Tracking the drop in U.S. childhood vaccinations: Our methodology

Editor’s note: This 2026 NBC News data investigation is an update of The Vaccine Divide, which was originally published in September 2025. The methodology for both projects is similar. The new data was collected from June to October 2026.

For “The Vaccine Divide,” NBC News gathered massive amounts of data from state governments and archives of public records. With the help of infectious disease researchers at Stanford University, NBC News filed scores of requests for documents, including materials obtained under the Freedom of Information Act, and wrestled different types of data into a standardized format to map and compare rates across thousands of counties.

Methodology

How NBC News collected the data

NBC News built a database of childhood vaccination and exemption information using data provided by state governments, primarily health agencies. The data combines public information available for some states and data provided directly to NBC News.

The data covers kindergartner vaccinations for each academic year.

After collection, the data was collated, and exemption rates were calculated in order to be analyzed. The analysis focuses on four facets of the data:

1. Medical exemptions: Exemptions granted when students have a medical condition that would make vaccination potentially harmful.
2. Nonmedical exemptions: Exemptions for students who opt out of required vaccinations due to personal beliefs or religious convictions.
3. MMR (measles-mumps-rubella) vaccinations: For states in which data is subdivided by number of doses, MMR vaccination is counted by the second dose. Some states track measles, mumps and rubella immunization separately; in these states, measles was used to represent MMR rates.
4. Up-to-date vaccinations: Students are considered “up to date” with their vaccinations if they have received the full vaccination series required for enrollment in their school.

How the data was processed

With a hodgepodge of formats, jurisdictions and collection practices across states, the task required extensive work to ensure that possible inconsistencies in the data were managed.

• Certain states, such as Ohio and New York, required further calculations to aggregate the state-supplied data.
• Other states required hand input of hundreds of rows of tabular data.
• All data was checked for accuracy.

Analyzing the data

This analysis of childhood MMR vaccination rates focused on states that provided MMR figures for the 2025-2026 school year. Accompanying this analysis were several variant analyses, which confirmed that the results were robust. The variant analyses all yielded similar results, mostly within 1 percentage point. We also examined the scale of MMR vaccination rate decline across counties, with almost 4 in 10 counties declining by over 5 percentage points since 2019 and approximately 1 in 8 declining by that amount since 2024. Fewer than 2% of counties declined by 0.1 percentage points or less since 2019.

Variations in the data

Many states collect and report their data differently. Some use health districts instead of counties. Some states provided over a decade of data; others provided only two years’ worth. Some states redacted certain data due to privacy concerns.

• Not all states have recent data. Florida did not provide data on kindergarten exemptions after the 2021-2022 school year. Montana stopped collecting vaccine and exemption data after 2019, first due to Covid and thereafter due to the passage of a state bill (HB 334) in 2021 that struck down the mandate for the state to collect data on immunizations.
• Certain states (such as Arizona and Minnesota) do not collect an overall “up-to-date” figure representing the percentage of kindergarteners who are compliant with all required vaccines. Others (such as Florida, New Hampshire and South Carolina) provided only “up-to-date” numbers.
• In other states — Mississippi, New Hampshire and South Carolina — data was provided only for years K-12, rather than specifically kindergarteners. West Virginia also falls into this category for its exemption data. Some states — Illinois and New York — do not collect data at grade level, so data is for elementary schools as a whole (up to K-8 for Illinois, and New York was based on headcounts, where elementary schools were defined as having less than 3% enrollment above grades K-6), rather than kindergarteners specifically. Hawaii also fell into this category (up to pre-K-5), as its school-level data was more consistent than its kindergarten-specific data.
• Other states did not provide data on a county level. Alaska and Nebraska provided data for other jurisdictions. Delaware and Rhode Island were unable to provide data at the county level, state-level figures were used instead. Indiana was unable to provide county-level exemption data. West Virginia was unable to provide county-level immunization data, but was able to provide exemption data per county, starting in January 2025.
• Some states (such as Michigan) collect data by calendar year, rather than school year.
• Wyoming did not consistently report data among kindergarteners, so the most consistent reported value was used instead: one-dose MMR among 19- to 35-month-olds. Accordingly, exemptions among children younger than 5 were used to correspond with this age group.
• Alaska was unable to provide regional exemption data for privacy reasons because of small sample sizes.