One in five people who has tested positive for Covid-19 could not be reached or would not comply with the Contact Tracing Service (CTS), the Public Health Agency has said.
Between June 3 and 10 the CTS added 101 cases to the contact tracing system, but the service was only able to trace 81 people, an 80% success rate.
This came as one-third of people who tested positive for the virus could not be reached by the contact tracing system in England between May 28 and June 3.
In Northern Ireland some of the reasons the CTS was unable to contact individuals included having no contact details, people not responding, or, in a small number of cases, they would not engage with the service.
The contact tracing programme, which aims to identify and alert people who have come into contact with a person infected with coronavirus, was piloted here from April 27 before being fully rolled out at the start of last month.
The region was the first part of the UK to run such a programme. Contact tracing in the UK largely stopped on March 12 as the number of cases soared and the country switched from a 'contain' to 'delay' strategy.
SDLP MLA and Stormont health committee member Colin McGrath hailed the 80% success rate, but said 100% should be the target.
He did concede that people are always going to come into contact with others whom they have no contact details for.
"The numbers are small there but 80% is good," he said.
"I would like to see it higher obviously, because 100% would be the ambition.
"There is going to be an opportunity where people are going to be in contact with people whom they maybe don't know or don't have contact details for."
Mr McGrath added he hoped those in the CTS will continue the good work.
"I would have hoped that there is some expertise amongst the people who are recruited for that process of contact tracing," he added.
"I know in the first instances they were looking at people who were, for example, environmental health officers and people from specialist backgrounds.
"I would hope that they don't leave any stone unturned in trying to get contact details, but if any of us were to sit down and do an audit of whom we've been in contact with in the last number of days, we're probably going to be able to list people but not list their contact details."
The PHA urged anyone who develops Covid-19 symptoms to self-isolate and book a test.
"If the result is positive, we encourage you to work with the Contact Tracing Service to identify close contacts so we can follow up with them," it added.
"If you are contacted by the Contact Tracing Service, we urge you to follow the advice you are given."