Comparison of the Wireless-Only and Landline Populations in a Small Pilot Immunization Study

The size of the wireless-telephone-only population has increased in recent years, raising concerns about the accuracy of RDD-based telephone surveys. Previous research on the impact of wireless-only on population estimates has found mixed results. In 2007, the National Immunization Survey (NIS) conducted a pilot study to determine the impact of wireless-only households on childhood vaccination coverage estimates. The NIS is a nationwide, list-assisted random digit dial (RDD) survey to monitor vaccination coverage among children aged 19 to 35 months. The pilot survey collected vaccination information from 99 wireless telephone users in Illinois. This paper compares vaccination coverage estimates of children in landline households, wireless-only households, and combined. We discuss the implications of these results for RDD surveys and, specifically, for the NIS. Given the extremely low response rate and small sample size of this study, the results presented here should be considered speculative, rather than conclusive, evidence.