|WHO-Recommended Standards for Surveillance of Selected Vaccine-Preventable Diseases (WHO - OMS, 1999, 40 p.)|
Rationale for surveillance
Measles is targeted for a reduction by 90% in incidence and by 95% in mortality (9GPW 6.2). Surveillance for measles is important to monitor/adjust strategies and should evolve based on level of control. Countries in the "control" phase are endemic and should concentrate on raising routine measles immunization coverage and focusing extra immunization efforts in areas with high measles mortality. Countries in the "measles elimination phase" are achieving high routine measles coverage and low incidence with periodic outbreaks. Surveillance in these countries should be used to identify high risk populations and to predict and prevent potential outbreaks. Those countries in which the objective is to completely interrupt measles transmission require very intensive case-based surveillance to detect, investigate, and confirm every suspect 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
A case that meets the clinical case definition
A case that meets the clinical case definition and that is laboratory confirmed***
A case that meets the clinical case definition and is linked epidemiologically to a laboratory confirmed case
A suspect case that does not meet the clinical or laboratory definition
* During the "control" phase, cases are confirmed (and classified) on clinical grounds.
** Laboratory confirmation is used during the "elimination phase" and to confirm an outbreak.
*** Lab confirmation is used during the "elimination phase" and to confirm an outbreak. If the case has been vaccinated within 6 weeks prior to serum collection refer to the Manual for the Laboratory Diagnosis of Measles Virus Infection for classification.
Recommended types of surveillance
· Control phase: When measles is endemic, routine monthly reporting of aggregated data of clinical measles cases is recommended by district, age group and immunization status. In order not to overburden the system, it is recommended that only outbreaks (not each case) be investigated to determine their causes.*
· Elimination phase: Case-based surveillance should be conducted and every case reported and investigated immediately (and also included in the weekly reporting system). Laboratory specimens should be collected from every suspect case. Suspected measles outbreaks should be confirmed by conducting serology on the first 5-10 cases only. Urine specimens (for virus isolation and genetic characterization) should be collected from sporadic/outbreak cases (~10 cases from each chain of transmission) to characterize viral circulation and importation patterns. All outbreaks should be thoroughly investigated.
· During both phases: zero reporting should be required at all levels.
* Laboratory confirmation may be attempted in control phase with sampling ~10 cases per outbreak
Recommended minimum data elements
Control phase (aggregated data):
· Number of cases by age group and immunization status, month and geographic area
· Number of measles vaccine doses administered to infants (< 12 months) and children 12-23 months
· Target population
Elimination phase (case-based data): in addition to data from the control phase
· Unique identifier
· Geographical area (e.g. district and province)
· Date of birth
· Date of rash onset
· 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 the 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 = indeterminate; 4 = no specimens processed; 9 = unknown
· Results of differential serology (make separate variable for each disease):
1 = positive; 2 = negative; 3 = 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; 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 and nature of source
Note: In every phase, completeness and timeliness of monthly (control phase) or weekly (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 is this is not too burdensome.
Recommended data analyses, presentation, reports
· Incidence rate by month, year, and geographic area
· Measles vaccine coverage by year and geographic area
· DTP1-measles or BCG-measles drop out rate
· Completeness/timeliness of monthly reporting
· Proportion of known outbreaks confirmed by the laboratory
· Age-specific incidence rate
· Proportion of cases by age group and immunization status: core age groups suggested 0-8 months, 9-11 months, 2-4 years, 5-9 years, 10-14 years, 15-19 years, 20-24 years, 25 years and over.
Measles elimination phase: same as control phase plus the following
- % of weekly reports received
- % of cases* notified £ 48 hours of rash onset
- % of cases* investigated £ 48 hours of notification
- % of cases* with adequate specimen** and lab results within 7 days
- % of confirmed cases with source of infection identified >80% % of outbreaks investigated
* All cases that meet the clinical case definition
** Adequate specimen is one blood specimen collected within 28 days of rash onset
Principal uses of data for decision-making
· Control phase: Monitor incidence and coverage to determine progress (i.e. decreasing incidence and increasing coverage), and to identify areas with poor programme performance. Describe the changing epidemiology of measles in terms of age, immunization status and inter-epidemic period.
· Elimination phase: Identify chains of transmission. Monitor the epidemiology (age groups at risk, inter-epidemic period, immunization status) of measles and determine when the next outbreak may occur due to a build up of susceptibles and accelerate immunization activities accordingly to avert a potential outbreak. Determine where measles virus is circulating or may circulate.
· Use epidemiologic data to classify cases (See special aspects section). Use performance indicators to assess the quality of surveillance to detect virus circulation or potential importation and identify areas that need strengthening.
· During all phases: Detect and investigate outbreaks to ensure proper case management, and determine why the outbreak occurred (e.g. failure to vaccinate, vaccine failure or inadequate strategies)
Use additional methods such as epidemiologic modelling and sero-epidemiology studies to monitor the build-up of susceptibles (guidelines on how to do this are under development)
Also see "manual for laboratory diagnosis of measles virus infection" and "guidelines on measles outbreaks"
Final classification of measles cases
Opportunities should be seized to integrate measles and rubella surveillance depending on control strategies and immunization programmes in place
See Annex 1
WHO Department of Vaccines and Biologicals
(See Annex 1 for address and fax number)
Tel: +41 22 791 4359