WHO-recommended surveillance standard of measles
Rationale for surveillance
The global Measles Mortality and Regional Elimination Strategic Plan 2001-2005 (WHO/V&B/01.13) seeks to reduce the number of measles deaths by half by 2005 (compared with 1999 estimates) and to achieve and maintain interruption of indegenous measles transmission in large geographical areas with established elimination goals. Surveillance for measles should evolve with each phase of measles control. Countries in the mortality reduction phase, where the disease is endemic should concentrate on raising routine measles immunization coverage and focusing supplemtal immunization efforts in areas with high measles mortality. Countries with more advanced measles control or in the elimination phase are achieving high levels of population immunity against measles and low incidence with or without periodic outbreaks. Surveillance in these countries should be used to identify high-rosk populations and to predict and prevent potential outbreaks. Countries in which the objective is to completely interrupt measles transmission (or countries with very low incidence) require intensive case-based surveillance to detect, investigate and confirm every suspected measles case in the community
Recommended case definition
Clinical case definition
Any person in whom a clinician suspects measles infection, or
Any person with fever and maculopapular rash (i.e. non-vesicular) and cough, coryza (i.e. runny nose) or conjunctivitis (i.e. red eyes)
Laboratory criteria for diagnosis
Presence of measles-specific IgM antibodies
Case classification
Countries are advised to use the clinical classification scheme until their programmes meet the following two criteria:
- low level of measles incidence;
- access to a proficient measles laboratory
The laboratory classification scheme should be used by countries in the low incidence or elimination phase
Clinical classification scheme
Clinically confirmed: A case that meets the clinical case definition
Discarded: A suspect case that does not meet the clinical case definition
Laboratory classification*
*Laboratory classification can also be used for outbreak investigation (see diagram in special aspects section below)
Laboratory-confirmed: A case that meets the clinical case definition and is laboratory-confirmed
Epidemiologically confirmed: A case that meets the clinical case definition and is linked to a laboratory-confirmed case
Clinically confirmed: A case that meets the clinical case definition and for which no adequate blood specimen was taken
Discarded: A suspect case that does not meet the clinical or laboratory definition
Recommended types of surveillance
Moratlity reduction phase
When measles is endemic, routine monthly reporting of aggregated data on clinical measles cases is recommended by district, age group and immunization status. Only outbreaks (not each case) should be investigated. During outbreaks it is useful to attempt to document measles mortality. Laboratory confirmation may be attempted by sampling approximately 10 cases per outbreak. Under special circumstances, the isolation of wild strains from selected cases occurring in outbreaks could be performed to enable genetic characterization of circulating measles virus and determine patterns of importation and exportation for countries in the low-incidence or elimination phase
Low-incidence or elimination phase
Case-based surveillance should be conducted and every case should be reported and investigated immediately (and also included in the weekly reporting system). Laboratory specimens should be collected from every sporadic suspect case. Suspected measles outbreaks should be confirmed by conducting serology on the first 5-10 cases only. Urine, nasopharyngeal or lymphocyte specimens (for virus detection and genetic characterization) should be collected from sporadic/outbreak cases (approximately 10 cases from each chain of transmission) to characterize viral circulation and importation patterns
During all phases
Designated reporting sites at all levels should report at a specified frequency (e.g. weekly or monthly) even if there are zero cases (often referred to as "zero reporting")
Recommended minimum data elements
Aggregated data
- Number of cases by age groups and immunization status
- Number of measles vaccine doses administered to infants aged under 12 months and children aged 12-23 months
Case-based data
- Unique identifier*
- Geographical area (e.g. district and province)*
- Date of birth*
- Sex 1 = male; 2 = female; 9 = unknown
- Date of onset of rash*
- Number of prior measles vaccine doses received: 9 = unknown*
- Date of receipt of last dose
- Date of notification
- Date of case investigation**
