The continued circulation of measles in a community depends on the generation of susceptible hosts by children's birth. In communities that generate insufficient new hosts, the disease will die out. This concept was first recognized in measles by Bartlett in 1957, who referred to the minimum number of people capable of supporting measles as the critical community size (CCS). Analysis of outbreaks in island communities suggested that the CCS for measles is about 250,000. In order to achieve herd immunity, more than 95% of the community must be vaccinated due to the ease with which measles is transmitted from individual to individual.
Measles is one of the first diseases to reappear when vaccination coverage rates decline. Ongoing measles outbreaks occur in European countries where vaccination coverage rates are lower than those in the United States.
Sustaining elimination requires high MMR vaccine coverage rates, particularly among preschool and school-aged children. High coverage levels provide herd immunity, decreasing everyone's risk for measles exposure and affording protection to individuals who cannot be vaccinated. However, herd immunity does not provide 100% protection, especially in communities with large numbers of unvaccinated individuals.
For the foreseeable future, measles importations into the United States will continue because measles is still prevalent in Europe and other world regions. Within the United States, the current national MMR vaccine coverage rate is adequate to prevent the sustained spread of measles. However, importations of measles likely will continue to cause outbreaks in communities with sizeable clusters of unvaccinated individuals. An outbreak of measles is defined as a chain of transmission with three or more confirmed cases.
A measles case is considered confirmed if it is laboratory-confirmed or meets the clinical case definition, i.e., an illness characterized by a generalized rash lasting 3 or more days, a temperature of ≥ 101°F [≥38.3°C], and cough, coryza, or conjunctivitis and is linked epidemiologically to a confirmed case. Confirmed measles cases in the United States are reported to the CDC by state and local health departments using standard case definitions and case classifications.
Measles cases are reported by state health departments to the CDC, and confirmed cases are reported via the National Notifiable Disease Surveillance System (NNDSS) using standard case definitions.
When the measles vaccine was first licensed in 1963, administering two doses of live-attenuated measles vaccine to children was implemented to prevent school outbreaks. The immunization program resulted in a decrease of more than 99% in the reported incidence of measles.
From 1989 to 1991, a significant resurgence of measles occurred, which affected primarily unvaccinated preschoolers. This measles resurgence resulted in 55,000 cases and 130 deaths. This resurgence prompted the recommendation that the second dose of the measles vaccine is given to preschoolers. This mass vaccination campaign led to the effective elimination of the endemic transmission of the measles virus in the United States.
By 1997-1999, the incidence of measles had been reduced to a historic low (less than 0.5 cases per million individuals). In 2000, endemic measles transmission was declared eliminated in the United States.
In 2004, 34 cases of measles were reported to the CDC. After that all-time low, however, the annual incidence began to increase, with most cases linked either directly or indirectly to international travel. Incomplete vaccination rates facilitated the spread of measles once the virus was imported to the United States.
January 1 - July 31, 2008, remains the highest year-to-date since 1996. 131 cases of measles were reported to the CDC from 15 states. This increase resulted from greater viral transmission after importation into the United States, leading to more importation-associated cases. These importation-associated cases occurred largely among school-aged children eligible for vaccination but whose parents chose not to have them vaccinated.
Summary 2001 through 2011: According to the CDC, cases continued to be caused by the measles virus being brought into the country by travelers from abroad, with spread occurring largely among unvaccinated individuals. 2 out of every 3 individuals who developed measles were unvaccinated. A review by the CDC in 2014 concluded that "the elimination of endemic measles, rubella, and Congenital Rubella Syndrome (CRS) has been sustained in the United States."
January 1 to August 24, 2013: The WHO European Region continues to be the source of imported cases, a popular destination for travelers from the United States and an area where measles continues to circulate. Measles importations were reported by United States residents, most of whom were 6 months or older and unvaccinated.
January 1 to May 23, 2014: 288 confirmed measles cases were reported to the CDC. Of the 288 cases, 280 (97%) were associated with importations from 18 countries. Almost half (22 or 49%) of these importations were travelers returning from the Philippines, where a large outbreak occurred in October 2013. Fifteen outbreaks accounted for 227 (79%) of the 288 cases.
December 2014 to February 2015: The CDPH issued a press release and an Epidemic Information Exchange (Epi-X) notification regarding the California outbreak. This outbreak originated in late December 2014 in Disney theme parks in Orange County, California. As of February 11, 2015, 125 measles cases with a rash had been confirmed in residents of the United States connected with the California outbreak (CDPH, 2015).