Serosurveillance of COVID-19 in Ireland

Seroepidemiology of COVID-19 in Ireland Data Hub - updated 21/11/2023

• About this release
• Main points and interpretation
• Background
• Further information

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 and interpretation

Overall, in blood donors and primary care, the sample seroprevalence remains very high at 99% for both sources.

Focusing on infection seroprevalence:

The latest blood donor infection seroprevalence is estimated to be 85%. This means that currently approximately 15% of blood donors do not have evidence of previous SARS-CoV-2 infection. This differs by age: the infection seroprevalence is 95% among donors aged 18-29 years, 83% among donors aged 30-49 years and 84% among donors aged 50+ years. This may reflect some waning of antibodies due to natural infection in the population by age.  

Latest infection seroprevalence from primary care sources is estimated to be 77%, indicating that currently approximately 23% of individuals attending primary care services do not have evidence of previous natural infection. Similar to the blood donor infection data, infection seroprevalence in older people is lower than that in younger people at 90%, 86% and 74% in the age groups 18-29, 30-49 and 50+ years respectively.

Focusing on quantitative spike antibody levels:

In both blood donor and primary care sources, median spike antibody levels overall (that indicate vaccination, infection, or both infection and vaccination) vary by age group, with antibody levels currently highest in the 50+ age group for blood donors and for primary care sources. For blood donors, median spike antibody levels over all ages appear to be trending slowly downwards since mid-April 2023.  For primary care sources, it is too early to report median spike antibody levels as there was a change in sample collection (over- sampling of people over 70 years) in late April.

When median spike antibody levels are separated between those with evidence of infection and/or vaccination (so-called hybrid immunity), and those who only have evidence of vaccination and not infection, the antibody levels are higher in those with hybrid immunity than in those with evidence of vaccination alone.

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 are only a part of the immune response.

Uptake of COVID-19 primary and 1st booster vaccination among adults (18+) in Ireland has been high (>90% uptake for primary course and >80% uptake for 1st booster course).  Currently, an Autumn-Winter booster dose is available to people at high risk of serious illness to COVID-19.  This includes people over 50 years of age, people over 5 years of age with a weak immune system, people with a condition which puts them at high risk of serious illness from COVID-19 and health & care workers.  As of the 5th of November 2023, uptake rates of Autumn-Winter booster in those over 5 years, over 18 years, over 50 years, over 60’s years and over 70 years respectively were 9.8%, 12.1%, 24.9%, 34.1%, 44.3%. 

 More information on vaccination can be found in HPSC’s COVID-19 vaccination reports here:


Residual primary care samples come from individuals that were attending healthcare and having blood taken, which means they may have different risk factors for COVID-19 than the general population that have no healthcare needs at a given point in time.

Residual blood donor samples come from blood donors who tend be healthier, on average, than the general population. Additionally, there have been varying 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. Please check the data hub for confidence intervals around and trends in the seroprevalence estimates.

Most, but not all, individuals who are infected with SARS-CoV-2 mount an antibody response. Among those who do, antibody levels may wane over time. As a result, antibody seroprevalence may underestimate population-level exposure to SARS-CoV-2.

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 October 2021 to 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. Samples are collected approximately every six weeks; SARS-CoV-2 testing was suspended between September 2022 and February 2023.

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 

To watch an animation detailing the work of the NSP and its partners, please visit: 

You can contact the National Serosurveillance Programme at: