National SARS-CoV-2 Wastewater Surveillance Programme
Week 43 2024 (20/10/2024 to 26/10/2024)
Report prepared on 30/10/2024
Summary
In week 431 2024 (week beginning 2024-10-20), samples were received and analysed from 29/30 catchment areas. SARS-CoV-2 RNA was detected in 29 of these (100%). Of the 29 catchment areas for which a change from the previous week could be calculated, the viral load remained stable (within +/- 10%) in 5 areas, while 24 experienced decreases of 10% or more, and 0 saw increases of at least 10%.
Please note as of 09/06/2024 the smoothing methods have been modified to give a better fit. Please see technical notes for more details.
Programme overview
People infected with SARS-CoV-2, the virus that causes COVID-19 disease, can shed the virus in their stool, which can then be detected in wastewater. This means that wastewater surveillance can be used to monitor the circulation of SARS-CoV-2 in the population.
The National SARS-CoV-2 Wastewater Surveillance Programme (NWSP) has been established through a partnership with Uisce Éireann (formerly Irish Water), the National Virus Reference Laboratory (NVRL), University College Dublin (UCD), the HSE Health Protection Surveillance Centre (HPSC) and the HSE Health Intelligence Unit (HIU). The NWSP samples 30 wastewater catchment areas across Ireland on a weekly basis and analyses for the presence of SARS-CoV-2, with the exception of Ringsend in Dublin which is sampled twice a week. These 30 wastewater catchment areas (see Figure 3) cover 70% of the population connected to public wastewater treatment facilities and are spread throughout every region of Ireland. Weekly SARS-CoV-2 sequencing started in November 2023 on one sample from the Ringsend wastewater treatment plant.
The NWSP is an additional tool in Ireland’s surveillance strategy for COVID-19. The NWSP aims to complement our case-based surveillance systems (i.e. our monitoring of the number of people testing positive or presenting to a healthcare provider with symptoms). The SARS-CoV-2 viral load in a wastewater catchment area correlates with other indicators of virus circulation.
Please note that stool and wastewater are not recognised sources of transmission for SARS-CoV-2. Standard hygiene measures should be used after using the toilet. Guidance from the WHO for water and sanitation providers recommends standard best practices, including PPE for those working in proximity to wastewater, be followed.
Methods
Results from Week 43 2024
SARS-CoV-2 viral load
In week 432 2024 (week beginning 2024-10-20), samples were received and analysed from 29/30 catchment areas. SARS-CoV-2 RNA was detected in 29 of these (100%). Figure 1 below gives the proportion of samples (including the two samples from Ringsend) with positive results for this and previous weeks, while Figure 2 summarises the catchment-specific classification of sample results for each week of the programme.
Figure 1 Wastewater SARS-CoV-2 detections in Ireland by week, for the year leading up to week 43 2024
Note: There are 31 possible samples from 30 catchment areas because Ringsend is sampled twice weekly. Samples with an undetected result may have SARS-CoV-2 present but the amount was too low to be detected by the test. It does not mean that SARS-CoV-2 is not circulating in the wastewater catchment area of the wastewater treatment plant (WWTP).
Figure 2 Weekly SARS-CoV-2 sample classification by wastewater catchment area, for the year leading up to week 43 2024
Of the 29 catchment areas for which a change from the previous week could be calculated, the viral load remained stable (within +/- 10%) in 5 areas, while 24 experienced decreases of 10% or more, and 0 saw increases of at least 10%. Of these, 0 had an increase between 10 to 50%, and 0 had an increase of more than 50%. Figure 3a below shows the locations of each catchment area and these relative changes in viral load from the previous week, while Figure 3b displays the absolute viral load for each catchment areas in week 43 2024. Figure 4 displays the SARS-CoV-2 viral loads (gc/day) by wastewater catchment area each week since the start of the NWSP in week 19 2021.
Figure 3a Approximate location of wastewater catchment areas and percent change in SARS-CoV-2 viral loads between weeks 42 and 43 2024, National Wastewater Surveillance Programme
Figure 3b Approximate location of wastewater catchment areas and SARS-CoV-2 viral loads (gc/day) in wastewater and percentage change compared to previous week, National Wastewater Surveillance Programme, week 43 2024.
