With firm vaccination campaigns, the US eliminated measles in 2000. The highly infectious virus was no longer constantly present in the country—no longer endemic. Since then, measles has only popped up when travelers carried it in, spurring mostly small outbreaks—ranging from a few dozen to a few hundred cases each year—that then fizzle out.

But all that may be about to change. With the rise of non-medical vaccine exemptions and delays, the country is backsliding toward endemic measles, Stanford and Baylor College of Medicine researchers warn this week. With extensive disease modeling, the researchers make clear just how close we are to seeing explosive, perhaps unshakeable, outbreaks.

According to results the researchers published in JAMA Pediatrics, a mere five-percent slip in measles-mumps-and-rubella (MMR) vaccination rates among kids aged two to 11 would triple measles cases in this age group and cost $2.1 million in public healthcare costs. And that’s just a small slice of the disease transmission outlook. Kids two to 11 years old only make up about 30 percent of the measles cases in current outbreaks. The number of cases would be much larger if the researchers had sufficient data to model the social mixing and immunization status of adults, teens, and infants under two.

“The results of our study find substantial public health and economic consequences with even minor reductions in MMR coverage due to vaccine hesitancy and directly confront the notion that measles is no longer a threat in the United States,” the authors conclude.

Researchers Nathan Lo and Dr. Peter Hotez were motivated to conduct the study after seeing data showing growing vaccine hesitancy and use of non-medical exemptions—largely due to lies and misinformation about the safety of vaccines and the threat of devastating diseases, such as measles. Currently, about two percent of kids aged two to 11 have a non-medical vaccine exemption.

Measles, in particular, requires vigilant vaccination. The highly infectious virus can linger in the air for hours after a cough or sneeze. Those sickened develop high fevers, rashes, inflamed eyes, and cold-like stuffy nose and cough. But people can spread the infection days before those symptoms appear. About 30 percent develop complications, such as pneumonia, brain swelling, and blindness.

Model disease

To thwart infections, a population must have between 90- and 95-percent vaccine coverage to maintain herd immunity. Many communities and counties in the US are already on the brink of dipping below that range and thereby losing their protection from a case of measles going, well, viral. And there’s room for those vaccination rates to continue to slip. Presently, 18 states allow for personal belief exemptions, and all but two states allow for religious or philosophical exemptions.

To estimate what will happen if vaccine exemptions continue to increase, Lo and Hotez came up with a mathematical model of measles transmission given current exemptions, data on vaccination rates from the Centers for Disease Control and Prevention, and social mixing patterns of kids aged two to 11 (that’s the age group for which they had the most data). They also estimated costs of measles outbreaks, accounting for medical staff wages, laboratory analyses, and outbreak surveillance.

The researchers calibrated and validated their model using past measles transmission and outbreak data from the US, England, and Wales. Then they ratcheted up the vaccine exemption rate—from one percent to eight percent—to see what would happen.

The researchers calculated that each measles case costs about $20,000. Unsurprisingly, the larger the exemption rate, the more cases and the bigger the outbreaks. Eliminating exemptions , on the other hand, would increase national MMR coverage to 95 percent, well within the range needed for herd immunity.

The researchers are hopeful that the data will inform policies on vaccine exemptions. “Every year, an increasing number of states are debating non-medical exemptions, which are a critical driver of vaccination coverage,” Lo said in a statement. “This study quantifies the consequences of a rise in measles cases and state dollars that will be spent if personal belief exemptions that can reduce vaccine coverage are in place.”

JAMA Pediatrics, 2017. DOI:10.1001/jamapediatrics.2017.1695 (About DOIs).