Almost one in 10 staff at a Dublin hospital have antibodies for Covid-19 despite having never tested positive for the virus.
A new study into staff at Tallaght University Hospital (TUH) also shows almost one in five of staff surveyed have at one stage caught coronavirus.
The study, which is the first of its kind carried out within a hospital setting in Ireland, looked at the SARS-CoV-2 seroprevelance of healthcare workers over a three-month period from July to October.
It found that, of the 1,200 staff members involved, antibodies for Covid-19 were detected in 18pc.
This increased for staff with roles involving more direct contact with patients, such as nurses and doctors, at 20pc, while it decreased to 13pc for workers who had a less direct role with patients.
Prior to the study, 12pc of the participants had been diagnosed with Covid-19 at some stage.
However, when the group with known infections was excluded, the seroprevalence among those never confirmed to have had the virus was almost one in 10 at 7.5pc - or around 88 people.
The study also noted that of these, more than half had suspected they had been infected at some point despite not having a positive test.
Dr Anna Rose Prior, consultant microbiologist at Tallaght University Hospital, said that the results demonstrate that a high proportion of Covid-19 infection in healthcare workers goes undetected.
This, she said, supports the requirement for universal mask use for all patient interactions during the pandemic.
This would both protect staff and minimise the risk of healthcare workers unknowingly transmitting infection to patients.
"This study enables us to identify colleagues who do and don't develop a measurable immune response to Covid-19.
"The study is a 12-month study, meaning we will follow all participants who want to remain in the study for 12 months, measuring their antibody levels at multiple time points," Dr Anna Rose Prior said.
"This will give us two key pieces of information.
"The first is understanding how long a measurable level of antibodies to Covid-19 is present in participants and secondly, how many staff develop antibodies during the coming waves of infection.
"I would also like to acknowledge the incredible work of the team in Laboratory Medicine who have dedicated their valuable spare time to conduct the study at what is a particularly busy time in the hospital," she added.
Of the nearly 1,200 staff that took part in the study, 943 were female and 233 were male.
The participants represented around one-third of the entire staff working in Tallaght University Hospital.
The hospital said that there was "a clear desire from staff to have their antibodies measured and this somewhat drove the project as areas and departments aided in the taking of samples to ensure their team members were involved."
It was open to all hospital staff including doctors, nurses, lab assistants, social workers, cleaning staff as well as electricians.
The study will last 12 months so antibody levels will continue to be tested at different times for willing participants.
In August, the HSE released a study on Covid-19 antibodies among the Irish population which found that 1.7pc had them.
The study, carried out by the Health Protection Surveillance Centre (HPSC) estimated at the time that 59,500 people in Ireland between the ages of 12 to 69 years had Covid-19 up to mid-July, three times the number of detected cases.