The chair of the High-Level Task Force on Covid-19 Vaccination has said the programme is making ''quite significant advances'' on the day the Janssen (Johnson & Johnson) jab is being rolled out to those aged 18-34.

More than 750 pharmacies around the country have begun administering the single-dose vaccine to those in this cohort who have opted to receive it.

Professor Brian MacCraith said that supplies of the Pfizer and Moderna vaccines are reliable for the coming months, but acknowledged that there are still ''some uncertainties'' around AstraZeneca supplies.

Beyond July ''it was not clear'' in terms of the arrival of supplies of AstraZeneca, he said.

However, Prof MacCraith said there are enough supplies of AstraZeneca to complete the second dose for people aged 60-69 and to commence the opt-in for people aged 18-34.

He also said that, by tomorrow morning, more than 50% of the adult population will be fully vaccinated - around 1.86 million people.

Earlier, the Health Service Executive confirmed that another 343,000 vaccines were administered last week.

On four days last week more than 55,000 doses were administered each day, Prof MacCraith said, adding ''that's equivalent of the entire population of Waterford city fully vaccinated in one day.''

He said that widening the age cohorts for the Janssen vaccine is also set to make an impact on the vaccination programme.

''From a percentage perspective 5.5% of the adult population will be fully vaccinated just through Janssen vaccines alone in July.''

The HSE has said 200,000 doses of the Janssen vaccine are available this month.

Prof Mac Craith said a further 285,000 doses of the one-shot vaccine are due to arrive next month.

HSE Chief Executive Paul Reid said pharmacies would play a "key role" this week in administering available Jannsen vaccines to those aged 18-34.

The accelerated roll-out for younger people in pharmacies is part of a multi-track approach to speed up the pace of adult vaccination to counter the threat of the Delta variant.

A pharmacist in Dungarvan, Co Waterford - where coronavirus incidence is the second highest in the country - has said there is "huge capacity" within community pharmacies to administer the Janssen dose.

Speaking on RTÉ's Morning Ireland, pharmacist Daragh Connolly said it was a "very exciting day" for pharmacists who were "delighted" to be at the heart of fighting Covid-19.

He said his own pharmacy had to close off its registration list for the 18-34 cohort as over 150 people had already registered.

"Without knowledge of the supply of the vaccine [in coming weeks], we didn't think it would be fair to keep taking names, so we're just asking people to be patient with us, and indeed they have. There's a fantastic enthusiasm from this cohort to get vaccinated," Mr Connolly said.

His pharmacy has capacity to "very comfortably and safely" administer up to 100 vaccines per week and if it can get the supply to match this, then "more and more people" will be vaccinated.

"We are reading that there may be another 200,000 Janssen vaccines coming into the country, so I think there is huge capacity within community pharmacies to administer those vaccines in probably four to six weeks with a fair wind," he said.

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Asked about the high Covid-19 incidence rate in Dungarvan, Mr Connolly said it was important to remember that Covid infection can strike "anywhere at any time", adding that Dungarvan had been statistically below the average case numbers for most of the pandemic.

He added: "It just seems to be one or two instances where people let down their guard.

"Particularly with the Delta variant, people might not know they are infectious and find themselves in scenarios where they infect other people, so I think that's how we came about it here in Dungarvan."