Health Service Executive CEO Paul Reid has said that only 1.1 million vaccine doses are likely to arrive in Ireland in the first three months of this year, against an expectation of 1.24m.
He told an Oireachtas Committee on Health that there was a large number of vaccines, around 175,000, expected on the last day of March and delays meant the 1.2m target would not be reached until the first week of April.
Mr Reid said that an original figure of 1.7m doses was based on advance purchase agreements with manufacturers.
This was then adjusted to a target of 1.4m and then lowered again to 1.24m.
Sinn Féin's Health spokesperson David Cullinane asked how many times the HSE has had to adjust its plan due to supply problems.
Mr Reid said the operating plan had already been changed 15 to 17 times due to supply problems and changes to sequencing and prioritisation.
"Our first quarter has been an experience of high levels of frustration on supply issues, to be frank. Pfizer has been more stable but we did have one change to their supply line. Moderna, of late, unstable and certainly AstraZeneca has been unstable," he said.
"That has been an EU-wide issue as well. We would expect the second quarter to have a higher level of predictability."
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Mr Reid said the first doses of the Johnson & Johnson Covid-19 vaccine - if approved - are expected to arrive in Ireland from mid-April and not early April as initially planned.
J&J's vaccine, which requires only one dose for protection, is expected to be approved on 11 March for use in the European Union by the European Medicines Agency and EU officials have said deliveries could start next month.
"The assumption on Johnson & Johnson is about 600k over quarter two, but primarily back-ended. Smaller numbers in mid-April," Mr Reid told the committee
Ireland has ordered 2.2 million of the Johnson & Johnson single dose vaccines in advance purchase orders.
A decision on whether to approve this vaccine is expected this week from the European Medicines Agency.
Overall, 3.8m vaccine doses are expected in the second quarter, including the 600,000 Johnson & Johnson doses.
One antigen test has been validated for use in meat plants and the HSE will keep this under review, he said.
So far, 15 cases of the variant first detected in South Africa have been identified in Ireland along with six of the Brazilian variant.
The HSE said that retrospective contact tracing is occurring in some cases, and will increase as case numbers reduce to under 600 consistently.
He said recent serial testing in nursing homes has revealed positivity rates of just 0.2% and the positivity rate among healthcare workers, which was at 16% in the first week of January, is now roughly 4%.
Mr Reid said this can be attributed to decreasing cases of the virus in the community but also to the vaccination programme.
Additional reporting Fergal Bowers, Tommy Meskill
