Enhanced surveillance of COVID-19 in Ireland

Population-based seroprevalence surveillance

Published: 3/10/2022

National Serosurveillance Programme

Seroepidemiology of COVID-19 in Ireland Data Hub

About this release 

This data hub release by the Health Protection Surveillance Centre Seroepidemiology Unit (SEU) provides the latest results and methodologies of the National Serosurveillance Programme. The SEU aims to estimate the proportion of people who have antibodies to SARS-CoV-2 in the general population, either from vaccination, previous infection, or both, and to see if this changes over time. In time the SEU envisages expanding to include other pathogens of public health importance. 

Main points 

Laboratory Surveillance Network (LSN):

The proportion of individuals attending primary care services associated with the LSN who had antibodies to SARS-CoV-2 is estimated to be 99% for residual sera samples collected between 08 August and 16 August 2022.

Among specimens sourced from primary care, the proportion who had antibodies indicating prior infection is estimated to be 72% (95% Confidence Intervals (CI): 70%-75%).

By age group, the proportions of antibodies indicating prior infection are highest in those aged 18-29 years (88%, 95% CI: 84%- 92%) and lowest in those aged 50+ years (60%, 95% CI: 56%-64%).

Median quantitative S+ antibody levels overall are highest in the 70-79 age group and lowest in the 18-29 age group.

S+ antibody levels for the S+ and N+ group (indicating previous infection) have a median value higher than the S+ and N- group (indicating vaccination only).

Irish Blood Transfusion Service (IBTS):

The proportion of individuals donating blood at participating IBTS clinics who had antibodies to coronavirus is estimated to be 99.5% for samples collected between 11 September 2022 and 17 September 2022.

The proportion of donors who had antibodies indicating prior infection is estimated to be 77% (95% CI: 73%-81%).

By age group, the proportions of antibodies indicating prior infection were highest in those aged 18-29 years (88%, 95% CI: 80%- 94%) and lowest in those aged 50-79 years (71%, 95% CI: 63%-77%).

Median quantitative S+ antibody levels overall were highest in the 70-79 age group and lowest in the 18-29 age group.

S+ antibody levels for the S+ and N+ group (indicating previous infection) are higher than S+ antibody levels for the S+ and N- group (indicating vaccination only).

Interpretation 

When interpreting these results, it can be helpful to focus on the confidence intervals rather than the point estimates.

Residual primary care samples (LSN): these individuals were attending health care and having blood taken, which means they may have different risk factors for COVID-19 from the general population with no healthcare needs at a given point in time. ​There is therefore uncertainty with regard to the representativeness of these samples compared to the general population.

Residual blood donor samples: blood donors tend be healthier, on average, than the general population. Additionally, there are currently restrictions on blood donation for people with COVID-19 infection or COVID-19 symptoms. There is therefore uncertainty with regard to the representativeness of these samples compared to the general population

The trends in both the IBTS and LSN sources are similar, although in general, seroprevalence sourced from primary care in the LSN data shows slightly lower seroprevalence rates. 

It is not yet known at what antibody level an individual is protected from future infection, symptomatic disease, or severe disease. IgG antibodies are a subset of antibodies that develop in response to the spike protein, and only a part of the immune system as a whole.

Background

The National Serosurveillance Programme (NSP) is led by the Health Protection Surveillance Centre’s (HPSC) Seroepidemiology Unit (SEU), working in partnership with the UCD National Virus Reference Laboratory (NVRL) Serosurveillance Unit (SSU), the acute hospital Laboratory Surveillance Network (LSN) and the Irish Blood Transfusion Service (IBTS). It is overseen by a national multi-disciplinary and multi-sectoral Steering Committee.

The SEU aims to estimate the proportion of people who have antibodies to SARS-CoV-2 in the general population, either from vaccination or previous infection and to see if this changes over time. In time the SEU envisages expanding to include other pathogens of public health importance. 

The NSP conducts systematic sampling of residual specimens from eight acute hospital clinical chemistry laboratories within the LSN, and from IBTS clinics at regular intervals. The SEU currently reports on the seroprevalence of SARS-CoV-2, and in the future will report on other infectious diseases of public health concern by age group, sex and region.

Residual sera specimens are blood samples that were originally collected for clinical testing and are now due to be discarded. The residual samples are anonymised and then tested for antibodies to SARS-CoV-2. There are currently two residual blood sample sources: the IBTS and the acute hospital LSN. Blood donor samples are tested on site in IBTS and in St James’s Hospital. Samples from the acute hospital LSN are tested in the NVRL.

All specimens are stored for the time period necessary to complete the testing and are discarded as per the IBTS and NVRL protocols.

  • IBTS samples: These are sourced from three fixed site IBTS blood donation clinics in Ireland, two of which are in Dublin and one in Cork. Sequential sampling of blood donors aged 20-79 takes place until a target of 500 valid specimens is reached per sample week. From 17 October 2021 to 4 March 2022, samples were collected weekly; after 13 March 2022 samples have been collected fortnightly.
  • NSP LSN samples: These are sourced from a network of acute hospital clinical chemistry laboratories. Samples aged 18+ years are collected from general practice sources for adults, and from emergency department, outpatient clinics, phlebotomy clinics, and urgent care centres for paediatric samples. Between 100 and 300 specimens are requested from each laboratory, depending on capacity for participation; the quota requested reflects the national population proportions by age group and sex in the general Irish population.

*Please note: Ages are displayed on the data hub from 18+ years to encompass both IBTS and NSP sample frames in the same plots, however IBTS ages range from 20-79 years only.

Four structural proteins are encoded by the SARS-CoV-2 genome, including the spike (S), envelope (E), membrane (M), and nucleocapsid (N) proteins. Selected specimens are first screened using the Abbott SARS-CoV-2 IgG II Quantitative Assay, which detects antibodies to SARS-CoV-2 spike protein (S).

Specimens with a result of at least 50.0 arbitrary units per millilitre (AU/mL) are considered positive (S+).

S positive specimens (S+) are subsequently tested using the Roche Elecsys Anti SARS- CoV-2 assay which qualitatively detects immunoglobulin G (IgG) antibodies to the SARS-CoV-2 nucleocapsid protein (N). Vaccines currently approved for use in Ireland target the S protein only, and it is not expected that individuals will produce an immunological response to N proteins following vaccination.

As such, specimens with a cut-off index of at least 1.0 are considered N positive (N+), indicating prior SARS-CoV-2 infection.

Test result interpretations:

S-:  No antibodies to the spike protein detected

S+: Antibodies detected to the spike protein (indicates prior infection or vaccination for COVID-19)

S+ and N-: Serological results consistent with vaccination for COVID-19 only

S+ and N+: Serological results consistent with prior infection with SARS-CoV-2 (+/- vaccination for COVID-19)

Further information

To view the Seroepidemiology of COVID-19 data hub, please visit https://seroepi-hpscireland.hub.arcgis.com/

You can contact the National Serosurveillance Programme at: seu.programme@hpsc.ie.