From Infectious Disease Special Edition

Why Do Vaccine-Preventable Disease Outbreaks Occur in the US?

James D. Cherry MD, MSc; Kathleen H. Harriman PhD, MPH

In This Article


Individuals infected with measles generally experience lifelong immunity. In the prevaccine era, approximately 98% of the US population (and other large-population countries) was immune by age 18. As a result, measles outbreaks only involved children and were caused by the annual introduction of a nonimmune birth cohort. The measles vaccine confers lifelong immunity for most recipients; thus, although more than 90% (estimates range from 83% to 95%) of the population must be immune to measles to interrupt transmission,[1,2,3] the United States achieved sufficient community immunity for measles to be declared eliminated in 2000.

Unlike pertussis vaccines, measles vaccines are highly efficacious. Two doses of the current vaccine for MMR (measles, mumps, and rubella) confers lifelong immunity to more than 99% of recipients.[17] Nevertheless, measles remains a concern because of travel into the United States from areas where the disease has not been eliminated, which creates opportunities for outbreaks and sustained transmission. Since the elimination of endemic measles transmission in the United States in 2000, a median of 60 measles cases were reported annually between 2001 and 2010. However, 222 cases of measles were reported in 2011, the majority of which were associated with importation. Many of the patients were unvaccinated or had unknown vaccine status. Of note, nearly half of the measles importations in 2011 occurred among individuals who initially were infected in Europe.[18]

Single-dose coverage with the MMR vaccine among US children aged 19 to 35 months has been higher than 90% since 1996; however, many European countries do not have sufficient community immunity to control transmission due to declining MMR immunization rates. Measles was nearly eliminated in much of Europe, but the disease has staged a comeback in recent years because safety concerns have stopped many parents from vaccinating their children. Although reported cases of measles have declined in Europe in 2012,[19] more than 30,000 cases were reported in 2011. Approximately 15,000 measles infections were reported in France alone, including 714 cases with pneumonia, 16 with encephalitis, and 6 measles-related deaths.[20] Measles will continue to pose a threat to the United States as long as it is persists in other countries, particularly because it has never been controlled throughout many regions of Asia and Africa.

Despite the high overall rate of measles immunity in the United States, pockets of under-vaccinated or unvaccinated individuals, along with the increasing rate of school children whose parents apply for personal belief exemptions, present a cause for concern. Over the past decade in California, the overall rate of personal belief exemptions for children in kindergarten has tripled, rising from 0.77% in 2000 to 2.33% in 2010—some schools reported personal belief exemption rates as high as 84% in 2010.[8]

As the cohort of unvaccinated children transitions into adulthood and travels to regions of the world where measles continues to circulate, the importation of the disease back into the United States will perpetuate. Immunization rates in the United States currently are high enough to prevent the sustained transmission of measles when it is reintroduced into the country. But the pockets of under-vaccinated or unvaccinated individuals pose a threat to those in the community who cannot be vaccinated, such as infants and immunocompromised people. However, if the number of unvaccinated individuals in the United States continues to grow, the country could experience a resurgence of measles similar to that observed in Europe.

We must remember that a resurgence of measles occurred in the United States between 1989 and 1991 that was primarily attributed to low vaccine coverage. During that period, more than 55,000 cases of measles and 123 deaths occurred;[7] 17,000 cases and 70 deaths were reported in the state of California alone.[8]