The time people who have contracted Covid-19 should be considered immune from reinfection has been extended to nine months, the Health Information and Quality Authority (HIQA) has said.
This extension in the presumed immunity period would increase the number of under-50s who only need one dose of Covid-19 vaccine to be considered fully vaccinated.
In its report published on Thursday, the HIQA advised the National Public Health Emergency Team (Nphet) that studies suggest that most people develop immune memory after infection that lasts for at least nine months.
The advice followed a review of international evidence including 19 large cohort studies of reinfection involving over 640,000 previously infected individuals.
It was also informed by 13 studies on immune memory response along with expert opinion, from the Covid-19 Expert Advisory Group, HIQA said.
HIQA also said that people who had the virus in the last nine months would be exempt from serial testing.
Dr Mairin Ryan, HIQA's deputy chief executive and director of health technology assessment, said: "We have advised Nphet that the period of presumptive immunity should be extended from six to nine months post-infection.
"Across all the studies we examined, the risk of Sars-CoV-2 reinfection was consistently low, with no increase in infection risk over time.
Dr Ryan continued: "Increasing the period of presumptive immunity from six to nine months has widespread positive implications for people.
"For example, a person who has Covid-19 in the last nine months would be exempt from serial testing.
"A change would also increase the number of under-50s who only need one dose of a Covid-19 vaccine to be considered fully vaccinated.
"It would also have implications for the implementation and rollout of the proposed 'green certificates'.
"It will be important that any policy changes and the evidence behind them are clearly communicated and consistently applied."
The potential impact of new variants on natural immunity is evolving rapidly and needs to be kept under review, HIQA added.