The CEO of the HSE Paul Reid has urged people to be "on their maximum level of watch" over the Christmas period.
Speaking at the HSE's weekly Covid-19 briefing, Mr Reid said this will be a "determining factor to what happens in hospitals in January".
He said there is a "high risk multiplier effect concerning three ingredients".
These are; an increased level of cases, with "a massive number of contacts"; the demands on the hospital system in the first weeks of January; and the disproportionate effect that Covid cases have on hospitals and the impact on bed capacity.
"This is a dangerous mix," he said.
He urged people to keep their contacts "as low as possible" and to keep house visits to "an absolute minimum" over Christmas.
Speaking about Covid-19 in Northern Ireland, HSE Chief Paul Reid says "it gives us a very stark reminder of what happens in a very rapidly changing manner" | Read more: https://t.co/pi7wmRzHok pic.twitter.com/jk7ov4QQIN— RTÉ News (@rtenews) December 17, 2020
Mr Reid said the HSE is seeing "very significant and concerning" trends of the virus with an increase each day in the last week in positive tests.
He said that week-on-week there has been a 30% increase Covid-19 swabs taken.
There has been a "significant" increase in tests carried out and on contact tracing calls made, he said.
'Intense' discussions ongoing over vaccine deliveries
Paul Reid said the first vaccine delivery will contain a minimum of 4,750 doses but "ideally more".
He said HSE vaccine teams will be working over the weekend and throughout the Christmas period to organise the delivery of first vaccinations.
He said "intense" discussions are ongoing between the HSE and Pfizer/BioNTech.
Paul Reid said the vaccination plan is for 2.2 million doses from Pfizer, starting with small volumes and building up throughout January and there should be significant quantities by the middle of February.
He said there are 180 staff on vaccination teams and 1,500 "peer vaccinators" all across the system who will be utilised to administer the vaccine.
The first mobilisation will be long-term care centres with mobile teams attending on site. After that, there will be a very different model used in Phase 3 which will be mass vaccination.
He said the cost of the vaccine was negotiated centrally by the EU and the issues being discussed with Pfizer are when the first delivery will arrive and how much will be in it.
He says the HSE is planning to work on the basis of regular deliveries every week or 10 days.
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Mr Reid told the briefing that the HSE has launched a "Contact Calculator", available on HSE.ie, which he said will help people to think about the number of people they meet.
He warned that people "will meet more people than they think" they will over Christmas.
No formal request for assistance from Northern Ireland
He said the situation in Northern Ireland demonstrates the volatility and lag between seeing a high number of cases and the "multiplier" effect on hospitals.
Paul Reid confirmed that the HSE has not received a formal request for significant resources or supports from the health service in Northern Ireland.
He said that if such a request comes, the HSE will consider what it can do to support them.
1,209 residential care facilities receiving support from the HSE
The HSE's Chief Operating Officer, Anne O'Connor has said attendances in Emergency Departments are increasing but are 15% below the number this time last year.
She said admissions to ED are increasing too, but are lower than this time last year.
Anne O'Connor said 1,209 residential care facilities are receiving support from the HSE. 129 are in receipt of specific supports,15 are in the amber category, and four are in the red category.
She said there are 65 open outbreaks in long-term care facilities.
On the 5th round of serial testing of nursing home staff the positivity rate is 0.33%
She said there are 57 staff affected by hospital outbreaks at St Luke's, Kilkenny, 11 in Waterford, 49 in Tallaght, 22 in Naas, and 19 in Letterkenny.
A 'critical period' for the spread of Covid-19
The HSE's Clinical lead on Covid-19, Dr Colm Henry said the positivity rate has "ticked up to 2.9% from 2.5%".
He told the weekly Covid-19 briefing that there are "trends causing concern" in relation to the spread of Covid-19 here.
He said there is a rise in the number of close contacts to 3.8, which he said is "not a surprise".
"This is a critical period," Dr Henry added.
"We know that if we don't reduce our number of contacts and suffocate the virus it will transmit in households, between generations and do so without restraint."
'Only meet people indoors who you trust with your life'
Professor Martin Cormican, the HSE's National Lead for Health Care Associated Infection urged people to be "honest with themselves about the risk they are taking".
He said for those intending to visit older relatives next week they need to reduce their contacts now.
"If you're taking risks this weekend, and you're visiting older relatives next week, you are putting their lives at risk," he said.
Prof Cormican said: "If you care for or plan to visit older or vulnerable people, you may put their life at risk."
"Only meet people indoors who you trust with your life," Dr Cormican said.
Professor Martin said: "When they arrive, welcome them with alcohol. This time it will be hand gel."
He warned people not to let their guard down over the course of Christmas Day.
"If you're going to be sitting around in groups all afternoon then wearing a mask in the morning isn't going to make much difference," he said.
Meanwhile, the National Public Health Emergency Team is meeting today, amid concern about the increase in cases of Covid-19.
The Chief Medical Officer, Dr Tony Holohan, said there were significant indicators that the disease was moving in the wrong direction.