Questions have been raised over the timeline of the rollout of Ireland's booster vaccination programme following the findings of a recent study.
The report conducted by experts tracking the effectiveness of the Pfizer vaccine found that the risk of contracting Covid-19 increases over time after receiving the second dose.
The findings published in the British Medical Journal concluded that the booster vaccine is needed to stop the likelihood of breakthrough infections.
The Research Institute of Leumit Health Services in Israel found that immunity from the vaccine gradually begins to wane after 90 days.
The study examined the health records of 80,057 adults who had a PCR test between mid-May and September at three weeks since their second vaccine.
None of those who took part in the study had previous evidence of a Covid-19 infection.
Of the 80,057 participants, 7,973 had a positive test result.
The report found that the rate of positive test results rose in line with the time period since their last injection.
The test results found that 1.3% of people tested positive 21 to 89 days after a second dose, but this increased to 2.4% after 90 to 119 days; 4.6% after 120 to 149 days; 10.3% after 150 to 179 days; and 15.5% after 180 days or more.
Experts concluded that immunity wanes three months after receiving the second dose of the vaccine.
They said: "Large scale Covid-19 vaccination campaigns can achieve control over the spread of the virus, but even in countries with high vaccination rates, breakthrough infections can occur.
"The risk of (Covid) infection in adults who received two vaccine injections increased with time elapsed since vaccination compared with the reference (individuals vaccinated in the last 90 days)."
Minister for Health Stephen Donnelly announced on Wednesday that people aged 16 and over with underlying health conditions would begin to receive their booster jabs in the coming week.
He said: "The HSE and the Department of Health are working through the details as to exactly how that will be phased."
He added: "Most people who are immunocompromised have either received or been offered a booster vaccine."
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