In mid-May, the Maryland Department of Health and Mental Hygiene confirmed that a patient had been diagnosed with measles after returning to the United States from overseas. Prior to diagnosis, the individual visited numerous public establishments—including two emergency departments—which has raised concerns of an outbreak in the greater D.C. area. While measles is no longer endemic in the U.S., unvaccinated travelers who acquire the disease abroad occasionally re-introduce the virus back into communities in the U.S., which can lead to outbreaks if exposed individuals have not been vaccinated. Events such as these have led to a number of small and large outbreaks since the official elimination of measles in the U.S. in 2000, resulting in annual case counts ranging from 37 cases in 2004 to 667 cases in 2014. According to the U.S. Centers for Disease Control and Prevention (CDC), this year alone has already seen 61 cases of measles from 10 different states (as of April 22). This number will certainly increase given the rising number of cases resulting from an outbreak in Minnesota, which reported 68 confirmed cases as of May 24.

While measles case numbers have fluctuated over the years, the greatest number of measles cases reported in the U.S. since its elimination occurred in 2014, when a record 667 cases from 27 states (across 23 total outbreaks) were reported to the CDC. One of these outbreaks accounted for over half of the total cases (383 in total), occurring primarily among an unvaccinated Amish community in Ohio. 2011 also saw a large spike in the number of measles cases (220 total), many of which were brought to the U.S. from France, which had been experiencing a large outbreak at that time. Perhaps the most memorable outbreak to impact the U.S. recently was a multistate outbreak that originated at Disneyland in California in 2015. While the source of the outbreak has not been identified, the virus genotype was found to be identical to the strain that caused a large measles outbreak in the Philippines in 2014, likely brought to the park by a foreign traveler.

Measles can lead to severe complications, including pneumonia and encephalitis, and one to two children out of 1,000 who get the disease will die. The measles vaccine—MMR, which also protects against mumps and rubella—has been proven safe and is 97% effective at preventing the disease. However, many caregivers have decided against vaccinating their children for a variety of personal reasons, and currently only 91.5% of children aged 19-35 months in the U.S. have received one or more doses of the vaccine. Low vaccination coverage rates can be particularly dangerous for individuals who cannot receive the vaccine due to medical conditions (eg, immunocompromised) and rely on herd immunity to keep them safe from diseases like measles. One recent literature review found that “a substantial proportion of the US measles cases in the era after elimination were intentionally unvaccinated.” While medical exemptions account for some of the unvaccinated populations, others who are vaccine-eligible go unvaccinated due to religious or philosophical reasons. For example, the current outbreak in Minnesota is largely concentrated in a Somali community, which has recently seen an increase in vaccine resistance due to autism fears. Skepticism of the vaccine began after their discovery of Andrew Wakefield’s now discredited theory that the MMR vaccine causes autism. In another example, an intentionally unvaccinated traveler returning to New York led to a large outbreak of measles in a orthodox Jewish community in 2013 that “was propagated by a few extended families that either refused the measles, mumps and rubella (MMR) vaccine or delayed receipt of the vaccine.”

In addition to the health impacts of measles outbreaks, they can have significant implications for the public health institutions that must respond. Outbreak responses require enormous resources given the high transmissibility of measles and the potential for severe consequences for those infected. These responses may require coordination across multiple states and may involve isolating a case, tracing their contacts and contacts’ vaccination status, testing contacts for immunity, and vaccinating or quarantining contacts, as necessary. Additionally, surveillance efforts may need to be enhanced, and increased coordination between healthcare providers, public health departments and the CDC is often required to ensure an effective response. One study estimated the personnel time and costs required to respond to measles outbreaks in 2011, finding that the “total number of personnel hours for the 16 outbreaks ranged from 42,645 to 83,133 and the corresponding total estimated costs for the public response accrued to local and state public health departments ranged from $2.7 million to $5.3 million U.S. dollars.” Another study found that it cost approximately $25,000 dollars to respond to a single preventable case of refugee-imported measles in Kentucky. These figures demonstrate that outbreaks will come at a cost even to those who choose/are able to vaccinate. To prevent further outbreaks and downstream economic impacts, efforts must continue to increase awareness and acceptance of the measles vaccine in all U.S. communities. While it may be difficult to prevent measles-infected individuals from entering into the U.S., having a highly vaccinated population and public health and healthcare systems with ample resources to support a response could ensure that no large outbreaks result.