The United States was able to eliminate measles because it has a highly effective measles vaccine, a strong vaccination program that achieves high vaccine coverage in children and a strong public health system for detecting and responding to measles cases and outbreaks.
–Centers for Disease Control and Prevention, measles FAQ
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A Race Against Measles: Evolution of Eradication Strategies in the U.S.
Measles can have detrimental effects on physiological functioning, morbidity, and mortality, thus illustrating a strong burden of disease within the United States population. Health officials recognized the deleterious potential of this virus during the early 20th century; therefore, took action to develop three elimination strategies between 1966 and 1996 that would achieve complete elimination of the disease (7). They heavily utilized the measles vaccine as a form of prevention despite shifting acceptability of the vaccine by the public during the 1980s. Each elimination strategy resulted in varying successes and failures, which were used to advance and strengthen upcoming eradication efforts.
Pre-elimination Initiatives: 1992-1995
Nationwide vaccination programs that increased access and affordability of vaccines did not exist prior to 1962. During this time, immunizations were provided through private physician practices and local health departments. Recipients were forced to pay out of pocket unless they qualified for federal Maternal and Child Health Block Grant funds. This system proved to be harmful because low population immunity and quick disease transmission were prevalent. This resulted from low rates of vaccination, a problem which the U.S. government acknowledged and responded to through the creation of the Vaccination Assistance Act in 1962. This act signified a national effort to rapidly increase protection of the population, especially preschool children, through massive vaccination action plans. It also supported vaccination campaigns through federal funding. In 1963, the act recommended vaccinations for measles, diphtheria and tetanus toxoids and pertussis, polio, and smallpox (6). The spread of measles during this time prioritized it as an important health issue and emphasized the need for massive public vaccination. These efforts also led to the creation of the Advisory Committee on Immunization Practices (ACIP) in 1964, which is composed of medical and public health experts that advise on vaccination techniques to control disease contagion (5). They promote the measles vaccine for the majority, especially for infants and children. These efforts set up the infrastructure for disease elimination in the 20th century (6).
First Elimination Strategy: 1966-1970
The first round of measles eradication efforts focused on four different strategies (7):
- Routine immunization of infants
- Immunization of susceptible school-aged children
- Epidemic control
Health officials working under the Vaccination Assistance Act and Advisory Committee on Immunization Practices (ACIP) prioritized delivery of the first dose of the measles vaccine, and worked hard to ensure that all age-appropriate infants and children received it (6). Vaccination campaigns received federal funding through the Vaccination Assistance Act during the first portion of this measles elimination strategy; however, within a few years funding decreased and ceased entirely by July 1969. This had an adverse effect on low measles prevalence and was cited as the reason why adequate vaccine coverage was not obtained among all infants and 1-4 year old children. Although complete eradication was not the primary focus in this strategy, measles prevalence declined by 95% in 1967-1969 when compared to pre-vaccine statistics (7).
Lessons Learned (7):
- High levels of vaccination coverage among 2-year age group are essential for measles elimination.
- Proper funding is crucial for complete implementation of vaccination programs.
Second Elimination Strategy: 1978-1988
A second nationwide childhood immunization initiative was launched in 1977 with the aim of increasing and maintaining immunization rates to 90% by October 1979. With this goal in mind, health officials focused their efforts on 3 strategies (7):
- Achieving and maintaining high rates of immunization coverage
- Strong and effective surveillance systems
- Aggressive outbreak response
This strong launch in federal and state initiatives encouraged school systems to become involved in disease eradication efforts. By 1981, all 50 states required proof of immunization before school admittance in primary schools, secondary schools, and licensed day care centers. This requirement significantly increased child measles immunity to more than 95%. While this was an important step in a positive direction, the requirement only applied towards the first dose of the measles vaccine. Lack of complete protection became an issue when children did not obtain the second dose at the appropriate time frame (4-6 years of age). This maintained a non-zero rate of transmission, and continued the spread of measles throughout the United States (7).
In an effort to increase acceptance and availability of the vaccine, Dr. Albert Sabin created an aerosol measles vaccine and initiated a trial to demonstrate its safety and immunogenicity in Mexico in 1981. Four million children were vaccinated with this form of immunization, and its trials demonstrated promising success. However, it was limited due to its lack of official testing and incorporation into routine immunization programs in the United States and throughout the world. His continued work has lead to the establishment of the Measles Aerosol Project, which aims to establish a method for respiratory delivery of the measles vaccine. Such an accomplishment would yield benefits relating to: increased acceptability of vaccination methods, decreased need for trained professionals to deliver vaccine, lower amounts of materials used, and greater accessibility for wide distribution. Increased movement towards disease eradication from Dr. Sabin and the World Health Assembly pressured the United States to advance its domestic attempt to reduce the rate of measles (8).
In 1986, the U.S. government established the National Childhood Vaccine Injury Act in order to solidify surveillance of the measles vaccine and prove its effectiveness in disease eradication. This act set up a mandatory process for which health providers and vaccine manufacturers were obligated to report to the Department of Health and Human Services details of specific detrimental events following the administration of the measles vaccine. They utilized the Vaccine Adverse Event Reporting System (VAERS), which was co-administered by the Food and Drug Administration and the Center for Disease Control and Prevention. Reports of suspected adverse events associated with all age groups were sent to this surveillance system for analysis. In 1989, they found that the first dose of the measles vaccine was not sufficient to produce solid immunity and therefore, resulted in vaccine failure (i.e. patients developed measles after vaccination). The Advisory Committee on Immunization Practices and the American Academy of Pediatrics responded by establishing recommendations for second dose routine immunization in order to establish solid individual immunity (10).
Lessons Learned (7):
- School immunization requirements promote high vaccination rates among school-aged children.
- Solid immunity is obtained after the second dose of the measles vaccine.
- School immunization requirements can play a critical role in ensuring vaccination of the second MMR dose.
- Evaluation of vaccine coverage is essential.
Third Elimination Strategy: 1993-1996
New programs created through additional federal funding were utilized to achieve the following measles eradication goals:
- Operative and useful surveillance systems
- Routine immunization of the second measles dose
- Establishment of laboratory diagnosis
The Healthy People 2000 Initiative through the US Public Health Service focused on maximizing population immunity through mass vaccination, establishing effective surveillance, and collaborating with other countries to obtain global measles control. The National Immunization Survey (NIS) also established guidelines to monitor vaccination among children aged 19-25 months. This form of systematic review increased awareness of coverage at the provider level and identified children who were behind in their vaccination schedule. State governments became involved and incorporated the second dose of the measles vaccine as a requirement for school immunization clearance. Vaccines for Children, a newly funded federal program, also distributed the first and second doses of the measles vaccines without charge to poor, uninsured, and underinsured children in their medical homes (7).
Additional federal funds were also allocated to states to guarantee proper surveillance of the elimination movement. This led to the revision of how a measles case was defined. Unlike before, a new case was confirmed and diagnosed through laboratory tests. All cases that did not originate from a different country were also considered indigenous. This changes in disease diagnosis shifted the focus of elimination efforts. Instead of focusing on eradicating indigenous measles cases, goals broadened to the elimination of endemic measles overall, regardless of their source (7).
This new strategy led the U.S. Center for Disease Control and Prevention to collaborate with the Pan American Health Organization in an effort to reduce measles prevalence in United States. They were successfully able to reduce the number of cases from >250,000 in 1990 to <1,000 in 2000. Fewer outbreaks resulting from foreign countries were reported during this established partnership. The CDC also created a specialized branch to deal with international measles issues, the Global Measles Branch. This was an organized effort to allocate U.S. resources towards vaccine purchases and technical support for international measles vaccination coverage. The number of measles cases imported from a different country decreased from >300 in 1990 to <40 cases a year during 1997-200 (7).
Lessons Learned (7):
- Developing, assessing, and refining measles eradication strategies requires surveillance.
- Laboratory diagnoses are critical for surveillance and accuracy.
- Analysis and awareness of measles virus genotype is important in determining disease circulation
- Internationally imported measles cases are responsible for small outbreaks in areas where measles is endemic.
- Measles reduction is achieved through collaborative efforts between countries.
Declaration of Success: Measles Elimination in 2000
The 2000 measles eradication success story was the result of these three elimination strategies, along with various public health, government, social, and economic movements around the world. Old prevention strategies used in the 20th century set up changes for new strategies in the 21st century. These improvements were so effective that the United States hit a record low rate of measles cases between 1997 and 2000. Less than 150 cases were reported each year, which signified a >90% reduction since 1983. This accomplishment led the CDC to officially announce measles eradication in 2000. To date, the United States is the largest country to have reported such a drastic interruption of endemic measles cases around the world (7).
Notice of such an accomplishment prompted the World Health Organization (WHO) and the United Nations Children’s Fund (UNICEF) to create a global measles strategic plan for 2001-2005. In 2001, this program sought to decrease the annual number of measles fatalities by 50% around the world compared to the 875,000 deaths that occurred in 1999. They also worked towards obstructing measles transmission through increased vaccination and convening a global consultation committee to review surveillance and progress. This strategic plan has been updated throughout the years and continues to function in many countries. Contemporary efforts are focused on (11):
- Establishing and maintaining strong routine vaccination systems and campaigns
- Laboratory-backed surveillance
- Outbreak preparedness and case management
- Research and development
These goals are similar to current strategies used by the United States (7):
- Maximize population immunity through mass immunization
- Establish strong surveillance systems
- Respond promptly and effectively to outbreaks
- Collaborate with other countries to improve global measles control
Preventing Outbreaks: How Herd Immunity Works
The overarching goal of these public health strategies was to obtain “herd immunity”. This term is defined in many ways, all which highlight the idea of obtaining a threshold proportion of immune individuals that prevent the spread of disease and protect populations from suffering through the invasion of an infection. Direct immunity is obtained by receiving a vaccine, which builds the body’s immune response to fight the virus in the event of infection. Indirect immunity or herd effect, on the other hand, is obtained when a large portion of the population is vaccinated, thus halting the spread of disease through the presence and proximity of immune people (12).
The proportion of individuals who need to be vaccinated in order to create a herd effect is described through the threshold theorem: The critical vaccination level (Vc) represents the fraction of people who need to be vaccinated to achieve herd immunity. Ro stands for the basic reproduction number, which is the number of secondary cases that result from an infectious individual. This value varies according to each virus, and equates to 12-18 people for measles. Immunity through vaccination must be delivered at random and occur within a random population for this equation to be true. Since most populations are mixed at random, this equation is utilized to determine the herd immunity threshold for most communities and analyze the
proportion of vaccines that should be distributed. If the proportion of immune individuals does not meet or exceed the herd immunity threshold, disease transmission will occur and result in measles outbreaks (12). All three elimination strategies initiated within the United States aimed at achieving the 95% herd immunity threshold, which is needed to stop the transmission of the measles virus.
The concept of herd immunity is established within the field of epidemiology; therefore, is highly utilized to determine the proportion of individuals who should be vaccinated. Protection is sought for those who can be vaccinated and for those risk groups who cannot be directly vaccinated (infants, immunocompromised individuals, pregnant women, and those allergic to the vaccine). Within the last century, the United States has obtained high vaccination rates, but the growth of migration and personal belief exemptions has increased the number of individuals that are susceptible to infection and disease outbreak.
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