Immunization, Vaccines and Biologicals

WHO-recommended surveillance standard of pertussis

Rationale for surveillance

Pertussis, caused by Bordetella pertussis, is a major cause of childhood morbidity and mortality. There is evidence of a high burden of pertussis in developing countries. It remains one of the worldís leading causes of vaccine-preventable deaths. An estimated 50 million cases and 300 000 deaths occur every year; case-fatality rates in developing countries are estimated to be as high as 4% in infants. High immunization coverage with an effective vaccine is the mainstay of prevention. The rationale for pertussis surveillance is to monitor the impact of the immunization system, identify high-risk areas and detect outbreaks (which must then be investigated). In countries where coverage is moderate to low, surveillance should simply monitor improving coverage and decreasing pertussis incidence. Once immunization coverage is high and pertussis incidence is low, surveillance should be enhanced to understand the changing epidemiology of the disease and thus to guide vaccination policy. Bordetella parapertussis, which causes milder disease in general and is not responsible for significant mortality, is not a priority for surveillance in most countries at present

Recommended case definition

Clinical case definition

A case diagnosed as pertussis by a physician or
A person with a cough lasting at least two weeks with at least one of the following symptoms:

  • Paroxysms (i.e. fits) of coughing
  • Inspiratory whooping
  • Post-tussive vomiting (i.e. vomiting immediately after coughing) without other apparent cause

Criteria for laboratory confirmation

  • Isolation of Bordetella pertussis or
  • Detection of genomic sequences by means of the polymerase chain reaction (PCR) or
  • Positive paired serology

Case clasification

Clinically confirmed: A case that meets the clinical case definition but is not laboratory-confirmed
Laboratory confirmed: A case that meets the clinical case definition and is laboratory-confirmed

Recommended types of surveillance
  • Routine surveillance (where DTP3 coverage is < 90%)
    Routine monthly reporting of aggregated data on clinical cases from the peripheral level to the intermediate and central levels is recommended. All levels should be encouraged to report cases stratified by age group (e.g. < 1 year, 1-4 years, >=5 years) and immunization status
  • Routine surveillance (where DTP3 coverage is >= 90%)
    Case-based surveillance is recommended when coverage reaches 90%. Information on age, immunization status and final outcome (i.e. alive or dead) should be collected
  • Investigation of outbreaks
    Every pertussis outbreak should be reported immediately to the appropriate WHO regional office, investigated to understand why it occurred, and confirmed by laboratory methods. Case-based information should be collected on: date of onset, age, immunization status, geographical location and final outcome
  • Sentinel surveillance
    Sentinel surveillance is recommended in a few major hospitals to collect more in-depth information than that obtained through routine surveillance. The data collected on each case should include: date of onset, immunization status, age, laboratory confirmation and final outcome (i.e. alive or dead). This provides additional information on the burden and epidemiology of pertussis (e.g. age-specific case-fatality rates). Sentinel surveillance should be linked to developments in laboratory diagnostics and networks
  • Regardless of the type of surveillance, 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 for reporting (when DTP3 coverage is < 90%)

  • Number of cases by age group (< 1 year, 1-4 years, >=5 years) and immunization status
  • Number of first and third doses of diphtheria-tetanus-pertussis (DTP) administered to infants
  • Number of DTP booster doses given (if part of the country schedule)

Note: Coverage surveys should collect and analyse data on the timeliness of DTP doses, because doses given on time rather than late can substantially reduce mortality

Case-based data

(when DTP3 coverage is >= 90%; also for sentinel surveillance and outbreak investigation)

  • Unique identifier
  • Geographical information of case (e.g. district and province)
  • Date of birth
  • Sex: 1 = male; 2 = female; 9 = unknown
  • Date of onset
  • Total number of pertussis vaccine doses; 99 = unknown
  • Date of latest pertussis vaccine dose; 99 = unknown
  • Outcome: 1 = alive; 2 = dead; 9 = unknown
  • Classification: 1 = clinical; 2 = laboratory-confirmed; 3 = discarded

Recommended data analyses, presentations, reports

Aggregated data

  • Number of cases and incidence rate by month, year and geographical area
  • Proportion of cases immunized, partially immunized and not immunized
  • DTP3 coverage by year and geographical area
  • DTP booster by year and geographical area
  • Dropout rate by year and geographical area from DTP1 to DTP3
  • Completeness/timeliness of monthly reporting by geographical area

Case-based data

Same as aggregated data plus the following:

  • Crude and age-specific case-fatality rate
  • Age-specific, sex-specific and district-specific incidence rates by month and year

Principal uses of data for decision-making
  • Monitor incidence rates to assess the impact of the immunization system and policy (e.g. immunization schedule)
  • Monitor incidence rates by geographical area to identify high-risk areas or those with poor system performance (so that corrective actions can be taken)
  • Monitor age-specific attack rates to identify age groups at risk (which may affect immunization policy)
  • Identify outbreaks, conduct investigations to determine causes and understand the epidemiology of pertussis, ensure good case management
  • Understand the changing epidemiology of pertussis (e.g. change in age group at risk, change in periodicity)
  • Monitor case-fatality ratios and, if they are high, determine the causes (poor/late diagnosis, poor case management, poor/late access to care)
  • On the basis of coverage surveys, analyse whether vaccination doses are given on time; determine the causes of late dosing (since this may affect both morbidity and mortality)