Magnitude of recall bias in the estimation of immunization coverage and its determinants.

OBJECTIVE: To study the magnitude of recall bias in the estimation of immunization coverage and to identify its determinants.
DESIGN: A follow-up study of cohort of children for one year; followed by a cross-sectional recall survey.
SUBJECTS: All live births in two contiguous PHC areas in Villupuram district, Tamil Nadu.
METHODS: The prospective data collected from mothers of 774 children was considered as 'Gold Standard' and the retrospective recall data from them was compared with it. This was carried out for individual immunization schedules, namely, DPT, OPV, BCG and measles and for the combined immunization status. Measures such as sensitivity, specificity, positive and negative predictive values were computed. Logistic Regression technique was employed for evaluating the determinants of agreement.
RESULTS: Less than 50% of mothers has immunization cards with them and more than 70% of the cards did not have complete information. The sensitivity of the recall method was 41.3% and the specificity was 79.5% when the complete immunization status was considered. For individual immunization schedules, the sensitivity ranged from 95% for BCG to 53% for measles vaccination and the specificity ranged from 30% for BCG to 68% for measles vaccination. Mother's age emerged out as a significant determinant in the agreement of two methods. Maintenance of immunization cards were very poor.
CONCLUSIONS: Method of obtaining immunization status through recall survey is not sensitive. Proper maintenance of immunization cards and ensuring the availability of them with mothers for inspection are recommended for obtaining accurate estimation of vaccine coverages.