Increase in SARS-CoV-2 seroprevalence in Irish HCWs during third wave of pandemic

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Healthcare workers and people they live with are at an increased risk of contracting severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Hospital healthcare workers (HCWs) worldwide have been found to have a higher number of SARS-CoV-2 infections and higher antibody seroprevalence compared to the general population.

A thorough understanding of SARS-CoV-2 transmission and potential immune response in hospitals is vital to mitigate transmission at the hospital level and offers valuable information on the viral transmission patterns among HCWs.

Comparing seroprevalence and changes in risk factors for seropositivity in HCWs during the first and third wave of the pandemic

SARS-CoV-2 seroprevalence among HCWs in 2 Irish hospitals was found to be 15% and 4.1%, respectively, in October 2020. Six months later, researchers compared the seroprevalence in the same HCWs and evaluated changes in risk factors for seropositivity as the pandemic progressed and serological response to COVID-19 vaccination.

Hospital 1 is a Dublin-based tertiary referral hospital with nearly 4,700 employees. It is one of the largest acute hospitals in Dublin, the capital city of Ireland. Hospital 2 is a tertiary referral hospital in Galway with about 4400 employees. It is the main acute hospital in Galway. Both these hospitals received COVID-19 patients during the first wave of the pandemic in Ireland.

This study is currently available on the medRxiv* preprint server while awaiting peer review.

Study: Prevalence of Antibodies to SARS-CoV-2 following natural infection and vaccination in Irish Hospital Healthcare Workers; changing epidemiology as the pandemic progresses. Image Credit: RomarioIen / Shutterstock

In April 2021, all 9,038 staff from both hospitals were invited to participate in an online survey and get tested for SARS-CoV-2 antibodies.

The researchers quantified anti-nucleocapsid and anti-spike antibodies and calculated frequencies and percentages for positive SARS-CoV-2 antibodies and adjusted relative risks for participant characteristics using multivariable regression analysis.

A repeat cross-sectional study was conducted to re-evaluate the prevalence of anti-SARS-CoV-2 antibodies in HCWs in these two hospitals following the third and largest wave of the COVID-19 pandemic in Ireland and monitor changes in risk factors related to demographics, work, and living arrangements.

The team also aimed to analyze serological response to COVID-19 vaccination in the fully vaccinated sub-group and measure breakthrough infections after vaccination.

Seroprevalence in HCWs from both hospitals increased during the third wave

A total of 5,085 HCWs participated in the study. The results showed that seroprevalence increased to 21% and 13%, respectively, in the two hospitals as the pandemic progressed, compared to 15% and 4.1%, respectively, in October 2020. Also, 26% of infections were previously undiagnosed.

Some factors significantly associated with seropositivity were black ethnicity, low literacy levels, and living with other HCWs. Direct patient contact was also a significant risk for healthcare assistants, nurses, or others who had daily contact with COVID-19 patients or with patients without confirmed COVID-19. Breakthrough infection was detected in 23 (0.6%) out of 4,111 fully vaccinated participants, all of whom had anti-S antibodies.

Ongoing testing and evaluation of vaccinated HCWs with confirmed infection is key to understanding the reasons for breakthrough infections

The increase in seroprevalence noted in this study highlights the magnitude of the third wave of the COVID-19 pandemic in Ireland. Genomic sequencing will help distribute the risk to the workplace vs. that to the household/community.

Although the antibody response to vaccination is promising, more studies are needed to establish a correlation between serological response and functional immunity and to determine the duration of protection from infection, especially in the context of the emergence of SARS-CoV-2 variants of concern.

The findings show that even at this stage of the pandemic, collaborative efforts are required to mitigate the risk factors related to ethnicity and low literacy levels.

According to the authors, breakthrough infections will continue in fully vaccinated individuals and must be further studied. Due to their increased exposure, HCWs are an ideal group to study breakthrough infections.

The undiagnosed and breakthrough infections found in this study highlight the need for ongoing infection prevention and control measures and HCW testing in close-contact settings. Vaccinated HCW with confirmed infection need to be actively evaluated using SARS-CoV-2 whole-genome sequencing, assessment of host determinants, and serology testing to gain a better understanding of the factors facilitating breakthrough infection.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:

  • Prevalence of Antibodies to SARS-CoV-2 following natural infection and vaccination in Irish Hospital Healthcare Workers; changing epidemiology as the pandemic progresses Niamh Allen, Melissa Brady, Una Ni Riain, Niall Conlon, Lisa Domegan, Antonio Isidro Carrion Martin, Cathal Walsh, Lorraine Doherty, Eibhlin Higgins, Colm Kerr, PRECISE Study Steering Group, Colm Bergin, Catherine Fleming medRxiv 2021.11.04.21265921; doi:,