Measles Outbreaks and Measle Vaccine Refusal
The endemic transmission of measles virus was declared eliminated from the United States in 2000 as a result of strong vaccination coverage. In recent years, measle vaccine coverage has waned due to parents’ lack of personal experience with the virus and their misperceptions of vaccine safety . Measles remains endemic abroad and therefore the risk of the virus being imported endures. An afflicted traveler is often the origin of a larger outbreak [2-8]. In 2011, 222 people in the US contracted measles: the most documented cases in a single year since 1996 [2, 4]. About 90 of these people were infected while in Europe and Asia. They carried the disease to the States and infected others, resulting in 17 outbreaks in various communities . Of these, the Minnesota outbreak is the largest outbreak in the US since 1991 [3, 4]. The outbreak originated from a 30-month-old toddler returning from a trip in Kenya. The toddler directly infected three others at a day-care facility and subsequently individuals were exposed in an emergency department, homeless shelters, and households . Some of those exposed include seven infants who were too young to receive MMR vaccine . Infants less than 12 months old are at particular risk of complications from the measles virus. Furthermore, many children of this Minnesota population were unvaccinated due to parental concerns about MMR vaccine safety . Often times and perhaps in this instance, the community’s measle vaccine refusal rate can be correlated with its susceptibility to a measles outbreak.
For example, a study of the 2008 measles outbreak in San Diego found that the outbreak was attributable to clusters of intentionally unvaccinated children . Many parents of this population believed that vaccinations could cause autism in their children . Recently, there have been numerous other vaccine preventable measles outbreaks, such as in Tucson, AZ ; Los Angeles County, CA ; and in Western and Central Pennsylvania [7, 8]. Treatment or prevention of these persisting outbreaks can carry unfavorable economic costs: up to $25,000 in the case of a single infected foreign refugee received in the US . In the Tucson outbreak, two hospitals spent almost $800,000 treating just 7 patients . Remarkably, it costs only $78 to vaccinate an individual for measles .
Similar to the US, Canada eliminated endemic measles transmission at the close of the 90s. Five years ago, Canada endured its first outbreak since 2000. The outbreak manifested through several unrelated networks of unvaccinated people, lasted 25 weeks, and resulted in 94 cases of measles . Countries overseas such as the United Kingdom have been striving to eliminate endemic measles transmission but are held back by low vaccination uptake in children. In Merseyside, UK, a vaccination uptake of only 85% for the second dose in children younger than five has allowed the manifestation of the largest outbreak in the Northwestern UK since 1996 . The outbreak began in January 2012 and as of June 30th 359 confirmed and 157 probable cases of measles were reported . Only 3% of the vaccine-eligible confirmed cases were fully immunized. As a result of this outbreak, existing vaccination initiatives in the area have been intensified .
Pertussis is another vaccine preventable disease whose spread in the US is supported by vaccine refusal. Nearly 32,000 cases of pertussis were reported this year as of August 8th, the largest occurrence in at least 12 years . Pertussis epidemic was declared in Washington on April 3 . Furthermore, recent outbreaks and high rates of the disease have occurred in Colorado, Minnesota, and Wisconsin . Risk factors for contracting pertussis include incomplete vaccinations and waning immunity [16-18]. Unlike measles, immunity from pertussis vaccine begins to wane 5 to 10 years after completion of the childhood immunization schedule and leaves adults and adolescents at high risk . Still, unvaccinated children are eight times more likely to contract pertussis . Similar to measles, clusters of intentionally unvaccinated children exist in regions around the US. One study of these clusters indicated that the risk of community-level pertussis outbreaks is greatly increased in the presence of these geographically concentrated exemptors . Another study found higher risk of pertussis associated with exposure to exemptors in school outbreaks and higher incidence of pertussis among vaccinated children living in counties with exemptors .
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3. Notes from the field: Measles outbreak–Hennepin County, Minnesota, February-March 2011. MMWR Morb Mortal Wkly Rep, 2011. 60(13): p. 421.
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