- Date of blood specimen collection*
- Date blood specimen sent to laboratory
- Date blood specimen received by laboratory
- Condition of blood specimen on receipt: 1 = adequate; 2 = inadequate; 9 = unknown
- Date measles serology results reported
- Results of measles serology: 1 = positive; 2 = negative; 3 = not tested 4 =indeterminate; 9 = unknown*
- Results of differential serology (make separate variable for each disease): 1 = positive; 2 = negative; 3 = not tested 4 =indeterminate; ; 9 = unknown
- Collection of specimen for viral culture/identification: 1 = yes; 2 = no
- Specimen type: 1 = urine; 2 = respiratory; 3 = lymphocytes
- Date specimen received for viral culture/identification
- Results of measles viral culture/identification: 1= positive; 2 = negative; 3 = not tested; 9 = unknown
- Final classification: 1 = clinically confirmed; 2 = laboratory-confirmed; 3 = epidemiologically linked to laboratory-confirmed case; 9 = discarded*
- Source of infection identified: 1 = yes; 2 = no; 9 = unknown
Note: In every phase, completeness and timeliness of monthly (mortality reduction phase) or weekly (low-incidence or elimination phase) measles reporting should be monitored. To avoid successive changes in forms and other data collection instruments, countries likely to move soon to the elimination phase may wish to move to case-based data while still in the control phase if this is not too burdensome
*Essential variable to be collected
**Investigation should include a household visit and search for additional cases in the household
Recommended data analyses, presentations, reports
Mortality reduction phase
- Number of cases and incidence rate by month and year, and geographical area
- Age-specific, sex-specific and district-specific incidence rates
- Measles vaccine coverage by year and geographical area.
- DTP1-measles or BCG-measles dropout rate
- Completeness/timeliness of monthly reporting
- Proportion of known outbreaks confirmed by the laboratory
- Proportion of cases by age group and immunization status. Core age groups suggested: 0-8 months, 9-11 months, 1-4 years, 5-9 years, 10-14 years, 15-19 years, 20-24 years, 25 years and over
Low-incidence or elimination phase
same as mortality reduction phase plus the following:
Performance indicators (target >=80%)
- % of weekly reports received
- % of cases* notified <= 48 hours after rash onset
- % of cases* investigated with house visit <= 48 hours after notification
- % of cases* with adequate specimen** and laboratory results within 7 days
- % of confirmed cases with source of infection identified
*All cases that meet the clinical case definition
** An adequate specimen is a blood specimen collected within 28 days of the onset of rash
Principal uses of data for decision-making
Mortality reduction phase
Monitor incidence and coverage to assess progress (i.e. decreasing incidence and increasing coverage) and identify areas at high risk or with poor programme performance. Describe the changing epidemiology of measles in terms of age, immunization status and interepidemic period. Assist in determination of optimal age groups to be targeted by second opportunity for measles vaccination (including mass vaccination campaigns)
Low-incidence or elimination phase
Identify chains of transmission. Monitor the epidemiology (age groups at risk, interepidemic period, immunization status) of measles and accelerate immunization activities accordingly to avert potential outbreaks
Use epidemiological data to classify cases (see special aspects section). Use performance indicators to assess the quality of surveillance and identify areas that need strengthening
During all phases
Detect and investigate outbreaks to ensure proper case management, and determine why outbreaks occurred (e.g. failure to vaccinate, vaccine failure or accumulation of susceptibles)
Special aspects
Final classification of measles cases
Note:
Adequate blood specimen: while IgM ELISA tests are more sensitive bteween days 4 and 28 after the onset of rash, a single serum sample obtained at the first contact with the health care system within 28 days after onset is considered adequate for measles surveillance
IgM positive: if the case has been vaccinated within 6 weeks before serum collection, refer to the Manual for laboratory diagnosis of measles infection, December 1999 (WHO/B&B/00.16). If an activate search in the community does not find evidence of measles transmission and there is no history of travelling to areas where measles virus is known to be circulating, the case should be discarded
Use additional methods such as epidemiological modelling and seroepidemiology studies to monitor the build-up of susceptibles (guidelines on doing this are under development)
