This post has been updated.
Public health officials have long known that the United States has pockets of vulnerability where the risk of measles and other vaccine-preventable childhood diseases is higher because parents hesitate or refuse to get their children immunized.
Eighteen states allow parents to opt their children out of school immunization requirements for nonmedical reasons, with exemptions for religious or philosophical beliefs. And in two-thirds of those states, a comprehensive new analysis finds a rising number of kindergartners who have not been vaccinated.
In a report published Tuesday in PLOS Medicine, researchers from several Texas academic centers tracked the increasing number of children with exemptions in all 18 states from the 2009-2010 to 2016-2017 school years. They characterized many rural counties, as well as urban areas, as “hotspots” because their high exemption rates put them at risk for epidemics of measles, whooping cough and other pediatric infectious diseases.
Idaho has eight of the top 10 counties with the highest exemption rates, the study found. In Camas County in southern Idaho, nearly 27 percent of the kindergarten population opted out of childhood vaccinations in the 2016-2017 school year, according to the study.
Generally, the 10 counties with the highest exemption rates have fewer than 50,000 people and are in rural areas.
But researchers also ranked urban areas. Among counties where more than 400 kindergartners received nonmedical exemptions in 2016-2017, the 15 metropolitan locations with the most exemptions were Seattle, Spokane and Tacoma, Wash.; Portland, Ore.; Phoenix; Salt Lake City and Provo, Utah; Houston, Fort Worth, Austin and Plano, Tex.; Detroit, Troy and Warren, Mich.; and Kansas City, Mo.
“We were able to identify some scary trends that were happening,” said Peter Hotez, dean of the National School of Tropical Medicine at the Baylor College of Medicine and one of the study authors.
The study initially included Allegheny County in Pennsylvania as one of the hot spots. But after the study was published, Pennsylvania officials noticed discrepancies in the county data. Researchers reviewed their data analysis and updated their ranking to remove Allegheny, which includes Pittsburgh, and add Pierce County in Washington, which includes Tacoma.
Federal health officials have said immunization rates have not changed much at national and state levels, Hotez noted. But the new county-level data shows conditions similar to those that resulted in recent measles outbreaks in California and Minnesota, he said. In such areas, anti-vaccination proponents have mounted aggressive and well-funded efforts.
Hotez said he did not know what factors were behind the high exemption rates in some of the rural places; researchers are hoping to conduct a follow-up study on social and demographic factors. But the findings, he said, should prompt federal health agencies such as the Centers for Disease Control and Prevention to undertake investigations.
The high numbers of exemptions in those densely populated urban centers “suggest that outbreaks of vaccine-preventable diseases could either originate from or spread rapidly throughout these populations of unimmunized, unprotected children,” the authors conclude in the report. Many communities are in cities with busy international airports, further increasing the risk of disease spread.
Saad Omer, a professor of global health, epidemiology and pediatrics at Emory University, said the analysis provides a deeper understanding of where pockets of vulnerability exist. “We may have known about hot spots, but this is a little more systematic look,” he said.
The analysis also shows a direct correlation between exemption rates and vaccination rates for the measles, mumps and rubella (MMR) vaccine and suggests other issues, such as access, are less of a factor in influencing vaccine coverage, according to Omer. Overall, the study found that states with more nonmedical exemptions had lower MMR vaccination rates. In contrast, the three states that have banned nonmedical exemptions — Mississippi, California and West Virginia — show the highest MMR vaccine uptake and lowest incidence of vaccine-preventable diseases.
After the 2014-2015 Disneyland measles outbreak was linked to low MMR vaccination coverage of children, California state legislators sought to increase vaccination rates by eliminating philosophical exemptions. In just one year, vaccination rates increased.
Although vaccination rates remain high overall nationally, some communities' vaccine coverage is slipping below the level of 90 to 95 percent that experts say is needed to prevent outbreaks.
“It’s alarming to see the rise in exemption rates across this country, putting communities at greater risk,” said Amy Pisani, director of the vaccine-advocacy organization Every Child By Two. “Parents need to understand that timely vaccines are critical to protecting children's health and should be at the top of the family’s to-do list.”
People who don’t get vaccinated are the most likely cause of the steady increase in the rate of measles and major outbreaks in the United States, according to a CDC analysis last fall.
Although public health officials declared measles eliminated from the United States in 2000, unvaccinated Americans or foreign visitors get measles every year while they are in other countries and bring the virus into the United States. Measles is one of the most contagious diseases on Earth, spreading when an infected person coughs or sneezes.
A child with a nonmedical exemption from the MMR vaccine is 35 times as likely to contract the disease as a vaccinated youngster, studies have shown. A 2016 study found a neurological disorder that is a complication of measles, killing children years after they have been infected, is more common than previously thought. The disorder lies dormant for years and then is 100 percent fatal. Researchers do not know what causes the virus to reactivate, and there is no cure once it does.
The first MMR dose is typically administered at 12 to 15 months of age. Babies may be vaccinated at 6 months or older if they are at risk of exposure to measles — for instance, if they are traveling to an area with an outbreak.
Clarification: A previous version of this post included a different list of cities from the researchers. They subsequently corrected their underlying data and revised their maps.