The Chief Medical Officer has said that the latest data shows a "concerning increase in transmission" of the Delta variant of Covid-19 in Ireland.

In a post on Twitter, Dr Tony Holohan said it is estimated that the variant accounts for up to 20% of Covid-19 cases reported last week, and that there have also been a number of outbreaks associated with the Delta variant in the last week.

It comes as the Department of Health reported 284 further cases of the virus. There are 53 patients with Covid-19 in hospital, 13 of whom are being treated in intensive care.

The HSE said that the number of cases may change due to future data validation following the disruption caused by the cyber attack on the HSE's system.

Dr Holohan said that the increase in the transmission of the Delta variant is similar to a pattern in a number of other EU members states.

He said it was important that people who were not fully vaccinated continue to follow all public health advice.

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Earlier, the Health Service Executive's national lead on the vaccine roll-out programme said that based on supply, he expects people in their 20s to be fully vaccinated by the end of September.

Speaking on RTÉ's News at One, Damien McCallion said they are seeing more stability with supply and that this is a particularly busy period for vaccinations.

He said more than 340,000 people were vaccinated last week, the biggest week so far, and supply permitting, they expect another 300,000 people to be vaccinated this week, next week and the following week.

He said two of the biggest deliveries of Pfizer vaccines, over 318,000, are due to arrive into the country over the next two weeks, which will give supplies a boost.

Mr McCallion said there are three busy weeks coming up and from then on, there will be a lull in supply as the forecast for Pfizer and Moderna vaccines supplies change.

People aged 38 can register for a Covid-19 vaccine today and Mr McCallion said he expects those in their 30s to receive their first dose in June into July and to be fully vaccinated at the end of August.

He said those in their 20s will receive their first dose in August and their second dose in September.

Mr McCallion said he expects that the full vaccination data, which has been disrupted as a result of the cyber attack, to be back at the end of this week.

He said close to 66% of the population are partially vaccinated, while around 35% are fully vaccinated.

Following advice from the National Immunisation Advisory Committee that those who have had Covid-19 can take one does of the vaccine, Mr McCallion said people can decide for themselves if they want to take a second dose or not.

He reminded people that those in older cohorts can still register for vaccines.

Mr McCallion urged people to take their vaccination appointment when they get it as it allows them to maintain momentum and get the majority of people vaccinated as quickly as possible.

Yesterday, the Covid-19 vaccine registration portal opened to people in their 30s, starting with people aged 39.

More than 47,000 people aged 39 registered on this portal yesterday, according to HSE CEO Paul Reid.

In a tweet, Mr Reid said that more than 52,000 vaccines were given on four of the last seven days. On Thursday, 59,000 were administered, he said.

"This week and next will continue at strong levels based on supply. More to go yet but good progress," he said.

The vaccine portal is open for people aged 38 from today and subsequent days will see it open for those aged 37, 36 and then 35.

Appeal to those out in Athlone on 11 June

Separately, the HSE has appealed for people who were socialising in Athlone on the evening of 11 June and who think they may have been exposed to Covid-19, to attend one of its test centres.

It comes as the Department of Health in the midlands is investigating Covid-19 cases in the Athlone area, which may be the Delta variant.

The HSE said that the cases are associated with socialising by the river on the west side of the Athlone area on 11 June.

It said that while it is not yet confirmed if the cases are of the Delta variant, the variant is very transmissible and the spread of it must be minimised.

Professor Sam McConkey said this is the first time that the physical location of an outbreak origin has been identified by public health doctors.

The Head of Department of International Health and Tropical Medicine at the Royal College of Surgeons said it was the first time that this information was being used to direct those involved to go for free testing.

Meanwhile, the US has lowered its Covid-related travel advisory for Ireland to "level 3 - reconsider travel" from the highest "level 4 - do not travel," the US State Department said.

In Northern Ireland, the Department of Health has reported no further coronavirus deaths in the past 24 hours. 133 new cases of the disease were recorded with 16 Covid patients in hospital.

There are no Covid patients in ICU.

The average 7-day incidence rate per 100,000 is 56.5. The area with the highest rate remains Derry & Strabane on 177.9, the lowest remains Ards & North Down on 14.9.