Health intelligence and disease surveillance

ESR’s team of epidemiologists, public health doctors, clinical microbiologists and laboratory scientists provide the Ministry of Health and the wider health sector with intelligence on notifiable diseases and other serious health threats, such as influenza and antimicrobial resistance. There are over 50 diseases in New Zealand that are notifiable, including COVID-19.

ESR develops and operates national surveillance programmes that collect data on these diseases. These programmes are carefully designed by our surveillance experts to ensure that they are robust and representative. This means ensuring that enough of the right clinical specimens are collected from the right people in the right locations across the country. Our experts analyse and interpret the data to provide intelligence including, identification of outbreaks, descriptions of disease spread and explanations for disease patterns and trends.

Ensuring that surveillance programmes are well designed is really important as the data being collected needs to be good enough to provide sound intelligence. This is critical when the intelligence is being used to make decisions about putting in place disease control measures or monitoring how well the measures are working.

What is a surveillance case definition?

A surveillance case definition is a set of criteria used to classify and count cases of a disease in a consistent way as part of a surveillance programme. For a patient to be counted as having the disease they must meet these criteria.

Surveillance case definitions include clinical criteria (signs and symptoms of disease), epidemiological criteria (linking patients to another person, a place or a time) and laboratory criteria (test results).

Surveillance case definitions change over time, particularly for new diseases, as more is understood about a disease or to meet changing public health objectives during a response.

The current COVID-19 case definition can be found here(external link)

Given New Zealand’s current aim to eliminate COVID-19, the case definition needs to be broad enough to capture all those who may have the disease. However, because the symptoms of COVID-19 are similar to many other viruses, such a broad case definition is also likely to include patients who are infected with other viruses.  

This means that case investigation by PHU’s is really important. After the investigation, the case status of a patient who may have initially been suspected of having COVID-19 may change category. The case status may change to “Not a case”, if COVID-19 is excluded and/or another infection explains their illness or a “Confirmed” case, if laboratory testing confirms COVID-19 infection.