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Classification of Measles Cases and Categorization of Measles Elimination Programs
Publication date: 01/07/1983
Table of Contents
SUMMARYWith 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 CASESI. Clinical classification: A clinical case of measles* is 1) a generalized maculopapular rash lasting 3 or more days, 2) temperature of 38.3 C (101 F) or greater, and 3) one of the following: cough, coryza, conjunctivitis. Such a standard case definition was adopted to avoid the requirement that cases be confirmed virologically or serologically before being considered reportable measles cases, thus enabling health departments to act promptly on cases of rash illnesses and institute control measures rapidly.
*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:
A. A suspect case is any rash illness with fever;
B. A probable case meets the clinical case definition, is not epidemiologically linked to another probable or confirmed case, and has no or noncontributory serologic testing;
C. A confirmed case meets the clinical case definition and is epidemiologically linked to another confirmed or probable case or is serologically confirmed. A serologically confirmed case does not need to meet the clinical case definition.
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.
A. An indigenous case is defined as a case of measles within a state unrelated to an imported case or with onset occurring more than two generations after an imported case to which it is epidemiologically linked. Any case that cannot be proven as imported or spread from an imported case should be classified as indigenous.
B. An imported case has its source outside the state. Rash onset occurs within 18 days of entering the state, and illness cannot be linked to local transmission. Imported cases are to be classified as:
C. An importation-spread case is directly traceable to a known imported case within two generations.
- International--Importation from another country.
- 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.
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:
A. A case is considered preventable if measles illness occurs in a U. S. citizen: (1) at least 16 months of age, (2) born after 1956, (3) lacking adequate evidence of immunity to measles,* (4) without a medical contraindication to receiving vaccine, and (5) with no religious or philosophical exemption under state law. occurs in a U. S. citizen: (1) at least 16 months of age, (2) born after 1956, (3) lacking adequate evidence of immunity to measles,* (4) without a medical contraindication to receiving vaccine, and (5) with no religious or philosophical exemption under state law.
*Documented receipt of live measles vaccine on or after the first birthday or a physician-diagnosed measles disease.
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.
- 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.
- 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.
- Other or unknown
CATEGORIZATION OF IMMUNIZATION PROGRAMSJust 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.
I. Program impact: No indigenous cases of measles have occurred within the preceding 12 months.
II. Program elements
A. Surveillance: A surveillance system is in place; all public and private health providers know measles is a reportable disease, understand the reporting procedures, and are aware of the need for prompt reporting.
1. Performance criteria:
a. At least a sample of schools and day-care centers are involved in routine reporting, which could include negative reporting in high-risk areas at least until December 31, 1983.
b. At least semi-annual contact is made with reporting sources to emphasize the need for rapid reporting and provide them with a specific telephone number to report suspect cases.
B. Case investigation, confirmation, and containment 1. Investigation
a. Performance criteria: Investigation of all suspect cases is initiated within 1 work day after initial report using standardized investigation forms. 2. Confirmation
a. Performance criteria: A uniform and standard clinical case definition is in use. Laboratory confirmation is sought on all cases not directly traceable to a source.
a. Performance criteria: Containment procedures are initiated within 3 calendar days of identification of a probable or confirmed case.
(1) School exclusion procedures are utilized and strictly 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
1. Enforcement of laws/regulations
a. Performance criteria:
(1) A licensed day-care facility immunization law/regulation is 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
a. Performance criteria:
(1) An annual assessment of the immunization status of all 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. 4. Service delivery
a. Performance criteria:
(1) Tickler/recall systems are in place in at least 50% of public 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. 5. Immunization records
a. Performance Criteria:
(1) Antigen-specific information is recorded at least by month 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 PROCESSA 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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