Recently, there has been increased attention paid to the resurgence of measles in the United States. But despite this uptick, it’s important to remember how effective the measles vaccine has been at reducing the prevalence of the disease. Since 1963, when John F. Enders helped develop the first vaccine against the measles, the number of cases of this respiratory disease has declined considerably. By the end the 1960s, according to Samuel L. Katz, Chairman Emeritus of Pediatrics at Duke University, “annual measles cases had been reduced by more than 90%.” Immediately before then, approximately 400 people died and more than 3 million people were infected by measles every year.
The chart at the top of the page displays the remarkable reduction in the measles incidence rate per 100,000 people in the United States. The incidence rate is defined by the CDC as the number of new cases of a disease over a given period that is then divided by the average population of a locality. So in this cases, the incidence rate measures the number of measles cases reported to the CDC adjusted for each state’s population. The chart is ordered from the states with the highest average incidence rate at the top and the states with the lowest rates at the bottom. I gathered the data for this visualization from Project Tycho, a database maintained by the University of Pittsburgh. The researchers at Project Tycho culled their data from weekly surveillance reports from the National Notifiable Diseases Surveillance System (NNDSS). Specifically, I used Level 1 data, which has been standardized into a common format for easier analysis.
The measles vaccine was so effective that the United States eliminated endemic measles at the end of the 20th century. This does not mean people are no longer infected by the disease; rather, it signifies that indigenous cases of measles are no longer a continuous threat. In 2004, the Task Force for Child Survival and Development determined that from 1997 to 1999, the measles incidence rate fell below 1 case per 1 million people. Most of the 338 cases reported during that period were either imported into the country or could be linked to international strains of the disease. Earlier this year, the elimination of endemic measles was extended into the 21st century. In a study published by JAMA Pediatrics, researchers found that between 2000 and 2011, the measles incidence rate remained below 1 case per 1 million people.
Despite the overall decrease in measles, imported cases of the disease remains an urgent issue — as the CDC announced last month. The CDC noted that during the first five months of 2014, the second most cases of measles were reported since the same period in 1994. Most of the 288 cases reported were imported from abroad — particularly from the Philippines. Between 2001 and 2011, around 89% of the measles cases reported to the CDC were imported from outside the United States. This has become an even larger problem because people are not getting their children vaccinated. The CDC noted that 9 in 10 cases of measles in 2014 were contracted by people who were either not vaccinated or did not know their vaccination status.
Update: I’d like to thank the amazing Susannah Locke for featuring my chart on Vox!