has been over 80%
for all but three of the past 10 years, but
only 11% of districts have reported 80% coverage
or higher in each of the last three years.
This suggests frailty in the immunization
system and demonstrates the need to consider
more than one year's data at any given time.
- A coverage estimate is only reliable if the data behind it are of reasonable quality. GAVI Data Quality Audits (DQA) are helping improve data collection and reporting in many countries. Nonetheless, different sources of immunization data often yield different coverage estimates, and all coverage data have certain limitations. For this reason, one must always look at the source of a coverage estimate to figure out what it really means and how it might be used.
Deciding Which Coverage
Estimates to Use
Coverage estimates from different sources are often
used for different purposes. Administrative data
are most readily available at each level of a health
system and, in lieu of more reliable data, they are
routinely used by program managers and donors to
monitor trends in coverage and pinpoint problems.
Surveys generate more reliable estimates of coverage,
but they are carried out less frequently and cannot
be used to monitor trends on a monthly or even an
annual basis. Also, national surveys do not generally
permit district-level estimates, which, as mentioned
above, are critical to understanding country performance.
What Can USAID Missions and Projects Do to Reduce
the Coverage Confusion?
- Use available coverage rates, but also question
them. Look behind the figures to understand
how they were calculated and what they mean.
- Seek information on coverage over time and
in smaller geographical areas. This will
give you the information you need to better
direct scarce resources toward low-performing
- When commissioning or conducting household
surveys, make sure that samples permit coverage
estimates by socioeconomic status and other
units of interest. This will help in identifying
special-needs populations, even in countries
and regions with relatively high immunization
- Support health systems initiatives to improve
the quality of immunization data and teach
health personnel to interpret and use their
own data to improve program performance.
- Visit the WHO
Vaccine Preventable Disesases Monitoring
System web page and become familiar
with the information it contains. This
site makes both official country estimates
and the WHO/UNICEF estimates of immunization
coverage since 1980 available in an interactive
CJ, Shengelia B, Gupta N, Moussavi S, Tandon
A, Thieren M. Validity
of reported vaccination coverage in 45 countries.
Lancet, 2003 Sep 27;362(9389):1022-7
Estimates of National Immunization Coverage,
need for equity-oriented health sector reforms." International
Journal of Epidemiology 2001;30:720-723
Essentials: A Practical Field Guide,
USAID, 2003 Chapter 4, pp.80-81.