Measles virus infection is one of the most important infectious diseases of humans and has caused millions of deaths since its emergence as a zoonosis thousands of years ago. Prior to the development and widespread use of measles vaccines, measles was estimated to cause between 5 and 8 million deaths worldwide each year. Remarkable progress in reducing measles incidence and mortality has been made through increased measles vaccine coverage. As an example, in the period between 2000 and 2008, there was substantial decrease in measles cases and measles-associated deaths in sub-Saharan Africa due to intensive efforts at increasing measles vaccination coverage through routine vaccination and provision of a second opportunity for measles vaccination through mass measles vaccination campaigns (called supplementary immunization activities [SIAs]). However starting in 2009, there were large outbreaks in these areas and the cause of this resurgence was attributed to suboptimal vaccine coverage due to complacency with the vaccination efforts, which increased the at-risk susceptible population. In the Americas, intensive immunization and surveillance efforts had stopped endemic transmission of measles virus, in part based on the successful Pan American Health Organization strategy of nationwide measles vaccination campaigns and high routine measles vaccine coverage. However, endemic transmission due to a large sustained outbreak of measles in Brazil in 2014 demonstrates that regional measles elimination can be fragile and dependent on global elimination efforts. In the United States, high coverage with 2 doses of measles vaccine has eliminated endemic measles virus transmission. These achievements attest to the enormous public health significance of measles vaccination.
As measles control efforts are increasingly successful, public perceptions of the risk of measles diminish and are replaced by concerns of possible adverse events associated with measles vaccine. As a consequence, numerous limited measles outbreaks have occurred in communities opposed to vaccination on religious or philosophical grounds or because of unfounded fears of serious adverse events. In 2014, the United States experienced a record number of measles cases, with 667 cases from 27 states reported to the Centers for Disease Control and Prevention (CDC), the greatest number of cases since measles elimination was documented in the United States in 2000. Most of these cases occurred in unvaccinated persons. The source of the 2014 outbreaks was primarily importations from endemic areas of the world, and a significant proportion were importations from travelers returning from the Philippines where a large outbreak had been occurring. In 2015, the United States experienced a large, multistate measles outbreak that originated in amusement parks in Orange County, California. Although the source of this outbreak was not identified, it was likely from a person infected overseas. Once again, the majority of patients with measles in this outbreak were either unvaccinated or had unknown vaccination status (total cases in 2015 were 188).
EPIDEMIOLOGY AND PATHOGENESIS
Measles virus is one of the most contagious human pathogens, and outbreaks can occur in populations in which less than ...