Since measles was eliminated from the United States in 2000, efforts to control reintroduction outbreaks have faced a variety of challenges. An outbreak in California was largely isolated to Los Angeles County and primarily affected a single social group in which anti-vaccination beliefs and behaviors prevailed. The outbreak resulted from a single individual who acquired infection from an unknown source, developed rash on Dec. 2, 2016, and subsequently exposed siblings and other contacts. This produced a clear chain of transmission. In total, 24 cases resulted from this outbreak, with 18 occurring in Los Angeles County and six occurring in nearby jurisdictions.
The need for early engagement with influential community leaders will be essential to suppressing outbreaks. Identifying additional measures as well as the need for community engagement to close identified vaccination gaps will be critical to limiting the impact of future outbreaks in the United States. Public beliefs and attitudes about vaccinations, in general, and measles vaccination, in particular, have changed over time. While the high effectiveness of two doses of measles vaccine is widely accepted, resistance to vaccination remains consistent and strong. In Los Angeles County, prior to the implementation of SB 277 in July 2016 (which eliminated religious and personal belief exemptions from vaccination requirements to attend school), school vaccination rates varied widely, largely due to high variance in parental use of the personal belief exemption. During the 2016-17 outbreak, cases occurred primarily in older-age adolescents, most of whom were beyond California’s mandatory vaccination requirements, which apply only to new school entrants or those entering the seventh grade. The primary driver of the spread of this outbreak was the increase of disease transmission in families where many children were unvaccinated.
In addition to refusals of vaccination, information-sharing was difficult. Members of the affected social group who had developed measles were reluctant to disclose information about their contacts during interviews with health department staff for fear of being stigmatized. Similarly, one out of three affected schools refused to provide a list of unvaccinated students in a timely manner, thwarting efforts to offer preventive treatment and quarantine unvaccinated contacts. To some extent, trusted spokespersons from within the community played a role in convincing some members of the community to disclose information to the health department, but this was limited.
There were a variety of challenges associated with this response. The number of contacts exceeded 2,000, which taxed both the health department’s ability to conduct contact interviews (to determine vaccination status) as well as its ability to analyze a large amount of information to identify and respond to any emerging difficulties. Some exposures occurred in healthcare facilities, requiring the health department to provide guidance and technical assistance to other health professionals who were unfamiliar with public health practices. In addition, laboratory testing, while available, required substantial resources and necessarily took time. Finally, because Los Angeles County’s borders blend with other counties in the region, the inevitable spread of disease across these borders led to the need for close interjurisdictional coordination as well as assistance from the California Department of Public Health.
Largely, the response relied on the well-established methods of case identification, contact investigation, and prophylaxis (when appropriate) and restriction of activities of susceptible individuals. Despite the identification of thousands of contacts and hundreds of susceptible individuals, the quarantining of people who eventually developed measles was rare. There may have been some reduction in transmission during the recent outbreak because of voluntary isolation in a tightly knit social group, but collectively, these traditional disease control methods probably had limited impact on the spread of disease. Rather, the eventual cessation of disease spread was likely determined by the presence of herd immunity. Cases in the last wave of the outbreak certainly exposed a number of individuals, but all must have been immune, and they stood as a barrier against further spread of the outbreak. It is difficult, and probably nearly impossible, to significantly increase levels of herd immunity during an active measles outbreak.
During the outbreak, substantial efforts were made to influence vaccine-related beliefs, attitudes, and behaviors in the affected social group. When initial efforts at the individual level appeared to be unsuccessful, a new strategy was adopted. Leaders of the affected social group were identified and engaged for education and support. The Los Angeles County Department of Public Health found these community leaders to be both receptive to and very supportive of disease control efforts. Several leaders made public statements, including posting information on the internet, in support of the department’s work and strongly encouraging everyone to cooperate and get vaccinated.
Development of relationships with leaders in communities where anti-vaccination beliefs and behaviors exist, both during and between outbreaks, is vital to enhancing the overall level of herd immunity in our communities. Direct efforts to meet with these leaders, both to provide accurate information about the benefits and effectiveness of vaccination as well as to highlight the importance of their voice in support of vaccinations, are critical to raising the level of health protection for the entire community. The potential impact of this approach may be high in light of the fact that reports of measles outbreaks in closed social groups continue to be frequent.
While laws such as California’s SB 277 will eventually have a large and positive impact on overall population vaccination rates, the large number of older adolescents and young adults who are currently susceptible and beyond the reach of mandatory school-based vaccination programs will play a major role in measles outbreaks in the United States for many decades to come. Strategies to address and close this gap are needed if nationwide achievement of elimination is to be sustained. In addition, in communities where anti-vaccination beliefs and behaviors prevail, proactive engagement with leaders is necessary to improve response efforts and protect health. While outbreaks afford and usually demand such engagement, this opportunity exists during inter-epidemic periods and should be taken.