[Table of Contents]

Volume: 25S5 - August 1999

Proceedings of the National Varicella Consensus Conference
Montreal, Quebec
May 5-7, 1999

Surveillance Needs

Participants agreed that there is a need for a baseline estimate of disease incidence before program implementation; some data already exist and need to be collated and, in some cases, validated. Recommendations for improved/additional data needs were proposed to address the current information gaps identified. In addition, it was recommended that all provinces/territories should have a surveillance system for varicella in place, although the systems do not necessarily need to be the same at the onset of immunization programs.

The surveillance system should attain seven goals: (1) allow descriptive analysis of varicella incidence; (2) detect changes in severe varicella disease; (3) monitor changes in the epidemiology of herpes zoster; (4) measure vaccine coverage; (5) monitor vaccine safety; (6) provide estimates of vaccine effectiveness in the field; and (7) enhance the capacity of the laboratory for varicella testing to support surveillance.

The surveillance methods used to estimate incidence must be flexible and dynamic so that as incidence drops, the reporting method may change, for example from aggregate reporting during the "implementation phase" (which includes the period before and during actual program implementation) to case-by-case reporting during a "control phase". The definition of the control phase was identified as a research need. Further research is also required to standardize the definition of "susceptibility"; this includes determining the levels of antibodies that correlate with protection against infection, establishing test reliability for various commercial test kits, and determining clearly what are positive and negative test results.

Once varicella control has been achieved, surveillance strategies for the "elimination phase" will need to be established, such as intensified outbreak control, contact tracing, and continued laboratory confirmation of cases and differentiation of wild versus vaccine virus strains.

Additional/improved data needs

Recommendation 5.1

All provinces should have a surveillance system that can provide information on the number and age distribution of cases of both varicella and herpes zoster.

Recommendation 5.2

Canadian data should be gathered on the epidemiology and burden of illness in pregnancy. This should include gestational varicella and herpes zoster; perinatal varicella (maternal and infant); and congenital varicella syndrome (add to the Canadian Paediatric Surveillance Program as active surveillance).

Recommendation 5.3

National population-based data should be gathered on the incidence of severe disease, particularly in adults, including hospitalizations and deaths.

Recommendation 5.4

Varicella and herpes zoster should be added to the Vital Statistics List of Rare and Infrequent Causes of Death.

Surveillance goals

Recommendation 5.5

A goal should be adopted to detect changes in the age-specific incidence of varicella.

The specific strategies/methods to achieve the specified goal are as follows:

• Implementation phase: aggregate reporting, using sources such as schools, nurseries, daycare centres, emergency departments, walk-in clinics, sentinel physicians, and health claims data
• Control phase: case-by-case reporting, including enhanced sentinel surveillance and outbreak investigation and control (with laboratory confirmation of cases, differentiation of wild versus vaccine virus strains, and serologic testing)

Recommendation 5.6

A goal should be adopted to describe changes in frequency and risk factors for severe varicella disease (defined as hospitalization or death due to varicella or an associated complication).

The specific strategies/methods to achieve the specified goal are as follows:

• A sentinel hospital-based system, including the following components
  • pediatric: use revised IMPACT surveillance
  • adult: use existing systems (e.g. Canadian Hospital Epidemiology Committee, Canadian Infectious Disease Society, Community Hospital Infection  Control Association - Canada) to identify patients and collect data
  • report cases to provincial/territorial epidemiologists
  • collate data at a national level
• Laboratory differentiation of VZV strains (wild type versus vaccine strain)
• For deaths, a review of the vital statistics database

Recommendation 5.7

A goal should be adopted to monitor changes in the epidemiology of herpes zoster.

The specific strategies/methods to achieve the specified goal are as follows:

• Implementation phase: to continue baseline monitoring and, for childhood herpes zoster, establish case-by-case reporting (through the Canadian Paediatric Surveillance Program) and laboratory differentiation of wild type versus vaccine virus strains
• Control phase: to continue baseline monitoring and initiate case-by-case reporting for adults aged 20 to 50 years through sentinel physicians and hospital-based reporting

Recommendation 5.8

A goal should be adopted to measure vaccine coverage in targeted populations.

The specific strategies/methods to achieve the specified goal are as follows

• to add varicella vaccine to the annual National Vaccine Coverage Survey
• to utilize existing registries such as the provincial immunization registries and the national network of registries
• to adjust coverage estimates for prior disease (which would result in falsely low coverage) if indicated, based on vaccine target population

Recommendation 5.9

A goal should be adopted to monitor vaccine safety.

The specific strategies/methods to achieve the specified goal are as follows:

• to use existing systems to monitor vaccine-associated adverse events
• to monitor childhood herpes zoster post-vaccination (see Recommendation 5.7)
• to monitor transmission of vaccine virus to contacts; if disease is severe, case-by-case investigation should be carried out (see Recommendation 5.6)
• to report inadvertent vaccination of pregnant women to existing pregnancy registries (e.g. manufacturer-based registries, Motherisk)
• to monitor the inadvertent vaccination of pregnant women by adding a question regarding prior vaccine exposure to the prenatal form

Recommendation 5.10

A goal should be adopted to measure vaccine effectiveness in the field.

The specific strategies/methods to achieve the specified goal are as follows:

• Implementation phase: selected outbreak investigation among vaccinated populations
• Implementation phase: special studies (seroepidemiology among immunized populations, immunogenicity under field conditions)
• Control phase: investigation of disease in vaccinated individuals

Recommendation 5.11

A goal should be adopted to ensure laboratory capacity for VZV testing to support surveillance.

The specific strategies/methods to achieve the specified goal are as follows:

• to have in place a readily available laboratory capacity (i.e. a provincial laboratory or a reference laboratory) to confirm the diagnosis of VZV infection as needed, and to determine susceptibility to VZV as needed
• to establish a national reference laboratory to perform strain characterization and to ensure quality assurance

Recommendation 5.12

A goal should be adopted to disseminate surveillance data.

The specific strategies/methods to achieve the specified goal are as follows:

• each province/territory should have a system in place for regular, timely dissemination of surveillance data
• to compile surveillance data at the national level as part of the annual immunization report
• to add varicella updates to LCDC's web site during the implementation phase

Recommendation 5.13

The surveillance methods should be evaluated:

• Ongoing evaluations of the surveillance systems should be built in by those responsible for the systems.
• Periodic special studies should be conducted to ensure the validity of the data.
• Regular surveys of users of surveillance systems should be conducted to evaluate ease of use of the system and the suitability and utility of the data.
• The systems should be modified or new systems developed as gaps are identified by users and expert groups.

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