Estimation of Measles Vaccination Coverage Using the Lot Quality Assurance Sampling (LQAS) Method --- Tamilnadu, India, 2002--2003
Saravanan Sivasankaran,1,2 P.
Manickam,1 R. Ramakrishnan,1 Y.
Hutin,1,3 M.D. Gupte1
Corresponding author: Saravanan Sivasankaran, Surveillance Medical Officer, National Polio Surveillance Project, 30 Chellan Nagar, Pondicherry, India 605011. Telephone: 91-413-2211113; Fax: 91-413-2354925; E-mail: firstname.lastname@example.org.
Disclosure of relationship: The contributors of this report have disclosed that they have no financial interest, relationship, affiliation, or other association with any organization that might represent a conflict of interest. In addition, this report does not contain any discussion of unlabeled use of commercial products or products for investigational use.
Introduction: As part of the global strategic plan to reduce the number of measles deaths in India, the state of Tamilnadu aims at >95% measles vaccination coverage. A study was conducted to measure overall coverage levels for the Poondi Primary Health Center (PPHC), a rural health-care facility in Tiruvallur District, and to determine whether any of the PPHC's six health subcenters had coverage levels <95%.
Methods: The Lot Quality Assurance Sampling (LQAS) method was used to identify health subcenters in the PPHC area with measles vaccination coverage levels <95% among children aged 12--23 months. Lemeshow and Taber sampling plans were used to determine that the measles vaccination status of 73 children aged 12--23 months had to be assessed in each health subcenter coverage area, with a 5% level of significance and a decision value of two. If more than two children were unvaccinated, the null hypothesis (i.e., that coverage in the health subcenter was low [<95%]) was not rejected. If the number of unvaccinated children was two or fewer, the null hypothesis was rejected, and coverage in the subcenter was considered to be good (i.e., >95%). All data were pooled in a stratified sample to estimate overall total coverage in the PPHC area.
Results: For two (33.3%) of the six health subcenters, more than two children were unvaccinated (i.e., coverage was <95%). Combining results from all six health subcenters generated a coverage estimate of 97.7% (95% confidence interval = 95.7--98.8) on the basis of 428 (97.7%) of 438 children identified as vaccinated.
Conclusion: LQAS techniques proved useful in identifying small health areas with lower vaccination coverage, which helps to target interventions. Monthly review of vaccination coverage by subcenter and village is recommended to identify pockets of unvaccinated children and to maintain uniform high coverage in the PPHC area.
The goal of the World Health Organization's global measles strategic plan is to reduce by half the estimated number of measles deaths by 2005 compared with 1999 estimates (1). The target goal set by the Indian Ministry of Health and Family Welfare is to reduce measles mortality 66% by 2010 compared with 2000 estimates (2). Increasing, sustaining, and documenting high routine coverage is essential to achieve sustainable reduction of measles mortality.
The southern Indian state of Tamilnadu has set three objectives for its measles control program: 1) measles vaccination coverage of >95% through routine vaccination of children aged 9--12 months, 2) ring vaccination after occurrence of measles cases, and 3) treatment of ill persons to prevent complications. To evaluate progress toward achieving the first objective, the Field Epidemiology Training Programme and local public health officials estimated coverage levels by using a Lot Quality Assurance Sampling (LQAS) survey (3). This evaluation was conducted for fiscal year 2002--2003 (April 1, 2002--March 31, 2003) to identify health subcenters with measles vaccination coverage of <95% among children aged 12--23 months and to estimate the overall measles vaccination coverage in the same age group in the primary health center area.
LQAS is a stratified random sampling method in which small samples randomly selected from each stratum are used to determine whether coverage in a stratum exceeds a specific threshold. Compared with the commonly used cluster survey technique, LQAS can be used to identify areas with low vaccination coverage (3). The operational feasibility of LQAS to evaluate vaccination coverage among limited population units in India has been demonstrated (4,5).
In April 2003, the LQAS survey was conducted in the Poondi Primary Health Center (PPHC) area, Tiruvallur District, Tamilnadu, India. This primary health center serves approximately 27,000 persons distributed in six health subcenter areas (approximately 4,500 persons per health subcenter).*
A simple random sample of the population served by PPHC was used, divided into six strata representing the six health subcenters. District health authorities expected measles coverage in the PPHC area to be >95%. A decision value (d) of two nonimmunized children and an acceptable alpha value of 0.05 were set. On the basis of this information, sample size (n) was estimated by using Lemeshow and Taber LQAS tables (6). A plan with n = 73 was used; this single-stage sampling plan accepted an alpha error of 5%. In each health subcenter, the total number of eligible children (i.e., those aged 12--23 months) available was assumed to be 100. Within each stratum, 73 children were assessed for measles vaccination status, for an overall sample of 438 children in the PPHC area.
Each health subcenter included multiple villages. To determine the number of children to be selected in each village in each stratum, a list of villages was constructed for each stratum with the number of households and the cumulative number of households; 73 random numbers were selected using random number tables. Once the number of children had been determined for each village, researchers randomly selected as many households as children were needed. For each selected household, any eligible child was included. When no eligible child was identified in the house, the next houses to the right were surveyed until an eligible child was identified. When more than one eligible child was present in a house, only the youngest one was included. Children's ages were estimated using birth certificates or any other records showing the date of birth. When no written documentation was available, the age given by the mother or the guardian was used.
