In a somewhat frightening illustration of anti-vaccine trends, a new report estimates that among groups affected in the recent measles outbreak, the rates of measles-mumps-rubella immunization might have been as low as 50 percent.
Earlier this month, a report published in JAMA Pediatrics concluded that MMR vaccination rates in many of the populations affected by the Disneyland-related measles outbreak are well below the necessary numbers to maintain herd immunity. Led by researchers at Boston Children’s Informatics Program, the project used disease data from the California Department of Public Health as well as current and historical data from the disease surveillance system known as HealthMap to estimate vaccination rates among the recent outbreak clusters. Researchers then used estimates of disease transmission among fully susceptible as well as immune populations to calculate their findings.
According to the Centers for Disease Control and Prevention, from January to March 20, 178 cases of measles have been reported in 17 states and Washington, D.C. The majority of the cases — 74 percent — were linked to the multistate outbreak traced back to Disneyland in Anaheim, California. CDC reports that the majority of those who have contracted the virus were not vaccinated. The JAMA Pediatrics report puts that point into even clearer focus, finding that inadequate vaccination rates were likely fueling the outbreak. (This may seem like a no-brainer, as public health officials made numerous pleas for people to get vaccinated in the wake of the outbreak. But in the world of public health, associations are rarely made without first seeing the science. This report is the first to positively link measles vaccination rates and the current outbreak.)
“Our data tell us a very straightforward story — that the way to stop this and future measles outbreaks is through vaccination,” said report co-author John Brownstein, a digital epidemiologist at Harvard and co-founder of HealthMap, in a news release. “The fundamental reason why we’re seeing the number of cases we are is inadequate vaccine coverage among the exposed.”
The report found that within measles outbreak clusters in California, Arizona and Illinois, the measles vaccination rate is likely between 50 percent and 86 percent. Both rates are well below what’s needed to protect the larger population. Because measles is so contagious, the report estimates that a vaccine rate of 96 percent to 99 percent is needed to preserve herd immunity and prevent future measles outbreaks. The report authors noted that their estimates only reflect vaccination rates among the exposed populations within each outbreak cluster and do not reflect nationwide rates or the overall rate for California. Report authors Brownstein, Maimuna Majumder, Emily Cohn, Sumiko Mekaru and Jane Huston write:
While data on MMR vaccination rates are available, coverage is often calculated at the state or county level and may not be granular enough to assess risk in an outbreak situation; this is especially the case for outbreaks originating at a tourist destination, where vaccination coverage among visitors is highly heterogeneous. Clearly, MMR vaccination rates in many of the communities that have been affected by this outbreak fall below the necessary threshold to sustain herd immunity, thus placing the greater population at risk as well.
In conjunction with the JAMA Pediatrics report, researchers at HealthMap also put together a dynamic model that shows how different vaccination rates affect the growth of a measles outbreak. The model shows that if a population is fully immunized against measles, one case of measles will result in only two additional cases within 70 days of the start of an outbreak. Using that same scenario and time period, the model found that a 90 percent vaccination rate would lead to 29 cases, an 80 percent vaccination rate would lead to just more than 200 cases, a 70 percent coverage rate would result in about 900 cases, and a 60 percent vaccination rate would lead to more than 2,800 cases.
Kim Krisberg is a freelance public health writer living in Austin, Texas, and has been writing about public health for more than a decade.