Epidemiology of COVID-19 in Ireland Frequently Asked Questions

Q. What does epidemiological data mean?
Epidemiology is the study of the distribution (patterns, frequency and determinants (causes, risk factors) of health and disease in specified populations. It is the study of how often different diseases occur and why. It is used to inform decisions about the control of health problems. Epidemiological data are also known as ‘epi-data’. COVID-19 epi data for Ireland describes the pattern and frequency of the COVID-19 within the Irish population.

Q. What epi data does HPSC provide relating to COVID-19?
HPSC publish a number of reports relating to COVID-19 which can be found here. The weekly report is published each week and provides information on cases notified in the past week (an ‘epi-week’), and cases since the start of the epidemic. HPSC also produce reports on:

Q. What is the ‘epi-date’?
The epi-date is the epidemiological date for a case. It is based on the earliest of dates available on the case and taken from date of onset of symptoms, date of diagnosis, laboratory specimen collection date, laboratory received date, laboratory reported date or event creation date/notification date on CIDR. By using this date rather than event creation/ notification date, adjusts for any delays in testing/notification.

Q. What is an “epi week”?
For epidemiology the 365 days of the year are split into 52 or 53 epidemiological weeks (epi weeks). This is to standardise time for epidemiological surveillance. This is important to allow for comparison of events that occurred in a given year, or a period of a year, with previous years. As these are internationally agreed they also facilitate comparison between countries. Epi weeks (epidemiological weeks) start on a Sunday and end on a Saturday. The first epidemiological week of the year ends on the first Saturday of January, as long as it falls at least four days into the month, even if it means that this first week starts in December.

A breakdown of epidemiological weeks is available here.

Q. What is CIDR?
CIDR stands for Computerised Infectious Disease Reporting (CIDR). It is an information system developed to manage the surveillance and control of infectious diseases in Ireland.

Q. Why may the number of COVID-19 notifications on CIDR differ from other HSE data sources?
The number of notifications on CIDR will differ from other HSE data sources, such as positive SARS CoV-2 results uploaded to the COVID Care Tracker, for several reasons including:

  • Deduplication of repeat positive tests in the same person
  • Differing upload schedules by the laboratories to CCT and CIDR – uploading of positive records on CIDR by laboratories is a more manual process than uploading to CCT. Uploading to CIDR typically occurs on the day or within one day of authorisation of the laboratory test result
  • The time required for the two-step method of processing notifications on CIDR - this typically occurs either on the day or within one day of laboratories uploading positive records on CIDR

Typically, the daily number of notifications to CIDR lag approximately one day behind the number of positive SARS CoV-2 results uploaded to the COVID Care Tracker. However, changes in the timeliness of the processing steps described above can increase or decrease this lag.

CIDR as the national surveillance system is the definitive source for validated data on COVID-19 cases in Ireland which meet Irish and European case definitions.

Q. Can you describe what death in confirmed/probable/possible case of COVID-19 means?
The case definition for COVID-19 in Ireland has been updated routinely during the pandemic in accordance with the European Centre for Disease Prevention and Control (ECDC) guidance and updates.

  • Deaths in confirmed COVID-19 case: A death in a person with laboratory confirmation of COVID-19 infection, irrespective of clinical signs and symptoms (including post mortem).
  • Deaths in probable COVID-19 case: A death in a person with probable COVID-19 infection as described in the COVID-19 case definition below
  • Deaths in possible/suspect COVID-19 case: See below scenarios for possible/suspect cases which should be reported as COVID-19 deaths.
    • All deaths in patients suspected of having COVID-19 i.e. patients with symptoms clinically compatible with COVID-19 illness. These suspect cases may or may not have been tested for COVID-19 prior to death. These possible COVID-19 deaths include patients with pending COVID-19 laboratory results. 
    • All unexplained deaths/sudden deaths in residential facilities with a confirmed/suspected COVID-19 cluster/outbreak of illness unless there is a clear alternative cause of death that cannot be related to COVID-19 disease (e.g. trauma)

A person can be classified as a probable or possible COVID-19 death without laboratory confirmation of COVID-19 based on the criteria for probable and possible cases of COVID-19 as outlined in the case definition.

Q. Is there a time-lag between when cases are confirmed and clusters are confirmed?
Sometimes, it can occur that there is a time-lag between when cases are confirmed and when clusters are confirmed. This can happen as positive individual cases may be identified before a cluster of cases is observed.

Q.  How many of the COVID-19 cases have been hospitalised/are currently in hospital?
The weekly COVID-19 epidemiology report which can be found here provide a breakdown of figures which include:
•    Number of cases
•    Number of cases hospitalised
•    Number of cases in ICU

Q. What is the definition of an Irish travel related case?
Travel related cases are defined as cases who acquire COVID-19 infection outside of Ireland (imported cases) and cases who acquire COVID-19 infection directly from imported cases.

Q.  Is a person who travels to Ireland and tests positive while in Ireland considered to be an Irish case?
If someone from another country is diagnosed with an infectious disease in Ireland, they are notified as an Irish case and included in the Irish case count. Their address of residence in Ireland is used as their geographic location within Ireland.

Q.  What are the sources of COVID-19 data on death?
HPSC receives information from a number of sources regarding deaths.

Its primary source of information is the national infectious disease surveillance system (CIDR). CIDR is populated by data from eight regional Departments of Public Health (DPHs). Staff in regional DPHs populate CIDR data regarding deaths, either directly from clinical information they receive when investigating cases and outbreaks of COVID-19 or from coroners files that the regional DPHs receive on a regular basis. HPSC may also receive coroner’s files to cross reference with regional DPH data. In addition, HPSC receives a daily file from the General Registrar’s Office (GRO) as an additional source of data regarding COVID-19 deaths.

Q. What are the dates of the pandemic waves in Ireland?
Below are the dates of the pandemic waves in Ireland:

Wave 1 (weeks 10-31 2020) 01/03/2020 – 01/08/2020
Wave 2 (week 32-47 2020) 02/08/2020 – 21/11/2020
Wave 3 (week 48 2020 – week 25 2021) 22/11/2020 – 26/06/2021
Wave 4 (week 26 2021 – week 50 2021) 27/06/2021 – 18/12/2021
Wave 5 (week 51 2021 onwards) 19/12/2021 onwards

Last updated: 04 July 2022

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