Figure 4 Weekly SARS-CoV-2 viral load (gc/day) in wastewater, and GAM smoothed trend, by wastewater catchment area, National Wastewater Surveillance Programme, week 19 2021 to week 43 2024
Figure 5 Weekly distribution of population-normalised SARS-CoV-2 viral load (gc/day/person), National Wastewater Surveillance Programme, week 19 2021 to week 43 2024
Figure 6 below is a phase plot that traces each catchment area forward in time in terms of the population-normalised viral load each week and the percent change from the previous week. This allows us to more easily identify catchment areas where the recent change in viral load has been noteworthy, and compare that to the history of such changes over the duration of the programme.
Figure 6 SARS-CoV-2 viral load phase plot by wastewater catchment area, National Wastewater Surveillance Programme, week 19 2021 to week 43 2024
Acknowledgements
Sincere thanks are extended to all those who are participating in the collection and processing of samples, processing data and reporting of data used by the NWSP. This includes the staff at participating wastewater treatment plants, Uisce Éireann (formerly Irish Water), laboratory staff (UCD and NVRL), epidemiologists, and administrative staff.
Report prepared by the Wastewater Team, HPSC
Glossary
Below detection limit (BDL): The limit of detection is the lowest concentration of viral material that can be detected in a wastewater sample. If the concentration of viral material in a sample is below the limit of detection, it does not mean that there is no virus present. Viral material may be present, but it cannot be detected by the test due to its very low concentration.
Below Quantifiable Limits (BQL): The limit of quantification is the lowest concentration of viral genetic material that can be reliably quantified. A result which is BQL means that viral RNA was detected but the concentration cannot be accurately determined.
Composite Samples: Wastewater sample consists of numerous individual discrete samples taken at regular intervals over a period of 24 hours.
Grab sample: A wastewater sample collected at a particular time and place can represent only the composition of the source at that time and place.
Flow rate: The total volume arising from the wastewater catchment area in the 24hours the sample is taken (m3 per day) as measured at the inlet of the wastewater treatment plant.
Population data: Population normalised results use 2022 estimates of the residential population of the wastewater catchment area provided by Uisce Éireann.
Technical notes
Wastewater sample collection: Staff of the participating wastewater treatment plants collect 24-hour composite samples on specified days. In some instances incomplete composites, or grab samples, are used instead.
Laboratory analysis of wastewater samples for SARS-CoV-2 viral load: Carried out by UCD. Following arrival of the wastewater samples in the laboratory, the virus is concentrated and the genetic material is extracted. The genetic material (RNA) is quantified using reverse transcriptase quantitative PCR which is used to determine the concentration of the virus in wastewater. The amount of viral genetic material per day entering a wastewater treatment plant can be calculated based on the concentration of the virus and the flow rate.
Selection of wastewater catchment areas for inclusion in the NWSP: Catchment areas were selected to provide the greatest coverage of the population and reasonable geographic coverage The 30 wastewater catchment areas included in the NWSP cover 70% of the population connected to public wastewater treatment plants, and are shown in Figure 1.
Smoothing: Smoothed trends were fit to observed data with generalised additive models (GAMs) using restricted maximum likelihood estimates of smoothing parameters with interpolation for weeks with missing sample data. From XX/XX/2024, the GAMs used to smooth the data were updated to increase the number of knots, allowing for more flexible trend estimates that better fit the observed data. Any differences in the smoothed trends from those previously reported are due to these updates. The resulting trend lines should only be viewed as one reasonable, visual summary of the observed data. Smoothed trends should be reviewed with caution for areas with several consecutive weeks of missing data.
Data limitations and uncertainties
In general, higher levels of SARS-CoV-2 in wastewater suggests more people with SARS-CoV-2 infection in the wastewater catchment area. However, people shed different amounts of virus during the period that they are infected which declines are as they are clearing the virus. For this and other reasons it is therefore difficult to reliably estimate the number of people with SARS-CoV-2 infection in the catchment area based on analysis of SARS-CoV-2 RNA in wastewater.
