On the surface, measles looks like a success story for the U.S. childhood immunization program, with vaccination rates topping targets in each of the past 5 years. Yet 2013 has seen a spate of measles outbreaks, resulting in one of the worst years since the U.S. declared it had eliminated the disease in 2000.
The seeming discrepancy between the immunization rate and prevalence of disease is a consequence of how the 10% of people who decline the measles vaccine are distributed. Most of the country is above the 90% immunization target, but in certain communities the vaccination rate is much lower. When a virus is introduced into these unprotected populations, it can cause several cases of measles in a short period of time.
This is what happened in the three big measles outbreaks of 2013, which between them account for nearly two-thirds of U.S. cases this year. "Clusters of people with like-minded beliefs leading them to forgo vaccines can leave them susceptible to outbreaks when measles is imported from elsewhere," National Center for Immunization and Respiratory Diseases director Dr. Anne Schuchat told reporters. While 2013 has reemphasized the risks faced by such clusters, similar events have happened for years. In 1999, the biggest measles outbreak involved a church community which became infected when its minister returned from traveling overseas.
In these outbreaks, the insular nature of the affected community and high immunization rate in the wider population stopped the virus from spreading. This is one reason the Center for Disease Control and Prevention (CDC) wants to maintain uptake of the MMR vaccine, which it sources from Merck ($MRK). If the proportion of people declining vaccines rises, expanding the unprotected pockets and lowering immunization rates in the broader population, the U.S. could again suffer from sustained, large-scale outbreaks.