Immunisation clinics using the AstraZeneca vaccine have been cancelled for the rest of the week, the Health Service Executive has said.

Earlier, the HSE had advised hospital groups and Community Healthcare Organisations (CHOs) to cancel clinics for today.

Thousands of AstraZeneca vaccinations were planned for today.

But in a statement released this evening, the HSE said: "We have today further advised Hospital Groups and CHOs to cancel all AstraZeneca clinics, except those arranged for certain people over the age of 60, for the remainder of this week as we work to reconfigure our roll-out plans to take into account the latest NIAC guidance.

"Some clinics will be in a position to proceed with the vaccination of patients over the age of 60 this week, in line with the new guidance, and in these cases individuals will be contacted directly by their vaccination centre to arrange their appointment.

"Anybody who is due to attend an Astra Zeneca clinic, and who is not contacted directly in advance, is therefore advised not to attend."

It said arrangements will be made in due course to rearrange appointments.

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The HSE said it was continuing to assess the impact of a decision by the National Immunisation Advisory Committee (NIAC) recommending that the AstraZeneca vaccine be limited to people over 60 years of age.

The committee has advised that the vaccine be limited to use in people aged over 60, following an assessment of the risk of rare blood clotting side effects versus benefits, based on age.

Health officials have said that the AstraZeneca vaccine remains a very effective vaccine.

Most people under 60, who have already received their first dose of AstraZeneca vaccine, are due to receive their second dose as planned.

However, most of the future AstraZeneca vaccines will be used just for people aged 60 to 69 years.

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This afternoon, a spokesperson for Minister for Health Stephen Donnelly said that timelines around administering vaccines may need to be adjusted.

The spokesperson said the HSE is working through the impact of the decision on AstraZeneca and this work should take no more than a couple of days.

However, the spokesperson said there is no change at this point on the volume of vaccines coming into the country.

NIAC chairperson Professor Karina Butler said the committee is trying to find who might be most at risk of Covid-19 and wind a safe path through the competing risks and balance it as best as possible.

"We absolutely want to defeat Covid on the one hand, but we don't want to raise any unnecessary risk on the other hand," she added.

Speaking on RTÉ's Morning Ireland, Prof Butler said if AstraZeneca was the only vaccine available, then we would not be having this conversation (about changing the vaccine roll-out) because the benefits hugely outweigh the risks.

However, she added, there are a number of vaccines that have proven incredibly efficient and the population is not as likely to contract Covid-19 now, compared to January, because of falling infection rates.

She said the "very rare" side effect of AstraZeneca is "of high consequence" and increases as you go down in age groups.

However, the risk of severe outcome from Covid decreases in the younger age groups, she added.

Prof Butler said NIAC was aware that this decision poses more logistical difficulties, but said there may be a dividend there because if a patient is now selected for the Pfizer vaccine, the time to complete full vaccination is actually shorter than if they had been selected for AstraZeneca.

She explained that four to ten in one million people might experience the clotting side effect of the vaccine, but just one in a million will die. However, she said, this is still a high risk.

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Prof Butler said NIAC is monitoring real-world data from the UK and elsewhere in making future decisions about the use of the AstraZeneca vaccine.

She said that a large trial in the US looking at the use of AstraZeneca in the over 65s along with real-world effectiveness data from the UK shows one dose is very effective at protecting people and keeping people out of hospital.

Prof Butler said that it has been used very effectively and safely in Scotland and a decision to delay second doses is to allow more information to be gathered.

She added that 800,000 second doses were given in the UK with none of these clotting events reported, but it does take time to gather information.

The rare side effects noted seem to be linked to first doses, rather than second, but NIAC is happy people will have sufficient immunity after 21 days for a 12-week period while it continues to assess information, she explained.

Prof Butler said that NIAC is also looking at the impact of increasing protection from variants by using a second dose of a different vaccine, which is the decision some countries are making.

She said that changes and modifications to advice is continuous and more "twists and turns" are likely to come along the way, which requires patience from the public as NIAC attempts to steer a safe path through the pandemic.

Prof Butler said that in some cases where a person under 60 is due the second dose of Astrazeneca, it will be recommended this goes ahead as 'the risk of Covid exceeds several fold, the tiny risk [of clotting]' for this person.