Aside from the number of people with SARS-CoV-2 infection, and the amount of virus they shed, there are other factors which influence the amount of SARS-CoV-2 detected in wastewater. For example, wastewater treatment plants also take in surface waters from rainfall, which alters the flow rate of influent entering the treatment plant. The concentration of SARS-CoV-2 is therefore adjusted to take the daily flow rate of the plant into account. When a flow rate is not available for the day the composite sample was taken, the average annual flow rate is used instead. Composite samples are the preferred sample type as the reflect wastewater passing through the sampling point for a 24-hour period. However, composite samples are not always possible. The NWSP will continue to monitor its methodology as part of its quality improvement process.
NWSP data should be reviewed in conjunction with data from other case-based surveillance systems.
Further information
A technical report by the Joint Research Centre (JRC) and the Directorate-General for Environment (DG-ENV) of the European Commission on wastewater surveillance for SARS-CoV-2 is available here.
Links to dashboards of other SARS-CoV-2 wastewater surveillance programmes are available here.
Appendices
Supplemental Table 1 National Wastewater Surveillance Programme sampling results by wastewater catchment area, week 43 2024
| County | Wastewater catchment area | Sample type | Sample date | Result category |
|---|---|---|---|---|
| Carlow | Carlow | Grab | 22/10/2024 | Positive |
| Cavan | Cavan | 24 Hrs | 21/10/2024 | Positive |
| Clare | Ennis North | 24 Hrs | 20/10/2024 | Positive |
| Cork | Ballincollig | 24 Hrs | 21/10/2024 | Positive |
| Cork City | 24 Hrs | 21/10/2024 | Positive | |
| Cork Lower Harbour | Incomp | NA | NA | |
| Donegal | Buncrana | 24 Hrs | 22/10/2024 | Positive |
| Letterkenny | 24 Hrs | 22/10/2024 | Weak Positive | |
| Dublin | Balbriggan | Grab | 22/10/2024 | Positive |
| Portrane Donabate | 24 Hrs | 21/10/2024 | Positive | |
| Ringsend 1 | 24 Hrs | 20/10/2024 | Positive | |
| Ringsend 2 | 24 Hrs | 23/10/2024 | Positive | |
| Shanganagh | Grab | 22/10/2024 | Positive | |
| Swords | 24 Hrs | 21/10/2024 | Positive | |
| Galway | Galway | 24 Hrs | 20/10/2024 | Positive |
| Kerry | Tralee | 24 Hrs | 21/10/2024 | Weak Positive |
| Kildare | Lower Liffey Valley Regional Sewerage Scheme (Leixlip) | 24 Hrs | 22/10/2024 | Positive |
| Upper Liffey Valley Sewerage Scheme (Osberstown) | 24 Hrs | 22/10/2024 | Positive | |
| Kilkenny | Kilkenny | 24 Hrs | 20/10/2024 | Positive |
| Laois | Portlaoise | 24 Hrs | 20/10/2024 | Positive |
| Limerick | Limerick City | 24 Hrs | 21/10/2024 | Positive |
| Louth | Drogheda | Grab | 21/10/2024 | Positive |
| Dundalk | 24 Hrs | 21/10/2024 | Positive | |
| Mayo | Castlebar | Unknown | 20/10/2024 | Positive |
| Meath | Navan | 24 Hrs | 21/10/2024 | Positive |
| Monaghan | Monaghan | 24 Hrs | 21/10/2024 | Positive |
| Sligo | Sligo | 24 Hrs | 22/10/2024 | Positive |
| Tipperary | Clonmel | 24 Hrs | 21/10/2024 | Positive |
| Waterford | Waterford | 24 Hrs | 20/10/2024 | Positive |
| Westmeath | Mullingar | 24 Hrs | 22/10/2024 | Positive |
| Wexford | Wexford | 24 Hrs | 20/10/2024 | Weak Positive |
Report prepared on 30/10/2024
Version 1.1
Dates of epidemiological weeks are available in the technical notes and at: https://www.hpsc.ie/notifiablediseases/resources/epidemiologicalweeks/↩︎
Dates of epidemiological weeks are available in the technical notes and at: https://www.hpsc.ie/notifiablediseases/resources/epidemiologicalweeks/↩︎