People who were infected with Covid-19 in December and January may not be protected against the latest mutation of the virus.
The warning was made by Dr Cillian de Gascun, the Director of the Irish Republic’s National Virus Reference Laboratory.
Speaking to RTE’s This Week programme, he explained that the BA.4 and BA.5 variants currently circulating in Ireland are “sufficiently different” than the B.A1 variant that resulted in a large spike in December and January.
"The problem is that people who were infected with B.A1 don't have protection against B.A4 and B.A5," he said, adding those people were at risk of getting the disease again.
He added that “some protection” was possible in the cases of those infected with the B.A2 variant in March or April.
It follows a rise in the number of Covid-19 patients in Irish hospitals.
From Sunday, there were 826 patients in hospital with Covid-19, compared to 746 people last Sunday.
It’s further reported the number has risen by 30% in two weeks, and more than doubled from three weeks ago.
This is said to be the highest figure since April 14, when there were 904 people with the virus in Irish hospitals.
Intensive care figures for Covid-19 patients is said to be stable, with 32 infected people in ICU, just one more than a week ago.
Health officials have also clarified that around half of those inpatients were admitted for other reasons.
Dr dr Gascun said that the duration of immunity from Covid-19 was “quite short...maybe four to six months.”
"The durability of immunity against people who were infected by Omicron variants is maybe even shorter than the previous variants, Alpha and Delta," he said.
On a more positive note, he said the variants currently circulating appeared to cause less severe disease for vaccinated people as they infect the upper airways instead of the lungs.
He has now said that an Autumn vaccination should be considered to give people the necessary protection for the winter.
"Based on what we've seen to date, it's not unreasonable to expect there will be an increase in cases, because human behaviour will change and more people will be indoors again," he said.
While an Omicron-specific vaccine is being prepared, he said it was unclear if it would pass regulatory checks in time to be ready for the winter in Ireland.
His advice was to continue taking precautions like wearing a mask on public transport or in crowded settings.
Dr Gerald Barry, assistant professor in virology at UCD, also told RTE that if the current pattern continued, it was likely there would be a new variant of Covid-19 every three to four months.
This made it distinctive from largely seasonal illnesses like the flu.
Meanwhile, the UK Health Security Agency chief Dame Jenny Harries said she also expected Covid-19 hospital numbers to rise.
She said it did not appear that the current wave of the virus had peaked, and urged people to go about their normal lives in a “precautionary way”.
This week, Covid-19 figures across the UK as a whole jumped by more than half a million.
Speaking to the BBC’s Sunday Morning programme, she said: "It doesn't look as though that wave has finished yet, so we would anticipate that hospital cases will rise. And it's possible, quite likely, that they will actually peak over the previous BA.2 wave.
"But I think the overall impact, we won't know. It's easy to say in retrospect, it's not so easy to model forward."
According to the Office for National Statistics (ONS), around 2.3 million people in private households were estimated to have the virus, up 32% from the previous week.
Corresponding to figures in Ireland, this is also the highest estimate since late April, but still largely behind the 4.9 million peak of the Omicron BA.2 wave at the end of the March.