Feb. 7, 2019 -- The continued spread of measles begs the questions: Where will it spread next? And when will it stop?
Just 6 weeks into 2019, at least nine U.S. states have confirmed cases of the disease, according to the CDC. Most of those states report multiple cases. The largest outbreaks are in Washington, where there are 50 confirmed cases and 11 more suspected, and in New York, which has confirmed at least 200 cases in five counties. It’s the largest outbreak in that state in decades.
“We are going to have more cases. That is clear,” says William Schaffner, MD, a professor in the division of infectious diseases and an expert in immunization policy at Vanderbilt University Medical Center in Nashville. “Measles is probably the single most transmissible virus that we have, so you need very high rates of immunization to end an outbreak.”
A Highly Contagious Virus
Before the measles vaccine, about 3 to 4 million people got the virus every year in the United States. About 400 to 500 of them died.
Among unvaccinated people, when one person has measles, 9 out of 10 people of all ages around the infected person will get it, too. If someone who has measles coughs or sneezes into the air, others can get infected when they breathe in the air around that person or touch a contaminated surface and then touch their eyes, nose, or mouth. To make matters worse, the virus can linger in the air or on surfaces for 2 hours after that cough or sneeze. Because you can transmit measles up to 4 days before the telltale rash appears, contagious people could spread the virus in public places before they know they are sick.
Clusters of Susceptibles
The measles-mumps-rubella (MMR) vaccine is 97% effective against measles after the 2nd dose. “It provides lifelong protection,” Schaffner says. “It’s one of our very best vaccines we’ve ever developed.”
High vaccination rates led the United States to declare measles eliminated in 2000. That means the virus no longer circulates in the country but that it could still come in from abroad. But expanding exemption policies in a number of states, through which parents can opt their children out of vaccines based on personal beliefs rather than medical causes, have led to declining immunization rates among children today. In a 2016 study of 1,416 measles cases that occurred after 2000, the researchers found that a disproportionate number of infected people, or their parents, had voluntarily refused vaccination rather than abstain for legitimate medical reasons.
“We anticipated importation of measles from other parts of the world,” Schaffner says, “but it has gained a foothold again because there are communities where a substantial proportion of children remain unimmunized.”
There are relatively few unvaccinated children in the country. Among kindergartners, the vaccine rate for two doses of the MMR vaccine that includes measles is 94.3%. If you took the remaining unvaccinated children and dispersed them equally around the country among vaccinated children, herd immunity would prevail: The unvaccinated child wouldn’t be exposed to measles because everyone around him or her would be protected. But unvaccinated children tend to cluster together for many reasons. For example, if one child in a small town had a bad reaction to a vaccine, word would spread around the community and many parents might then seek exemptions for their own children.
Clusters of unvaccinated people get dangerous when they also interact with the general population. “It’s a function of both clustering and interaction with the public,” says Saad Omer, PhD, a professor of epidemiology and pediatrics at Emory University in Atlanta. “There are some more isolated, unvaccinated communities, for example the Amish, where we haven’t seen a major outbreak.”
But when children who are unvaccinated shop in the same stores and go to the same events as everyone else, that’s when they can get exposed. The current Washington outbreak has been traced back to someone from outside the country. The people who have since contracted the virus have reported visiting crowded public places including Ikea, Costco, and a Trailblazers basketball game in nearby Portland, OR.
“Clusters of vaccine refusers are serving as tinder for these fires,” Omer says.
More Cases to Come
The perfect storm of unvaccinated communities and a highly contagious disease mean more cases are certain.
Experts can’t say for sure where the next outbreak might be. But they have their eyes on the 17 states, including many already dealing with outbreaks today, that allow vaccine exemptions for both religious and personal beliefs -- expanding the pool of unvaccinated beyond those who are simply medically ineligible for them.
“Regardless of where the next outbreak is reported, we do know that these clusters are high-risk areas where public health should focus,” Omer says.
At 7.5%, Oregon has the highest rate of vaccine exemptions, according to CDC data. Other states with high rates include Idaho (6.7%), Alaska (6.1%), Arizona (5.3%), and Wisconsin and Utah (5.2%). Some states already dealing with measles expect additional cases, too. “This is a changing situation and we would not be surprised if we see more cases in Oregon," says Daisy Hernandez, a spokesperson for the Oregon Health Authority.
Hernandez says their single case is linked to the Washington outbreak.
Regaining Our Herd Immunity
As the number of unvaccinated children continues to grow, the immunized herd loses strength. “We are hovering close to the threshold of (losing) herd immunity in the U.S.,” Omer says. A loss of herd immunity, he says, would mean larger and more frequent measles outbreaks. The only way to stop the outbreaks is to increase vaccination rates.
State and local health officials in New York have taken a multi-pronged approach to increase vaccine rates and stop the outbreak. An estimated 6,000 unvaccinated children have been kept out of schools that are either located in close proximity to cases or that have vaccination rates below 95%. Numerous vaccination clinics, held in response to the outbreak, have allowed an additional 14,000 people to get the MMR vaccine and avoid school exclusion.
California, Mississippi, and West Virginia keep their vaccination rates high -- the highest vaccination rates in the country -- as the only states that do not allow religious or personal exemptions. “Those are states that are unlikely to have an outbreak,” Schaffner says.
But states don’t have to ban the exemptions outright. Research shows that when parents must jump a few hurdles to get a vaccine exemption, vaccine rates rise and vaccine-preventable disease rates go down. “I’m a big believer in the Goldilocks approach to vaccine legislation,” Omer says, “which is that getting an exemption should not be easier than getting a vaccine.”