Only those children who received measles vaccination at age 9--12 months were considered vaccinated. The measles vaccination status of the child was obtained by reviewing vaccination cards if available or by interviewing the mother or guardian when cards were not available. Children were excluded from the survey if a parent or guardian was not available.
Analysis of the Survey
Two analyses were conducted, an LQAS analysis in each stratum and a pooled analysis for the total sample. Card availability in subcenters varied (range: 7%--37%). The LQAS analysis in each stratum was based on testing a hypothesis. The null hypothesis (Ho) was that the coverage in the health subcenter was <95% (Ho: p<95%, low performance). The alternate hypothesis (Ha) was that the coverage in the health subcenter was >95% (Ha: p>95%, high performance). In each stratum of 73 persons, two (2.7%) persons were considered as the decision value (i.e., the threshold for analysis). In practice, when more than two of 73 children were identified as unvaccinated, coverage in the stratum was considered to be <95%. If no more than two unvaccinated children were identified, coverage in the stratum was considered to be >95%. All data were pooled to estimate overall total PPHC coverage. Overall measles vaccination coverage was calculated from the total number of eligible children vaccinated compared with the total number of eligible children surveyed. Because this was a simple random sample, with no design effect, best estimates and 95% confidence intervals (CIs) were calculated using standard methods.
In two health subcenters (Neyveli and Vellathukkottai), more than two children were unvaccinated, and vaccination coverage was considered to be <95% (Table). In the other four subcenters (Chitampakkam, Meyyur, Nambakkam, and Poondi), the number of unvaccinated children did not exceed two, and vaccination coverage was considered to be >95% (Table).
The pooled analysis was made on the basis of an overall sample of 438 children (73 from each subcenter). Of these, 428 (97.7%) had been vaccinated against measles, and overall measles vaccination coverage in Poondi was considered to be 97.7% (CI = 95.7%--98.8%) (Table).
This study determined that the overall vaccination coverage level in the PPHC area was 97.7%, which is consistent with the state target level of >95%. Two (33.3%) subcenters had coverage levels below the target level of >95%; these results were used to target interventions to these low-coverage areas.
LQAS techniques provide a rapid and simple determination of output quality and are used in industry for quality-assurance purposes. The strategy and goals of LQAS in the health field are similar to those in the manufacturing field (7). LQAS analysis is based on testing a hypothesis rather than on estimating a proportion. Because LQAS is based on stratified random sampling, results from lot samples can be combined to obtain a point estimate for the entire population, allowing for a small sample size. LQAS procedures were useful for identifying small health areas with lower measles vaccination coverage. This information, combined with further assessment of performance problems and timely corrective action, has been used to improve vaccination coverage in the district. LQAS techniques also could be used to assess performance as part of routine monitoring or supervisory activities of routine vaccination.
For a vaccination program to achieve its goal, a sufficient number of doses must be administered at the appropriate ages. Coverage levels are therefore a key process indicator of performance. Monitoring this indicator at the population level provides an overall assessment of program performance. Operational units with poorer coverage should be identified so performance can be improved (8). LQAS techniques are a particularly useful way of monitoring indicators of coverage, as these techniques provide a rapid and simple determination of output quality.
On the basis of the results of the study, an analysis of vaccination coverage by health subcenter and village during regular monthly review meetings was recommended to identify missed pockets of unvaccinated children and to continue efforts to maintain uniform high vaccination coverage in the PPHC area. The recommendation was followed by all the medical officers and helped them identify groups of unvaccinated children and improve the measles vaccination coverage. No outbreaks of measles were reported subsequently in the study area. However, measles surveillance was limited by the unavailability of long-term trend data, and the validity of the surveillance system has not been estimated.
The findings of this study are subject to at least one limitation. Vaccination cards were available for only 91 (21%) of 438 children surveyed, which could have resulted in overestimating measles vaccination coverage. The supply of vaccination cards was limited, and those parents who did receive cards often did not preserve them. To aid in future surveys, a sufficient quantity of vaccination cards should be made available to health workers in each health subcenter. This will facilitate monitoring and evaluation efforts. Health workers should be trained to understand the importance of the cards and how to use them properly, and families should be educated to understand the need to keep the cards in a safe place.
In December 2004, an outbreak of measles was reported in the Cuddalore district of Tamilnadu. Estimated measles vaccination coverage was approximately 96% (9), indicating that a measles outbreak can occur among a well-vaccinated population when a single-dose measles vaccination strategy is employed. A substantial measles outbreak also was reported during 1999--2000 in Sri Lanka, where single-dose measles vaccination coverage since 1996 was >90% (10). For future measles outbreaks in an area with a single-dose measles schedule to be prevented, a possible strategy to provide a second dose might be considered.
* A primary health center is a basic health unit staffed by a medical officer and health team that provides integrated curative and preventive health-care services to a rural population of approximately 20,000--30,000 persons. A health subcenter is a peripheral outpost staffed by an auxiliary nurse midwife who provides primary health-care services (e.g., mother-and-child care, family planning, and vaccination) for a population of approximately 3,000--5,000 persons.
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Date last reviewed: 4/6/2006