National Immunization Surveys (NIS)

Overview

NIS is a group of nationwide telephone surveys that provide vaccination coverage rates among children aged 19–35 months (NIS–Child) and adolescents aged 13–17 years (NIS–Teen). Data collection for children aged 19–35 months started in 1994, and data collection for adolescents aged 13–17 years started in 2006.

Coverage

Children aged 19–35 months and adolescents aged 13–17 years in the civilian noninstitutionalized U.S. population are represented in these surveys. Estimates of vaccine-specific coverage are available for the nation, each state, the District of Columbia, Guam, Puerto Rico, U.S. Virgin Islands, and selected urban areas.

Methodology

Each survey within NIS is a nationwide telephone sample survey of households with age-eligible children that uses a two-phase sample design. First, a random-digit-dialing sample of telephone numbers is drawn. When a household with at least one age-eligible child (or adolescent) is contacted, the interviewer collects demographic and access-related information on all children aged 19–35 months or one randomly selected adolescent aged 13–17 years, the mother, and the household, and obtains permission to contact the children’s vaccination providers. Second, identified providers are sent vaccination history questionnaires by mail. Final weighted estimates are adjusted for households without telephones and for nonresponse. All vaccination coverage estimates are based on provider-reported vaccination histories.

Starting in 2011, the NIS sampling frames were expanded from a single-landline frame to dual-landline and cellular telephone sampling frames. This change increased the representativeness of the sample characteristics but had little effect on the final 2011 NIS–Child and NIS–Teen national estimates of vaccination coverage overall and when stratified by poverty status. Complex statistical methods were used to adjust vaccination estimates to account for refusals, households without telephones, and children or adolescents whose vaccination histories could not be verified through their providers. In 2018, NIS shifted from a dual landline and cell-phone frame to a single cell-phone frame sampling design to increase efficiency. Data users should take note that small changes in vaccination coverage may occur with the change in sampling design, and direct comparisons with coverage estimates from previous years should be made with caution. Further information on NIS–Child is available from Transition from a Dual-Frame (Cell-phone and Landline) to a Single-Frame (Cell-Phone) Sample Design: Impact on Vaccination Coverage Estimates, National Immunization Survey-Child, 2014-2018, and on NIS–Teen from: Impact of a methodological change from a dual-frame landline and cell-phone sample design to a single-frame cell-phone sample design on vaccination coverage estimates among adolescents 13-17 years, National Immunization Survey-Teen, 2016-2017.

Sample Size and Response Rate

NIS–Child is currently reporting data by birth year rather than survey year. Data for birth year are based on the 3 subsequent survey years. For example, 2016 birth year data are based on 2017, 2018, and 2019 survey data. Vaccination coverage by age 24 months was estimated using Kaplan-Meier (time-to-event) analysis to account for children who were aged under 24 months on the date vaccination status was assessed.

The overall Council of American Survey Research Organizations (CASRO) survey year response rates were 26.1% in 2017, 24.6% in 2018, and 21.1% in 2019. Also in 2019, 16,365 of the 33,137 eligible children with completed household interviews had adequate provider data (49.4%). The rate is lower than that achieved in 2018 (54.1%) and 2017 (53.9%) due to the COVID-19 pandemic, which forced an early end to Provider Record Check data collection operations—data collection ceased on March 19, 2020, whereas it usually extends into April.

The overall CASRO response rate for the 2019 NIS–Teen was 19.7%. From the cell-phone sample, 18,788 of the 42,668 eligible adolescents with completed household interviews (44.0%) had adequate provider data.

Issues Affecting Interpretation

The estimates are subject to several limitations. Data year 2011 was the first year that a dual-frame sampling scheme included landline and cell-phone households. Starting with data year 2018, the switch from dual landline and cell-phone frame to a single cell-phone frame occurred. Estimates from 2011 through 2017 and 2018 and subsequent years may not be comparable with each other and those before 2011. NIS is a telephone survey, and statistical adjustments may not fully compensate for nonresponse and for households without  telephones or cell phones (for data years 2018 and later). Underestimates of vaccination coverage may have resulted in exclusive use of provider-reported vaccination histories because completeness of records is unknown.

Before January 2009, NIS did not distinguish between Hib vaccine production types; as a result, children who received three doses of a vaccine product that requires four doses were misclassified as fully vaccinated. For more information, see: Santibanez TA, Singleton JA, Shefer A, Cohn A. Changes in measurement of Haemophilus influenzae serotype b (Hib) vaccination coverage—National Immunization Survey, United States, 2009. MMWR Morb Mortal Wkly Rep 59(33):1069–72. 2010.

Starting in 2014, NIS–Teen defined an adolescent’s vaccination record as having adequate provider data if that adolescent had vaccination history data from one or more of the named vaccination providers, or if the parent reported that the adolescent was completely unvaccinated. Before 2014, the adequate provider data definition had more criteria, and it was based on a comparison of provider report of vaccination history with parental report of vaccination history, either by shot card report or recall.

To assess the effect of the change in the adequate provider definition criteria on vaccination coverage estimates, NIS recomputed estimates from the 2006 through 2013 surveys. In general, 2013 NIS–Teen vaccination coverage estimates using the revised adequate provider data definition were different, and generally lower, than original 2013 NIS–Teen estimates. Differences between revised and original 2013 national vaccination estimates ranged from -0.1 percentage point to -2.2 percentage points. For more information, see revised adequate provider data criteria and revised 2013 estimates based on the 2014 criteria. Because of the revision in the adequate provider definition, NIS–Teen vaccination coverage estimates for 2013 and beyond cannot be directly compared with previously published 2006–2013 NIS–Teen survey vaccination coverage estimates based on the previous adequate provider definition.

References

• Hill HA, Yankey D, Elam-Evans LD, Singleton JA, Pingali SC, Santibanez TA. Vaccination coverage by age 24 months among children born in 2016 and 2017—National Immunization Survey-Child, United States, 2017–2019. MMWR Morb Mortal Wkly Rep 69(42):1505–11. 2020.
• Reagan-Steiner S, Yankey D, Jeyarajah J, Elam-Evans LD, Singleton JA, Curtis CR, et al. National, regional, state, and selected local area vaccination coverage among adolescents aged 13–17 years—United States, 2014. MMWR Morb Mortal Wkly Rep 64(29):784–92. 2015.
• Elam-Evans LD, Yankey D, Singleton JA, Sterrett N, Markowitz LE, Williams CL, et al. National, regional, state, and selected local area vaccination coverage among adolescents aged 13–17 years—United States, 2019. MMWR Morb Mortal Wkly Rep 69(33);1109–16. 2020.

For more information, see the NIS website at: ChildVaxView and TeenVaxView.