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Classification of Measles Cases and Categorization of Measles Elimination Programs
With next week's issue of the MMWR, Tables I and III will reflect a new system of classification for measles cases. This system, and the categorization of immunization programs, has been developed by the Division of Immunization in conjunction with the directors of each immunization program and the Conference of State and Territorial Epidemiologists.
Because the number of reported measles cases has decreased dramatically during the last several years, the need to develop clinical, epidemiologic, and programmatic classifications of measles cases to more effectively implement measles elimination programs has increased. Careful differentiation of cases as to their preventability and accessibility, and whether they represent spread within a state or from another state or importation from abroad, becomes important. It is essential to establish uniform criteria for categorization of measles elimination programs to determine whether programs have interrupted indigenous measles transmission and whether their program elements are sufficiently institutionalized to permit maintenance of the gains. CLASSIFICATION OF MEASLES CASES
*In April 1979, the Conference of State and Territorial Epidemiologists agreed to adopt a standard case definition of measles to permit more uniformity in their reporting of clinically confirmed measles cases.
Not all states have accepted this definition. Some have suggested a more specific clinical case definition. Several require serologic confirmation before reporting cases officially. As the measles incidence rate falls, serologic confirmation becomes increasingly important to the diagnosis. Dependence on seroconfirmation for reporting, however, may decrease the number of cases reported and might delay outbreak control and interstate notification. The Immunization Practices Advisory Committee recommends that all rash illness with fever be investigated and a single case meeting the clinical case definition be considered an outbreak and sufficient reason to begin control measures. The consequences of overdiagnosing or accepting non-measles cases of rash illness with fever are less serious than those of not acting promptly on a true case. With the availability of the capillary filter paper technique, seroconfirmation has become progressively more accessible; however, seroconfirmation should continue to be used only as a retrospective tool to confirm measles. Generally, serology cannot be used to rule out measles. A diagnosis should be considered confirmed in the presence of good clinical and/or epidemiologic evidence, even in the absence of confirmatory serology.
Since the measles case definition is sensitive but not very specific, the need for quantifying the degree of certainty in the diagnosis of measles becomes increasingly important as cases become fewer. Cases are to be classified as:
It is possible that two epidemiologically linked cases may occur without serologic confirmation and would appropriately be considered confirmed. However, ideally there should be at least one serologically confirmed case in each chain of identified transmission, and each isolated case should be serologically confirmed. As of January 1, 1983, weekly telephone reports to the MMWR should include only confirmed cases; they may be designated as either imported or indigenous.
II. Epidemiologic classification: The differentiation between
imported cases and indigenous cases becomes increasingly important as success at interrupting indigenous transmission of measles is assessed. For a given state, a case that may be indigenous to the United States may, in fact, represent an out-or-state importation.
International--Importation from another country. 2. Out-of-state--Importation from another state. Designation of an out-of-state importation requires documented face-to-face contact with a person with measles outside the state or documented evidence that the person was out-of-state for the entire period during which she or he might have become infected. A resident of one state who acquires infection from another state (but becomes ill in the state of residence) should be reported as imported by the state of residence.
C. An importation-spread case is directly traceable to a
known imported case within two generations. Table I will list total measles cases and note the number that represents international importations and their spread cases. Table III will include out-of-state importations and their spread cases, along with international importations and their spread cases.
Differentiation between indigenous, imported, and importation-spread cases will permit analysis of sources in a systematic fashion. Classifications are limited because the potential exists for misclassification of cases (e.g. when an undetected importation leads to spread that will, of necessity, be classified as indigenous). III. Programmatic classification: To assess program effectiveness
under current guidelines, it is necessary to determine the proportion of cases preventable by immunization and to examine the reasons these cases were not prevented. CDC differentiates between preventable and nonpreventable cases and determines the accessibility and relationship of the case to the health care system. Cases are to be classified as follows:
*Documented receipt of live measles vaccine on or after the first birthday or a physician-diagnosed measles disease.
