Behind the data: Tracking the rate of vaccination and school exemptions

How an NBC News data investigation tracked vaccination and school exemption rates across thousands of counties in the U.S.

This year's lethal spread of measles in rural West Texas and other pockets of the U.S. was a strong warning about how protection against deadly childhood diseases is eroding around the nation.

This was a key conclusion of a six-month NBC News investigation, in collaboration with Stanford University, resulting in the most comprehensive analysis of vaccinations and school exemptions to date.

NBC News gathered massive amounts of data from state governments and archives of public records reaching back years or decades. With the help of infectious disease researchers at Stanford, 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.

The analysis found that a large swath of the U.S. currently does not have the basic, ground-level immunity medical experts believe is necessary to stop the spread of measles, which had once receded into the past.

The idea for this project originated from a conversation about how to cover changes in vaccine policy and opinion between the NBC News Health and Medical unit and Dr. Peter Hotez, a vaccine expert and dean of the National School of Tropical Medicine at Baylor College of Medicine in Dallas.