NPHET's Professor Philip Nolan has warned that a wave of COVID-19 infections in young people will cause "severe illness and long-term complications in a sizeable minority".
He also says such a wave could also 'break through' vaccination protection to cause dangerous infections in some older and vulnerable people.
He was speaking after the fourth day in a row of COVID-19 case numbers above 1,000.
The five-day moving average of new cases now stands at 1,159 cases per day - the highest since early February.
Professor Nolan - chair of NPHET's modelling advisory group - said the "very steep rise" in cases shows the need for caution when faced with the Delta variant of the virus.
Weekend effects typically give low case counts on Sunday and early in the week; case counts over the next few days will be an important indicator. 3/9 pic.twitter.com/uusQHgZUpD
— Professor Philip Nolan (@President_MU) July 19, 2021
He said: "Incidence in those aged 16-29 years is rising rapidly, is higher than we have seen since end of January 2021, and higher than the October 2020 peak.
"We need to limit transmission: a wave of infections in young people will cause severe illness and long-term complications in a sizeable minority, and will break through the protection of vaccination to cause dangerous infections in older and vulnerable people."
He said this wave of the virus will not lead to the same level of severe disease and deaths as previous waves.
However, he said hospital numbers are rising - with around 16 hospital admissions per 1000 cases of the virus.
Professor Nolan said it's now vital to support those who aren't yet fully vaccinated to socialise "safely outdoors and distanced".