Northern Ireland's Public Health Agency (PHA) has said there were 612 confirmed or probable cases of the Delta variant of Covid-19 detected throughout all 11 local council areas by yesterday.
The PHA said that around half of Covid-19 positive cases sampled in Northern Ireland are now indicative of the Delta variant.
The variant was first identified in India, and is now dominant in the UK.
Up to Monday, a total of 449 people in Omagh and 689 in Ballymoney were tested after a number of probable cases were identified in the two towns.
Early analysis of results shows a total of 31 positive cases of Covid-19 were detected, the PHA said.
All have been contacted and advised to self-isolate in line with public health guidance.
The PHA said that no additional public health interventions are required at present.
Dr Gerry Waldron, head of health protection at the PHA, said: "The spread of the Delta variant underpins the need for continued vigilance and adherence to the public health guidance - it is essential that people continue to work within the regulations and advice.
"We ask that the public remain vigilant, keep their interactions with others down, continue following the health advice to limit the spread of Covid-19, and engage with the Contact Tracing Service if identified as a confirmed case or a close contact, and follow the advice given.
PHA's surveillance systems are indicating that as of yesterday, 612 confirmed or probable cases of the Delta variant of COVID-19 have been detected throughout all 11 local council areas in NI. https://t.co/6I66NjOItd pic.twitter.com/XqmK51t70C— Public Health Agency (@publichealthni) June 23, 2021
"There is a range of testing sites across Northern Ireland, so if you develop symptoms or are asked to get a PCR test because you are a close contact, please book one as it will help reduce further spread and assist us in identifying any additional variants.
"We have also activated enhanced testing in areas where we have identified a number of probable cases of the Delta variant.
"In order to help slow down the transition of the variant, we have been asking asymptomatic people to get tested in selected, targeted neighbourhoods where cases have been identified.
"We would urge anyone who is invited to get tested under one of these localised programmes to do so."
If you test positive for COVID-19 or are identified as a close contact of someone who has tested positive, you must self-isolate to help stop the virus spreading any further.— Public Health Agency (@publichealthni) June 23, 2021
Find out more at https://t.co/lSisoYIOPP@healthdpt pic.twitter.com/R6PX7xuCpS
An Armagh-based GP, who is also Deputy Chair of the British Medical Association in Northern Ireland, has said a swift response is being taken to track the Delta variant there.
Speaking on RTÉ's Drivetime, Dr Frances O'Hagan said that the variant is being "tracked in real-time" by the Public Health Agency.
"We know where it was yesterday so that public health then can send pop-up testing to that area and encourage testing in those areas so that we can try and minimalise it. It is higher in Newry and Mourne area, and Derry and Strabane area."
Dr O’Hagan said some people who have received one dose of a Covid vaccine have contracted the Delta variant of the virus.
She said there are also positive cases in younger age groups in Northern Ireland. However, she said the rise in cases have not yet converted into a greater number of hospital admissions.
57% of the adult population have been fully vaccinated in Northern Ireland, Dr O’Hagan said.
Another 188 people have tested positive for coronavirus in Northern Ireland in the past 24 hours. No new deaths were recorded.
The Department of Health also said that 17 people are being treated for the virus in hospital, an increase of four since yesterday. None of the patients are in intensive care units.