A return to restrictions cannot be ruled out despite the success of the vaccination programme, chief medical officer Tony Holohan is warning.
Dr Holohan has thrown a significant curveball just days ahead of the Government's planned further easing of restrictions.
The Government has already set out a phased relaxation of restrictions this month and next, including allowing up to 100 people to attend indoor activities such as dance and sports classes from Monday.
However, the rise of the Delta variant means that vaccination alone is unlikely to control the spread of the virus, Dr Holohan will tell politicians on Wednesday when he appears before an Oireachtas Committee.
The chair of the National Immunisation Advisory Committee (Niac), Dr Karina Butler, will also tell the Oireachtas Health Committee that older people in nursing homes are having a “poorer response” to the first two vaccine doses and their immunity is “more rapidly waning because of their age and underlying medical conditions”.
Dr Holohan warned of a significant number of infections in older, vaccinated people due to the spread of the virus.
"In the context of this highly transmissible variant, it is unlikely that vaccination alone, even at the high levels of vaccine coverage that we have now achieved, will bring the effective reproduction number below 1 such that we will achieve suppression of the disease," he said.
"This means that through this coming autumn and winter, possibly in the face of high levels of infection, we will remain dependent upon public understanding and buy-in to the basic public health measures in order to minimise opportunities for this virus to transmit."
Mask-wearing, self-isolation with the help of employers, and "robust public health surveillance" will be needed into the winter, but Dr Holohan will say that further public health measures cannot be ruled out on top of these actions.
"We cannot predict with certainty the future trajectory of the disease and, consequently, we cannot fully rule out the possibility that the reintroduction of measures may be required in the future."
This would scupper the Government's roadmap for reopening which sets out specific dates for the resumption of activities and events.
From September 20, indoor and after-school activities like dance and drama classes for children will resume.
However, the Government has indicated that virtually all remaining restrictions will be lifted from October 22, if 90% of people aged over 16 are fully vaccinated and if Covid-19 related hospital admissions are manageable.
Dr Holohan's statement says the vaccine programme, which has now reached 90% of adults with at least one dose, would allow for a regime focused less on restrictions and more on personal judgement in the medium term.
Meanwhile, Dr Butler will tell the committee that Niac continues to examine evidence regarding the need for a booster vaccine for specific groups.
"These include those at increased risk of severe Covid-19 disease, other older persons, and healthcare workers because of their role in providing essential health services," she will say.
She is also expected to tell the committee that boosters and flu shots should be given at the same time.
It comes as around 1,200 children a day are being identified as close contacts, prompting a leading public health expert to call for better supports for primary schools.
Professor of Health Systems at Dublin City University Anthony Staines said vaccinations will make “a big difference” for secondary schools but concerns remain for younger children.
“It is about keeping schools open,” he said. “It’s not going to happen by magic. It will happen if we bring in the right measures to reduce outbreaks; ventilation, masks for primary school as well," he said.