Serosurveillance reveals population insights for targeted health interventions


In a recent study published in the journal European observation, the researchers describe how they set up a surveillance system in Wales, United Kingdom (UK), using residual samples from the Welsh Blood Service (WBS) to gather information on community exposure to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). They provided data on the seroprevalence of SARS-CoV-2 antibodies in these donors stratified by time, region of residence, and demography.

Study: Serosurveillance for SARS-CoV-2 in Welsh blood donors: establishment of the monitoring system and results through November 2022. Image credit: LightField Studios/Shutterstock

Stady: Serosurveillance for SARS-CoV-2 in Welsh blood donors: establishment of the surveillance system and outcomes to November 2022. Image credit: LightField Studios/Shutterstock


Understanding differences in immunity across a geographic area or within a country facilitates targeted public health actions. To this end, serosurveillance can help to effectively monitor exposure to SARS-CoV-2 in a population and to identify distinct trends in natural exposure to the virus versus vaccination.

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In this study, researchers collaborated with three organizations to sample and test blood samples collected by WBS between June 2020 and January 2022 and then analyze the data and present the findings to solicit feedback from key stakeholders involved in the response to the pandemic in Wales.

The sampling group consisted of 17-year-old volunteer blood donors who donated blood to WBS in Wales between 29 June 2020 and 20 November 2022. Public Health Wales (PHW) recorded demographic and clinical information for each donor and assigned a donor key to the population . Information to help take recurring donations from an individual.

Identification of recurrent donors helped investigate changes in SARS-CoV-2 antibody positivity over time. WBS also recorded the age, gender, ethnicity, and region of residence of all donors.

The team performed testing for SARS-CoV-2 antibodies using the Roche immunoassay on blood samples that will be disposed of as waste products from blood processing at the biochemistry laboratories of the Swansea Bay University Health Board (SBUHB) and Cwm Taf Morgannwg University Health Board (CTMUHB) .

Earlier, the team tested all samples for SARS-CoV-2 nucleocapsid (N) antigen antibodies. However, for samples taken after April 12, 2021, they also tested for the presence of naturally occurring or vaccine induced antibodies to the spike (S) antigen.

The positive cut-off index (COI) for both N and S. antigens It was 1 and 0.8 U/ml, respectively. Samples that tested positive for anti-N and anti-S antibodies indicated recent natural SARS-CoV-2 infection, whereas samples positive for anti-S antibodies indicated recent vaccination but no previous infection. Next, the researchers used the e801 platform to locally validate the antibody assays, assessing accuracy by performing identical measurements on 25 groups of positive and negative patients over the course of five days.

Finally, PHW staff retrieved data from the portal, researchers uploaded and compiled data files at four-week intervals for dissemination among all key stakeholders.


In this study, researchers used a low-cost method of obtaining blood samples to perform SARS-CoV-2 antibody testing (serodissemination) of the Welsh population. Anti-N and anti-S antibody data indicated the cause of seroconversion and provided much needed information about the distribution of individuals susceptible to penetrating infection.

The observed temporal differences in the prevalence of SARS-CoV-2 antibodies significantly indicated transmission of SARS-CoV-2 and coronavirus disease 2019 (COVID-19) in Wales. For example, a consistently high prevalence of anti-S antibodies is consistent with a consistently high vaccine uptake in Wales. Likewise, the sharp increase in anti-N antibodies coincided with the appearance of Omicron in November 2021.

In agreement with previous reports, the authors observed a higher incidence of SARS-CoV-2 antibodies in young adults acquired from natural infection. In addition, they demonstrated that the rate of antibody waning was lowest in people between the ages of 18 and 24 and maximum in people between the ages of 70. The age-related gap between seropositivity was due to staggered eligibility for vaccination. Globally, governments initially prioritized older people for COVID-19 vaccines, anticipating higher morbidity and mortality rates in this vulnerable population.

Because of the higher vaccine frequency in some ethnic minorities, white donors had a higher seroprevalence rate of anti-S antibodies than blacks and Asians. On the contrary, they had a higher prevalence of antibodies due to natural infection than white donors. Notably, only 2% of blood donor samples belonged to people who self-reported as black or Asian.

On the contrary, the majority (43%) of the population sample lives in South Wales and, therefore, fully represents the Welsh population in relation to residential demographics. Furthermore, anti-N antibodies were more prevalent in South Wales and Gwent regions, suggesting localization of COVID-19 outbreaks to specific postcodes.

Because infection and vaccination records are not available for repeat donors, it is possible for these individuals to become reinfected between donations. Thus, the authors were unable to identify true reversal among these people. They note that 55% of repeat donors tested negative for N antigen at all study visits, indicating that although viral transmission peaked several times; To date, many individuals have avoided infection.

The observed lower seroconversion rate of anti-N antibodies compared to anti-S antibodies favored previous findings that anti-N antibodies dilute faster than anti-S antibodies, thus serving as a surrogate for re-infection in this study.


To summarise, the proportion of Welsh blood donors who had vaccine-stimulated antibodies rose over time, and most had acquired immunity to SARS-CoV-2 by the end of the study. The percentage of donors with N antibodies due to COVID-19 peaked at 80% by November 2022, indicating a high rate of disease transmission. Remarkably, except for the gender, age, ethnicity, and place of residence of the donor, significantly affected SARS-CoV-2 antibody levels.

During the pandemic caused by COVID-19, blood donor-based serosurveillance provided continuous information about population immunity and could continue to do so after the pandemic, which could help policymakers design future public health strategies. This serosurveillance system continues to identify cases of COVID-19 despite the discontinuation of free testing of the general population in Wales since June 2022. Furthermore, linking data from Wales’ serosurveillance systems to public health databases can help interpret this data.

However, information from the blood donor surveillance system successfully detected temporally variable seroprevalence of antibodies to SARS-CoV-2 N and S antigens, complementing other surveillance system data to provide a complete picture of the COVID-19 epidemic in Wales. Policymakers can use this data to devise mitigation and prevention strategies that roll out targeted SARS-CoV-2 testing and COVID-19 vaccination initiatives.


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