An accessible case occurs in a child enrolled in a school (ages 5-19 years) or day-care center who, with appropriate application of state law, could have been vaccinated.
2. A hard-to-reach case occurs in a child at least 16 months of age, born after 1956, who has received at least one other immunization in the United States but who is not in a school or day-care center affected by state law. This child is considered to have once entered the health care system, and, therefore, could have been vaccinated.
3. Other or unknown
B. A case is considered not preventable under current
program guidelines if measles illness occurs in a person: (1) less than 16 months of age, (2) born before 1957, (3) with adequate evidence of immunity, (4) with a medical contraindication to receiving vaccine, or (5) with a religious or philosophical exemption under state law. CATEGORIZATION OF IMMUNIZATION PROGRAMS
Just as it is important to standardize measles case definitions, it is essential to establish uniform criteria to categorize measles elimination programs. As measles elimination is accomplished, a mechanism for "certifying" or categorizing programs' achievements is necessary. The absence of measles does not in itself guarantee the maintenance of a measles-free state; therefore, a series of performance criteria that are integral to the elimination of measles and assure the maintenance of that achievement has been developed. In addition, these elements provide a basis for the control of other vaccine-preventable diseases.
B. Case investigation, confirmation, and containment
enforced. (2) Interstate notification occurs within 1 work day of
identification of a probable or confirmed case. (3) A specific outbreak control manual is available and routinely
followed in containment activities. (4) Spread is limited to two generations beyond the index case.* (5) Serologic data are interpreted accurately. (6) Source** identification is made in at least 50% of index
cases. (7) Cases are correctly classified and reported as outlined in
the previous section. (8) Routine tracking of time lapses between onset, reporting, and
action phases is maintained. *Index case: A probable or confirmed case of measles that is not part of a previously identified chain of transmission.
C. Operational elements
Enforcement of laws/regulations
in place, requiring proof of immunity of all enrollees.*** (2) A school enterer/attendance law is in place, requiring proof
of immunity of all kindergarten and/or first grade students in both public and private schools.*** (3) The school and day-care laws/regulations are strictly
enforcedS by the state and local authorities so that a child is denied admission or excluded for noncompliance. **Source: The case or outbreak to which a case of measles is epidemiologically linked. A foreign country may be considered a source even without identifying specific exposure to an outbreak within that country. ***Where state law permits or exists
2. Assessment of immunization status
kindergarten and/or first grade attendees is conducted. (2) An annual assessment of the immunization status of all
day-care center attendees is conducted. (3) Kindergarten and/or first grade assessments are completed by
December 31 of each year. (4) Licensed day-care center assessments are completed annually. (5) These assessments demonstrate more than 95% vaccinated
according to state requirements. (6) Validation surveys are conducted annually to verify
assessment data in at least a sample of kindergarten and/or first grade and licensed day-care facilities.
3. Service delivery
clinics to ensure that all children under 2 years of age are appropriately immunized in the project area. (2) Prototype tickler/recall systems are introduced and promoted
through state and local chapters of appropriate professional organizations. (3) Immunization services are promoted for susceptible
adolescents in high school and college.
4. Immunization records
and year on standard records for all new school and licensed day-care center enrollees. (2) Standard parent-maintained personal immunization records are
presented to parents in all public clinics and made available to private physicians and promoted as part of education activities. EVALUATION PROCESS
A two-stage evaluation process is planned for the certification of immunization programs related to the absence of indigenous measles: an internal and an external evaluation. For the internal assessment, a self-assessment protocol is to be made available for an immunization program to evaluate itself. This will be followed by an external evaluation.
The infrastructure for a complete immunization program may not be in place at the time of evaluation. Therefore, minimal performance standards will be applied against the above described criteria to delineate the degree to which each has been carried out and the validity of each assessment. Reported by Div of Immunization, Center for Prevention Svcs, CDC.
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