Data investigation: Childhood vaccination rates are backsliding across the U.S.

An NBC News data investigation finds that much of the U.S. currently doesn't have the protection needed to stop the spread of deadly diseases. St. Louis is a window into hurdles communities face nationwide.

This is Part 1 of The Vaccine Divide, a data investigation into how an escalating movement against immunization is challenging public education, widening health disparities and threatening the health of America’s children.

ST. LOUIS — For more than a half-century, vaccines have had remarkable success eradicating the most lethal and devastating childhood infectious diseases, saving millions of lives and ushering in a relative golden era of global public health.

But now, America is dangerously backsliding.

The vast majority of counties across the United States are experiencing declining rates of childhood vaccination and have been for years, according to an NBC News data investigation, the most comprehensive analysis of vaccinations and school exemptions to date.

NBC News investigation finds steep decline in childhood vaccination rates03:45This six-month investigation, done in collaboration with Stanford University, gathered massive amounts of data from state governments and archives of public records reaching back years or decades. The data focused on core childhood vaccines that, together, regard someone to be "up to date" on immunizations; these are the measles, mumps, rubella, polio, whooping cough and diphtheria shots.